# Searching for very low brightness, red LED headlamp for astronomy



## stevetaylor199 (Jan 13, 2013)

I'm looking for a headlamp intended to preserve night vision under dark skies. I have had a few Energizer headlamps, and I just acquired a Petzl Taktikka Plus, but they all fall far short of my expectations. I've browsed through astronomy vendors' sites, but so far it seems that none of the headlamp manufacturers seem to get it when it comes to making an astronomy headlamp.

I just found this forum when searching for "moonlight" modes that have an output of fractions of a lumen, so I'm hoping to get a new perspective on LED lights here. (I've always enjoyed flashlights, too.)

Some of the ideal features in a headlamp would be a continuously adjustable output, starting either at the lowest or last-used setting; no chance of turning on a white or other non-red LED by accident; a red light that really *is* red, not orange (as I'm getting out of my Taktikka); output substantially less than 1.0 lumen; and perhaps even with some kind of adjustable beam. Any suggestions?


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## RNDDUDE (Jan 14, 2013)

The Surefire Minimus is now available with red snap-on filters, and a starting output of about 1 lumen, probably half that with the filter in place.


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## jonathanluu2 (Jan 15, 2013)

Zebralight makes a dedicated RED LED headlamp, several actually. They have two model catergories, one that takes CR123 batteries (H31) and one that takes AA types (H51). They are further subdivided into two beam patterns: a "floody" (H31Fr/H51Fr) or "spot/spill" version (H31r/H51r). I like the floody versions because they are good for working on things at arms length. However they struggle to illuminate far off features. This may be just fine for you though if most of your work will be walking and handling things at arms length in the dark.

I do not have the RED headlamp, but I believe that all other features are the same. I like the headlamp a lot and the user interface is easy to learn, but I am not happy with the company right now. Most of their stuff is on back order or sold out indefinitely... I think there is still the H51Fr in stock. Hope that helps!


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## lampeDépêche (Jan 15, 2013)

For the output range you are interested in, I really think your best solution is a Photon Freedom with a covert nose.

I own the Zebralight H51r that jonathan refers to. It's an awesome light--I use it as a rear light on my bike, because it kicks out such a flood of red photons.

But that's not what you are looking for. You want something for astronomy, something really dim.

The ZL H51r on its lowest setting *might* be dim enough for you, but what's the point of that? If a cook asks me for a good way to simmer a delicate dish on low heat, I don't say "buy an oxy-acetylene torch--you can always turn it down!" If you ask me for a way to play your baby some gentle lullabies so it can fall asleep, I don't say "buy a 5,000 watt sound system with mega-woofers--you can always turn it down!"

So you don't need a powerful light to begin with. What you need is a light that has wide flexibility in the low range, e.g. 2 lumens down to zero lumens.

The Photon Freedom has that, to perfection. Infinitely adjustable, and really, really dim. And on lower light-levels, a single 2032 coin cell will last 50+hours, no problem.

It comes with a great clip that will let you put it on a ball-cap, or a head-band, or anything you like. I often clip it to my glasses frame--it's light enough that I don't notice it there.

The covert nose makes sure that you don't shine it in anyone else's eyes, either.

Again, I love ZL, and if you have a spare $50 bucks or so, and you want a red light that you can light up a huge area with, then get it. But for ultra lows, you could just spend $10 on a Freedom and get something much closer to what you want.


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## jonathanluu2 (Jan 15, 2013)

Good show lampe. I am not familiar with smaller lights, though this looks to fit the bill much better. I will keep the H51r in mind if I want to upgrade my rear bike light.


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## Gregozedobe (Jan 16, 2013)

lampeDépêche said:


> For the output range you are interested in, I really think your best solution is a Photon Freedom with a covert nose.



+1

If you really want a specific "red" then if you can buy a std 5mm LED in the "red" you want it is fairly easy to swap that new LED into your PFM.


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## eh4 (Jan 17, 2013)

With the photon freedom the only way you'd accidentally turn it on high is by negligently clicking it... holding the button down starts the light on lowest level and ramps up till you release the button.


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## cland72 (Jan 17, 2013)

RNDDUDE said:


> The Surefire Minimus is now available with red snap-on filters, and a starting output of about 1 lumen, probably half that with the filter in place.



I was coming here to post this.


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## B0wz3r (Jan 17, 2013)

This is something I know a great deal about, and I've posted extensively on it in various threads here in the headlamp forum, and in the general LED flashlight forum. You should be able to find posts I've made about it by checking my post history in my profile.

To cut to the chase, there aren't currently any headlamps on the market that I'm aware of that are _true_ night vision lights. All of them use a wavelength in the 620 - 630 nm range of the visible spectrum, which is well into the orange range. Technically they are specified as red-orange in their color. To get a red light that is a true night vision light, you need a wavelength that is 660 nm or greater, in the 'deep-red' range of the visible spectrum. The only light that I know of that is a true deep red is made by a company called Rigel Systems, an astronomy accessories company. I forget what the light itself is called, but they make two or three different versions of it, and it's in the $30 - $40 range. It runs off of a 9v battery, and is a little too bulky to be converted into a headlamp. In general, a Photon Micro with a covert nose is probably your best bet, as they come with a nifty little clip that makes them easy to mount on the bill of a cap, or stick on anything magnetic. 

Also, rods, the photoreceptors in the retina that are responsible for our low-light vision (called scotopic vision) are maximally sensitive at a wavelength of about 505 nm, which is why a deep red light won't affect them. However, a red-orange light will. Despite their wavelength sensitivity, color information from the light they are sensitive to is not encoded by the brain, so we are in effect color blind to the information they give us. 

This gives a simple rule of thumb for night vision lights; if you can see the color of the light, it's too bright, and you are compromising your scotopic vision. A good way to see this is to use one of the Photon 'night-vision green' or NVG lights. With your eyes dark adapted, ramp up the light from off and at first it will look white; as the intensity increases you'll reach a point where the light begins to look green. That is the point at which your cones are activated and you are compromising your dark adaptation. 

In short, any light will work as a night vision light, provided it is of a low enough intensity. So far, the only light I've got that allows me personally to experience that is my Zebralight H502d with the L2 mode programmed to the lowest, and next to lowest settings. In general though, I use Photon Micro coverts for things such as star parties and the like. A pair of NVG ones, one on my cap, and one on a cord around my neck, and a red one also on my cap via the clip. The NVG ones work best for me personally, but I use the red one too because it avoids arguments with people who aren't properly informed about the subtleties of visual perception and night vision.


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## cland72 (Jan 17, 2013)

Holy crap.


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## jonathanluu2 (Jan 28, 2013)

Hats off to you sir. I see that you have really educated yourself in the matter.


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## Bicycleflyer (Jan 28, 2013)

I have a Princeton Tec "Byte" It has a single red led that is really too dim to be of use to me, so it could work for you.


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## Ajraservices (Feb 9, 2013)

B0wz3r said:


> This is something I know a great deal about, and I've posted extensively on it in various threads here in the headlamp forum, and in the general LED flashlight forum. You should be able to find posts I've made about it by checking my post history in my profile.



That is great information. I didn't realize that about our eyes and the wave length. I dont believe I have seen any light list the wavelength of the red and/or blue lights. 
Thanks for the information.


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## EscapeVelocity (Feb 9, 2013)

Thank you B0wz3r. Hat's off to you.


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## B0wz3r (Feb 11, 2013)

Glad to help guys. I don't normally announce my credentials on internet forums, but I hold a PhD in experimental cognitive psychology, am a college professor, and I teach classes in visual neurophysiology, sensory perception, physiological psychology, and so on as part of my job. I've also worked as a neurophysiology researcher using rhesus monkeys for the study of brain based mechanisms of visual attention, spatial perception, and color perception. In other words, I've cut holes in monkey's heads, and then stuck needles in their brains to record what their brains are doing in response to various kinds of visual and perceptual tasks. Although, I only work with human subjects now, and no more brain-needles kinds of things. It's a lot less messy, and a lot more rewarding.


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## MikeAusC (Feb 12, 2013)

stevetaylor199 said:


> I'm looking for a headlamp intended to preserve night vision under dark skies. . . . .


OK, you've made it clear what the light should NOT do, but you haven't said what you want to do when it's on.SEE LARGE OBJECTS - if you just need to see large objects with detail vision needed then extremely low lights levels will work. You can use any colour light because the Rods are sensitive across the full spectrum, but you need to keep the level as low as possible to minimise unnecssary desensitisation of the Rods. Everything will appear bluey-green because the Rods cannot provide any colour information. If the objects appear Red, then the light is so bright it's triggering the Cones, indicating the brightness is well above the level needed to trigger Rods, causing night-vision desensitisation.SEE DETAIL OR READ NORMAL TEXT - if you want to able to read text smaller than 1cm high at arm's length or see detail like marker lines, you need enough light to trigger the Cones. Deep red (close to 700nm) is the best colour because that's where there is the greatest DIFFERENCE in sensitivity between the Rods which can detect starlight-level light and the Cones which you need to see detail or to see colour. Both types will respond to light across the full 400 to 700nm, but close to


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## MikeAusC (Feb 12, 2013)

stevetaylor199 said:


> I'm looking for a headlamp intended to preserve night vision under dark skies. . . . .



OK, you've made it clear what the light should NOT do, but you haven't said what you want to do with the light when it's on.

*SEE OBJECTS* - if you just need to see objects with no detail vision needed, then extremely low lights levels will work. You can use any colour light because the Rods are sensitive across the full spectrum, but you need to keep the level as low as possible to minimise unnecessary desensitisation of the Rods. Everything will appear bluey-green because the Rods cannot provide any colour information.
If the objects appear Red, then the light is so bright it's triggering the Cones, indicating the brightness is well above the level needed to just trigger Rods, thereby causing night-vision desensitisation.

*SEE DETAIL OR READ NORMAL TEXT* - if you want to able to read text smaller than 1cm high when at half arm's length, or see detail like marker lines, you need enough light to trigger the Cones. Rods are not able to provide the detailed vision we are used to and depend on to read normal text.

So you need more than the minimal levels for barely triggering the Rods, but just enough to trigger the Cones for detailed vision.

Deep red (close to 700nm) is the best colour because that's where, across the spectrum, there is the greatest DIFFERENCE in sensitivity between the Rods which can detect starlight-level light and the Cones which you need to see detail and to see colour. Rods and Cones will respond to light across the full 400 to 700nm, but close to Deep Red a low level of light will cause triggering of Cones for detail vision - while having the LEAST leaching of the light sensitive chemicals in the Rods, thereby allowing maximum sensitivity to starlight levels, AFTER you've turned the Red light off.

*RECOGNISE COLOURS* - if you need to recognise colours (not just see them in monochrome) e.g. to read a map or a coloured panel, you need a higher level of white light to triggger the Red, Green and Blue Cones.


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## lightdelight (Feb 12, 2013)

So, is it possible to modify an H51r with a deep red rebel? That would be the light to have..


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## B0wz3r (Feb 12, 2013)

MikeAusC said:


> *SEE DETAIL OR READ NORMAL TEXT* - if you want to able to read text smaller than 1cm high when at half arm's length, or see detail like marker lines, you need enough light to trigger the Cones. Rods are not able to provide the detailed vision we are used to and depend on to read normal text.
> 
> So you need more than the minimal levels for barely triggering the Rods, but just enough to trigger the Cones for detailed vision.
> 
> Deep red (close to 700nm) is the best colour because that's where, across the spectrum, there is the greatest DIFFERENCE in sensitivity between the Rods which can detect starlight-level light and the Cones which you need to see detail and to see colour. Rods and Cones will respond to light across the full 400 to 700nm, but close to Deep Red a low level of light will cause triggering of Cones for detail vision - while having the LEAST leaching of the light sensitive chemicals in the Rods, thereby allowing maximum sensitivity to starlight levels, AFTER you've turned the Red light off.



I disagree that deep red is best for reading under low light conditions. In order to activate the cones the light intensity needs to be at the mesopic level, the intensity level that's at the boundary of the photopic/scotopic transition. A night-vision green light, with a wavelength of about 505 nm will be best because there's a fair amount of overlap between the wavelength sensitivity range between the medium and long wavelength cones, and will produce a stronger response in them.

The reason this is better for reading detail isn't because of the difference in wavelengths and senstivities, it's actually the opposite. High acuity vision is encoded by the low ratio of cones to parvocellular retinal ganglion cells in and around the macula, and in order to see detail the parvocellular system needs to be activated. A red light, particularly in the deep red zone of 700+ nm would have to be very bright to be able to produce the necessary activation of the parvocellular system to allow acuity and detail vision, and more likely to compromise scotopic vision as a result. Because of the large amount of overlap in the sensitivity ranges of the medium and long wavelength cones, the 500 nm light will produce greater activation at a lower objective intensity than a red light will. This will produce greater acuity with a dimmer light as a result. 

Overall, the most important factor in being able to read and/or see detail when dark adapted (which isn't the same as being in scotopic vision) is activating the parvocellular retinal ganglion system, and this can be done with a dimmer light at the 500 nm wavelength than with a light at the 700 nm wavelength.


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## EscapeVelocity (Feb 12, 2013)

Traditionally in the astronomy crowd, low level red lights have been the way to go. Ive heard the US Military is going to green though, for ultra low light preserving night vision use....and Im sure that they are well informed in the research to make this change from red.


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## B0wz3r (Feb 12, 2013)

EscapeVelocity said:


> Traditionally in the astronomy crowd, low level red lights have been the way to go. Ive heard the US Military is going to green though, for ultra low light preserving night vision use....and Im sure that they are well informed in the research to make this change from red.



That is one reason that I know of. I'm also of the understanding that deep red night vision lights can produce quite a bright signature on IR scanning/detection equipment, making it quite easy to find people/vehicles using them as a result. The 505 nm NV lights don't have that same problem.


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## MikeAusC (Feb 12, 2013)

EscapeVelocity said:


> Traditionally in the astronomy crowd, low level red lights have been the way to go. Ive heard the US Military is going to green though, for ultra low light preserving night vision use....and Im sure that they are well informed in the research to make this change from red.



The reason the military use Green for night time illumination is because of Night Vision Goggle use is widespread.

If you use Red lighting you will stand out like a beacon to the enemy's NVGs. You will also cause blinding glare if any of your colleagues using NVGs are watching an area illuminated by your red light.

"Always use Red light at night" is just an oversimplification. What colour and how bright depends totally on what you need to do - and that was the point of my message.

On board ships, the ONLY colour lighting that is permitted on deck at night is red (unless you need to read a chart !)


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## EscapeVelocity (Feb 12, 2013)

Thanks for the further explanation.


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## B0wz3r (Feb 13, 2013)

EscapeVelocity said:


> Traditionally in the astronomy crowd, low level red lights have been the way to go. Ive heard the US Military is going to green though, for ultra low light preserving night vision use....and Im sure that they are well informed in the research to make this change from red.



I've run into issues with people some times because of this. They object to my use of my NVG lights (I typically use a pair of Covert Photon Micros, one on my cap and the other on a neck cord), instead of a traditional red light. After a while, I got tired of explaining it to them, and simply bought a red one that I also use on my cap at star parties and the like. So most of the time, I use the red one, even though it's actually a red-orange rather than a true deep-red, and use my NVG ones when I need to read or write and the like. I like to think I'm helping to educate people about this, though it's definitely a slow process!


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## SemiMan (Feb 13, 2013)

B0wz3r said:


> I disagree that deep red is best for reading under low light conditions. In order to activate the cones the light intensity needs to be at the mesopic level, the intensity level that's at the boundary of the photopic/scotopic transition. A night-vision green light, with a wavelength of about 505 nm will be best because there's a fair amount of overlap between the wavelength sensitivity range between the medium and long wavelength cones, and will produce a stronger response in them.
> 
> The reason this is better for reading detail isn't because of the difference in wavelengths and senstivities, it's actually the opposite. High acuity vision is encoded by the low ratio of cones to parvocellular retinal ganglion cells in and around the macula, and in order to see detail the parvocellular system needs to be activated. A red light, particularly in the deep red zone of 700+ nm would have to be very bright to be able to produce the necessary activation of the parvocellular system to allow acuity and detail vision, and more likely to compromise scotopic vision as a result. Because of the large amount of overlap in the sensitivity ranges of the medium and long wavelength cones, the 500 nm light will produce greater activation at a lower objective intensity than a red light will. This will produce greater acuity with a dimmer light as a result.
> 
> Overall, the most important factor in being able to read and/or see detail when dark adapted (which isn't the same as being in scotopic vision) is activating the parvocellular retinal ganglion system, and this can be done with a dimmer light at the 500 nm wavelength than with a light at the 700 nm wavelength.







B0wz3r said:


> I disagree that deep red is best for reading under low light conditions. In order to activate the cones the light intensity needs to be at the mesopic level, the intensity level that's at the boundary of the photopic/scotopic transition. A night-vision green light, with a wavelength of about 505 nm will be best because there's a fair amount of overlap between the wavelength sensitivity range between the medium and long wavelength cones, and will produce a stronger response in them.
> 
> The reason this is better for reading detail isn't because of the difference in wavelengths and senstivities, it's actually the opposite. High acuity vision is encoded by the low ratio of cones to parvocellular retinal ganglion cells in and around the macula, and in order to see detail the parvocellular system needs to be activated. A red light, particularly in the deep red zone of 700+ nm would have to be very bright to be able to produce the necessary activation of the parvocellular system to allow acuity and detail vision, and more likely to compromise scotopic vision as a result. Because of the large amount of overlap in the sensitivity ranges of the medium and long wavelength cones, the 500 nm light will produce greater activation at a lower objective intensity than a red light will. This will produce greater acuity with a dimmer light as a result.
> 
> Overall, the most important factor in being able to read and/or see detail when dark adapted (which isn't the same as being in scotopic vision) is activating the parvocellular retinal ganglion system, and this can be done with a dimmer light at the 500 nm wavelength than with a light at the 700 nm wavelength.




Going to write some stuff for a discussion:

- The ratio of scotopic sensitivity to photopic sensitivity is minimized beyond 660nm and is within 20% of this at 650nm. Given spectrum width limitations, 680nm sources are about the lowest that will not stimulate below 660nm.

- It has been shown in the lab detection that the detection threshold for 680nm light flat lines after about 10 minutes (similar to all cones), but for wavelengths 630nm and below, the detection threshold keeps increasing for up to 40 minutes. That implies (or shows) that 680nm light is most effective for not causing loss of sensitivity of the rods.

- Below mesopic levels, say 0.1lux, it does not make any difference what color light you use as long as they are of the same scotopic brightness, they will have the same impact on desensitizing the rods in the eye. From an electronic detection standpoint (military), you are best to pick the color that uses the least radiant power (500nm) as most electronic detectors are pretty flat across the visible spectrum and if anything dip at shorter wavelengths.

- BOwz3r, you used the term "bright" but perhaps you needed to clarify. You don't need a light that is any brighter at 680nm than at 500nm for photopic vision. By bright, I will clarify as photopic brightness, not radiant power. You would need a lot more radiant power at 680, but that is not what we are talking about here.

- If we determine that we need PHOTOPIC levels, then 680nm IS the right choice as there will be the highest stimulation of the cones for the corresponding least stimulation of the rods and least impact on threshold detection sensitivity. The question becomes what about Mesopic levels.

Now by saying the parvocellular retinal ganglion system needs to be activated are you not just using fancy terms to say we need to be at least at mesopic lighting levels .... i.e. we need to be at a lighting level above the cone detection threshold such that their signals feed into the midget cells and combine with the signals from the rods? 

If that is so, then you definitely have not proven your argument that 680nm is not best and certainly not that 500nm is best.

In order to be at mesopic lighting levels, you need to be above the detection threshold of the cones. From a photopic "brightness" standpoint, that level is independent of wavelength. I.e. it will take 0.5 lux of blue, green or red. The radiant power varies, but the photopic brightness is consistent. However, there is a big difference in the scotopic brightness at those difference wavelengths varying between high for blue/green down to low for red.

The question becomes, does rod response have any impact on whether we are at a state of mesopic vision? I am not asking if rod response contributes to mesopic vision, I know it does, I am asking if rod response impacts it. I.e. if I am at a level high enough for the cones to be activated, does it matter what the corresponding stimulation of the rods is?

I will forward that for the purposes of this discussion, where we are talking about reading, we are only taking about the fovea which is near devoid of rods. Hence, it only matters what the photopic response of the cones is.

As stated above, the ideal wavelength(s) for achieving stimulation of the cones without corresponding stimulation of the rods is 660nm and above. Hence purely for the purposes of reading, i.e. central vision cone, it appears the argument put forward does not apply and that 680nm is best. If there was 0 optical scatter in or outside the eye, in theory you could light up just what you wanted to read with any color due to the lack of rods in the fovea, but that is practically not possible.

There is also a lot more red cones in the fovea, but at mesopic light levels, I am not sure the benefit from this will be achieved though it may result in higher contrast sensitivity at low light levels.

People may want to look up photochromatic interval if interested.

Semiman


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## Illum (Feb 13, 2013)

From experience a cluster of three 5mm LEDs driven at 20mA is TOO BRIGHT to work with when its dark enough to see the milky way. My best light so far is this: http://www.candlepowerforums.com/vb/showthread.php?286907

single red piranha LED, 6.8ohm resistor, 2xNiMH, no reflector and a diffused window. 

I too am looking for a headlight for astronomy. I currently "star surf" using the minimag, setup and breakdown using an E2L and a red filter.


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## SemiMan (Feb 13, 2013)

MikeAusC said:


> The reason the military use Green for night time illumination is because of Night Vision Goggle use is widespread.



Further to this, a big reason for the military shift away from red for in-vehicle lighting (land, air, sea), is the need for fully adapted night vision has been reduced in importance compared the ability to read the complex instrument panels that are present in today's vehicles. Those panels normally cover way more than just the central vision and hence place different requirements on the lighting. Deep red would be poor in this case as you would lose peripheral vision.


Semiman


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## EscapeVelocity (Feb 13, 2013)

I currently use an Energizer 7 LED headlamp, however you have to close your eyes to cycle through 3 modes to get to the two red LED mode....and if you are imaging, it's best to cover the headlamp with your hand when doing so. It isnt ideal.

I saw this the other day and thought it might be good. 

Petzl E-Lite Headlamp


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## B0wz3r (Feb 13, 2013)

SM; I'm going to delete the portions of your post that aren't directly relevant to the comments I'll make, or that I agree with and/or think I've already stated clearly, and/or the things you state that are in agreement with what I've already posted.



SemiMan said:


> Going to write some stuff for a discussion:
> 
> - The ratio of scotopic sensitivity to photopic sensitivity is minimized beyond 660nm and is within 20% of this at 650nm. Given spectrum width limitations, 680nm sources are about the lowest that will not stimulate below 660nm.
> 
> - It has been shown in the lab detection that the detection threshold for 680nm light flat lines after about 10 minutes (similar to all cones), but for wavelengths 630nm and below, the detection threshold keeps increasing for up to 40 minutes. That implies (or shows) that 680nm light is most effective for not causing loss of sensitivity of the rods.



The problem here is that this occurs only at the level of the individual receptor. The eye/brain encode brightness based on total incoming signal strength, which means neurophysiologically speaking, the total number of action potentials received by the primary visual cortical areas. (I'll ignore for now the V1-2 vs V4 distinction for what types of information are encoded in different areas of occipital cortex, because that's not relevant to my point.) 

While there can be a fair amount of individual variation, neurophysiological and psychophysical studies have shown that about 60% of all cones are long wavelength, and about 30% are medium wavelength, measured as averages across a large sample of normal humans as the population of interest. Thus, even though the detection levels may differ at the level of the individual cone receptor, that makes little difference when you're looking at the mass action of all the red cones working together. 

As evidence of this, it's been shown in psychophysical studies that two narrow spectrum lights, say one at 645 nm, and another at 425 nm, both of equal objective brightness (lux, candela, whatever, but let's just say total photons for now to be clear), the red one will appear perceptually brighter than the blue one, simply because of the fact there are more red cones being stimulated than blue cones, even though the actual amount of photons entering the eye are identical. Thus, even though the detection threshold of the red cones is higher than that of the the other two types, the far greater signal gain they produce through their mass action will easily outweigh the gain of the signal from the cones with lower detection thresholds.



SemiMan said:


> - BOwz3r, you used the term "bright" but perhaps you needed to clarify. You don't need a light that is any brighter at 680nm than at 500nm for photopic vision. By bright, I will clarify as photopic brightness, not radiant power. You would need a lot more radiant power at 680, but that is not what we are talking about here.



Yes, I was using my terminology loosely and I apologize for that. _Intensity_ is the term used to specify the object amount of stimulation in the light, as above, it's easiest to think of this as the total number of individual photons. The psychological correlate of this is _brightness_, the perception of the physical intensity of the stimulus. Similarly, _illuminance_ refers to the amount of light emitted from a generating source, such as a bulb filament or an LED, while _luminance_ refers to the amount of light reflected off of a surface. The percentage of light reflected off a surface is referred to as _albedo_; this is seen directly in things like printer paper brightness ratings, where a rating of 92 means 92% of light falling on the surface is reflected and so on.

So yes, strictly speaking, I should have distinguished between intensity and brightness.



SemiMan said:


> - If we determine that we need PHOTOPIC levels, then 680nm IS the right choice as there will be the highest stimulation of the cones for the corresponding least stimulation of the rods and least impact on threshold detection sensitivity. The question becomes what about Mesopic levels.



As above, the total amount of light needed for mesopic vision is more strongly influenced by signal gain of the different kinds of receptors than it is by their individual detection thresholds. Strictly speaking, there's going to be a fuzzy area around the mesopic cut-off due to individual differences and environmental conditions. In general, and given the overall sensitivity of the retina as a whole as a detection device, I'd say 5% above the transition point should be more than sufficient to invoke photopic activity. Overall though, we can only speak in generalities about this, mainly due to individual differences between observers.



SemiMan said:


> Now by saying the parvocellular retinal ganglion system needs to be activated are you not just using fancy terms to say we need to be at least at mesopic lighting levels .... i.e. we need to be at a lighting level above the cone detection threshold such that their signals feed into the midget cells and combine with the signals from the rods?
> 
> If that is so, then you definitely have not proven your argument that 680nm is not best and certainly not that 500nm is best.



The parvocellular retinal ganglion cells are the same cells you're referring to as the "midget" cells. I think your familiarity with that term compared to the one I'm familiar with indicates generational differences in our educational backgrounds.  I use it to keep clear distinctions between parvo cells, magno cells, konio cells, and so on in discussion. 

That said, IIRC (I'll have to look it up to be sure), parvo input doesn't integrate with magno input at least until the blue/yellow opponent process. It might not be until V4 or V5; as I said, I'll have to look it up to be sure. Regardless, in either case, the issue of signal gain again comes up, and is more important particularly if the parvo and magno integration occurs in the opponent process and in lateral inhibition between receptors than at the cortical level. In that case, the far greater gain of the red signal compared to the green and blue signals is going to overcome any differences in individual sensitivity of the different types of cones, and a bright 680nm light is going to have a greater desensitization effect than a much dimmer light at 505nm IMHO.



SemiMan said:


> The question becomes, does rod response have any impact on whether we are at a state of mesopic vision? I am not asking if rod response contributes to mesopic vision, I know it does, I am asking if rod response impacts it. I.e. if I am at a level high enough for the cones to be activated, does it matter what the corresponding stimulation of the rods is?



In short, no. As long as the rods are not overwhelmed as they typically are during normal photopic vision, so they aren't responding at all, they'll still be able to respond to levels of light well below the mesopic transition. To my knowledge, rods aren't differentially affected by varying levels of intensity in the scotopic range. (Again, keep in mind that what constitutes scotopic will be very different for different people; my scotopic threshold is noticeably higher than my wife's, and she regularly complains about the brightness of light I need when we're at the telescope). In short, the rods should have the same response characteristics once the amount of light they are exposed to is low enough to allow their proper functioning, and variations in intensity will only affect the strength of their response and not their sensitivity or threshold.



SemiMan said:


> If there was 0 optical scatter in or outside the eye, in theory you could light up just what you wanted to read with any color due to the lack of rods in the fovea, but that is practically not possible.



Of course not; chromatic aberration of blue vs red light through the lens means that we'd have a much more difficult time reading something in blue text against a colored background than say black text against a white background. Blue text on a white background illuminated by red light would appear black though, and would be legible because of the response of the red cones in the macula despite the lack of blue cones in the macula.

Overall, your points are well made, but to my knowledge they don't consider the larger issues of the perceptual correlates that go along with the objective measures of stimulus intensity, and your discussion of the individual sensitivity characteristics of the different types of cones at the individual level is counteracted by the differences in signal gain that lead to differences in perceptual correlates because of the different population sizes of the different cone receptor types. 

As a case in point, look at the purkinje shift in perceived brightness between red and blue. As I mentioned earlier, we normally see a red light as brighter than a blue light despite them being equally objectively intense. As overall light levels dim in the atmosphere (I'm going to ignore the issue of Rayleigh Scatting here), and there's less red light available and more blue light, we experience a qualitative perceptual shift and experience blue as brighter than red because of the lack of red light in the environment. This again points to the fact that it is the gain of the signal between the three types of receptors that is more important in determining not only the perceptual correlate of the physiological response, but also impacts the physiological response as well (which it should, of course). So when you have enough blue light compared to red light, we then experience blue as brighter than red, the opposite of what normally occurs during photopic vision. Further, the purkinje shift is document to occur only at intensity levels in the photopic zone that are close to the photopic-scotopic transition. 

Another issue that you haven't touched on, and I have only mentioned in passing so far, is that of dark adaptation vs scotopic adaptation, as they aren't the same thing. Dark adaptation involves the ocular responses of the iris and retina to dimming levels of light. The iris is actually an involuntary muscle that responds to changes in overall light levels and is independent of receptor responses (both rods and cones). Contraction of the muscle results in contraction of the pupil, and relaxation of the iris results in dilation of the pupil. This is why it takes so long for full dark adaptation to occur, as there is no counteracting extensor muscle in the iris that I am aware of to assist the dilation of the pupil. 

As far as I know, by the time light levels reduce enough to progress to the photopic/scotopic transition, pupillary dilation has already reached its maximal level, and plays no role in the effectiveness of scotopic vision. However, when a bright light is suddenly flashed, Whytte's Reflex is invoked causing a very rapid (in the range of 20 - 30 ms) contraction of the pupil attenuating the total amount of light entering the eye. This also is not a progressive change; if the light isn't _bright enough_, Whytte's Reflex won't be invoked. Thus, the time course of the transition between photopic and scotopic vision is governed almost exclusively by the time it takes for dark adaptation to occur. Scotopic adaptation on the other hand, were it not limited by the optical light gathering limitations of the eye, would take no longer than the average latency of the signal processing between the two major divisions of the photoreceptors and how their corresponding ganglion cells response characteristics work. In short, we'd only need about half a second to fully transition from photopic to scotopic, because that's the time course of the signal processing in the neural pathway from retinal ganglion cell to the occipital cortex.

To sum up, all the evidence I am aware of, that I've tried to explain above as best I can in this limited format, indicates that despite differences in the sensitivity and thresholds of the individual types of cone receptors, issues of gain and how the brain encodes brightness easily trump differences arising from threshold sensitivity when activating parvocellular vision. Thus, as I said before, to the best of my understanding, a very bright red light is far more likely to compromise night vision ability (particularly because of things like Whytte's Reflex) than is a lower level light teal NVG light that is ramped up just enough to bring an observer into their mesopic zone and activate the parvocellular system enough to allow reading to occur.


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## SemiMan (Feb 13, 2013)

Don't take this the wrong way, but you have said a lot but not really addressed my points those being:

- No rods in fovea so purely cones
- Photochromatic interval is 0 past 650 nm, i.e. pretty much only red cone response, without little to no rod response. Therefore I can stimulate cones (photopic vision) with limited impact on rods and hence no impact of dark adaption of the rods

On some other points:

- In terms of midget cells, this is outside my area of expertise, but the rods/cones do not feed directly into the parvocellular ganglion cells but into an intermediary cell group first

- In terms of iris response, it is not dependent specifically on overall light levels, but predominantly on light levels below 520nm (peaking at 480) there is response above 520, but that only comes into play at very high light levels. While initial adaption of the ocular system is fast (seconds) for a certain level of response, the full dark adaption of the cones (10 minutes) is much slower than the iris response and the full dark adaptation of the rods is up to 40 minutes (with some wavelength sensitivity).

- It was not the difference of intensity and brightness per se. that was needed, but of radiometric intensity versus photopic intensity. Candela, lux, and total photons are quite different. The first two are scaled to the response curve of the photoreceptors, the last is not and it is critical to make the distinction when talking about brightness.

- Interestingly enough, if you have two "equal" lights in a narrow field, one at 630 say and one at 480, the 630 will appear brighter. However, if you light the whole visual field (or at least +/- 20 degrees of center), the 480 will appear brighter ... assuming a white background of course.

"Overall, your points are well made, but to my knowledge they don't consider the larger issues of the perceptual correlates that go along with the objective measures of stimulus intensity, and your discussion of the individual sensitivity characteristics of the different types of cones at the individual level is counteracted by the differences in signal gain that lead to differences in perceptual correlates because of the different population sizes of the different cone receptor types. "

I actually understand your points, but you seem to be missing the overall point ....

a) This is not about perceptive brightness... scenes with lots of blue in the spectrum are perceptively brighter, but are not in reality "brighter". In terms of population sizes of different cone receptor types, you stated there are more red and I will be stimulating more red ... though I don't think that plays as much into it.

b) "a very bright red light is far more likely to compromise night vision ability (particularly because of things like Whytte's Reflex) than is a lower level light teal NVG light that is ramped up just enough to bring an observer into their mesopic zone and activate the parvocellular system enough to allow reading to occur. " It may seem like it, but you have provided no objective evidence, only subjective evidence to support this and I think you are missing some critical points in your argument.


1) Whytte's Reflex does not apply to deep red light. The iris response has been shown to be driven by melanopsin sensitive ganglion cells and their response is heavily weighted to shorter wavelengths. Glare perception, which has been shown to be detached from iris response at least directly is tied to blue cones which may have something to do with blue cones being distributed wider in the eye and better adapted to cause a protection mechanism across a wider area.

2) There is nothing you have written to really support your statement of a "very bright red light" is far more likely to compromise night vision than a lower level teal light. First, we define brightness in photopic units, say candela or lux. We would not be using a "very bright red light" but a red light of almost exact equal brightness to the teal light, say 0.5 lux on the surface of whatever we are lighting. By your writing above, we may need even less red, but I do not think that is the case as that is already taken into account in the photopic response curve. Now that red light may be radiometrically "brighter" but photopically it is not. To that end, at least for dark adaption of the cones, there will be no difference.

3) Now looking at the rods/scotopic response, your "lower level teal light" is actually scotopically MUCH brighter than my "very bright red light" which photopically is of the same brightness. This is the critical point. Your "low level teal light" is actually going to provide a lot more stimulation of the rods than my photopically similar brightness deep red light. Again radiometrically the red light may have much more power, but the chemical response of both the red cones and rods is very limited at this wavelength. In the case of the rods, there is almost no response hence why there is NO photochromatic interval at over 650nm.


BOwz3R, I am enjoying the conversation and one of us is obviously missing something  .... I think in this case I may be right. I can see where you arguing from, but I think you may be getting caught up in radiometric brightness and photopic/scotopic brightness which take into account chemical response sensitivity and to some degree neural gain. The teal light you are proposing will have almost exactly the same chemical activity as the red light I am proposing for the cones, though your light will stimulate different cones from mine. At the parvocellular level, the perception of the light sources will be similar in brightness. However, at the rod level, your teal light, even though radiometrically lower in power, will create significantly more chemical activity in the rods than my 680nm deep red light. Both our lights will be at mesopic levels, but yours will create enough chemical activity in the rods to impact dark adaption of the rods. Due to the very limited chemical activity of the rods at 680nm, even though I have higher radiometric power, I will create much less chemical activity and hence have far less impact on dark adaption of the rods. 


I look forward to your reply, but you need to look specifically at my last paragraph as that is the best summary of the argument.

Semiman


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## MikeAusC (Feb 13, 2013)

I've missed what a great source of knowledge CPF used to be years ago. 

You can learn SO much more here, than "yeah, the SuperUltraFire is definitely the greatest torch"


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## MikeAusC (Feb 13, 2013)

If you're thinking of buying LEDs of a specific colour close to the edges of the eye's response i.e. close to 400nm or 700nm, be aware that there are two ways of specifying the colour of non-white LEDs by their wavelength in nm (nanometres).

The numbers here are for Kingbright L-934SRC-G.

LEDs aren't lasers and the spectral response is rather wide, i.e they don't emit light of just one wavelength e.g. 20nm away from the peak, the intensity is still half.

The PEAK wavelength indicates the colour which is strongest, when measured by a radiation meter e.g. 660nm.

This LED is also specified as having a DOMINANT wavelength of 640nm, because to the human eye, the radiation at 640nm APPEARS brightest. This is because at the edges of our colour response, there is a steep slope and even though the ENERGY from the LED at 640nm is half of that at 660nm, the human eye has a better sensitivity, making this part APPEAR to be the brightest.


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## B0wz3r (Feb 14, 2013)

Semi, I don't think I am missing the point you're making, I think you're missing mine. I don't have time to give an in-depth response to your latest post because I am going out of town for the next five days on a camping trip with my wife and kids, so a more thorough response will have to wait. As I said before, regardless of issues of threshold sensitivity, the total gain produced by each set of cones is what determines perceived brightness. Yes, this is a physiologically based mechanism, but the problem is you're looking at the wrong level of description. When I can post on this again, I'll make sure to explain how the phenomena of color and brightness constancy work, along with things like lateral inhibition and the opponent process. The opponent process itself is one that takes place at multiple levels in the visual system, not only at the level of the competing inputs to parvo retinal ganglion cells, but also at the level of the cortex in V4 and V5, which actually has a far stronger effect on the determination of perceived brightness than does the retinal response.

As I indicated before, I think you're missing my point because you're focusing too much on the specifics of neurophysiology in the retina, and aren't looking at it more wholistically, at the level of the primary visual pathway as a whole, or at the level of the perceptual correlate and the conscious experience, which is where my specific training is; the psychophysics of visual perception, attention, color, spatial perception, and motion perception. The example I gave of the purkinje shift is demonstrative of my larger point about how the gain of each set of cones influences perceived brightness, based on the amount of gain each cone channel produces. For now, I won't even bother to try and explain how this gets vastly more complicated once you start looking at individual differences and have to invoke signal detection theory in order to be able to tease out individual bias from actual threshold sensitivity; that's a whole Pandora's box in itself in some ways, and can incredibly muddy the issue of trying to draw causal conclusions about perceptual correlates.

As for citations, I gave none and didn't bother to look anything up as you didn't either in your response to my first post. I could but that would be a lot of time and I frankly don't have it; if you do, please feel free to check my claims, you'll be doing both of us a favor. 

I'm going to speculate and assume your background is in visual neurophysiology, and clearly you are more knowledgeable on some of these specific issues than I. However, my background of perceptual psychology and psychophysics is the level at which I'm working here, and as such I have implicitly adopted Karl Lashley's famous dictum; to paraphrase, he said psychology is a more fundamental science than neuroscience because the former provides organizational principles of behavior that must be understood in a larger context, while the latter does not. We don't poke neurons and then formulate laws to which behavior must conform to the effect of the action of the neuron; rather, we develop laws of behavior and then see how the neurons must perform as the basis of those behaviors. In short, just because a neurophysiological effect occurs at the level of the retina doesn't mean it has a direct perceptual correlate; many perceptual correlates can be shown to be the product of the integration of multiple different stimulus inputs which produce non-linear results, most easily seen in things such as Weber Fractions and Stevens' Exponents and the like. 

As a brief aside, I did a post-doc at the Smith Kettlewell Eye Research Institute in San Francisco the year after I received my doctorate in experimental cognitive psychology, and did some very interesting in-vivo work with single cell recordings in macaque frontal eye fields and superior colliculus. The reason I mention it is because what I saw in the literature in that field, which came almost exclusively from people trained as pure neurophysiologists, was the tasks they used in such experiments, saying they were "an attentional discrimination task" wasn't specifically true with respect to the definitions given by the larger theories they were using as inspiration for their stimulus design and perceptual tasks. Their perspective acts as a set of blinders leading them to make assumptions about perceptual correlates based only on the behavior of the individual or small populations of neurons that doesn't accurately describe the complexity of the behavior in the larger context. This is a trend I've seen growing in the neuroscience literature on visual perception, an increasing lack of attention to psychophysical and perceptual issues that introduces significant confounds into the research being done because of even simple things, like not taking the acuity/sensitivity trade-off into account as you move away from the macula into the periphery. Bad, bad, methodology...  Neurophysiologists and perceptual psychologists (I'm assuming like you and I) could stand to learn a lot from each other as a result.

In the end, regardless of the specifics of this particular discussion, I'd say we've got what could be a very good start on formulating a very interesting research project here... If you're interested, I'd be happy to exchange contact information with you, spend a little time getting to know more about your work and background, tell you about mine, and perhaps we might even be able to make this into something we could turn into a paper. Let me know if you're interested in that.

I'll be back next week on Tuesday, so I should be able to take this up again then.


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## B0wz3r (Feb 14, 2013)

MikeAusC said:


> If you're thinking of buying LEDs of a specific colour close to the edges of the eye's response i.e. close to 400nm or 700nm, be aware that there are two ways of specifying the colour of non-white LEDs by their wavelength in nm (nanometres).
> 
> The numbers here are for Kingbright L-934SRC-G.
> 
> ...



Yeah, this issue of the spectral purity of a stimulus is a big issue in this kind of work. My experience with it has been that colleagues I've known who've worked specifically with color perception and its neurophysiological underpinnings go to great lengths to acquire light sources that are as narrow in their spectrum as possible. This usually means using lasers rather than LEDs. If the power spectrum of your stimulus is too fuzzy, you're going to have a hell of a time differentiating different cells responses from one another. It's analogous to doing single cell recording in macaque brains... you want to use the finest possible electrode to facilitate getting the cleanest signal from a single neuron when recording its spike train, and it takes a lot of practice to learn to hear the fuzziness of two neurons responding similarly when they are very close to one another and both participating in a larger cell assembly to create a particular phase sequence.


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## uk_caver (Feb 14, 2013)

Without wanting to drag the thread further OT, it does remind me of something I noticed many years ago.
I was living in a place where I had covered a wall with 12x8 prints of colour photos I'd taken, about half underground and half landscapes, all very familiar to me.
I'd been playing with making red LED backup lights with 5mm LEDs, and when using them for navigating around the house I'd got used to the red-tinted monochorme effect.
At first when I lit the photos with the LED light in an otherwise dark room they had the same red-monochorme appearance, but after many days, I started to see the 'correct' colours. Not quite in the full-on normal way, but almost like I was seeing them simultaneously with the red.

I know the red light wasn't monochromatic, but I'm assuming what I saw was at least significantly a reconstruction from memory of the specific pictures and knowledge of what colours should have been (caving clothing typically being yellow PVC suits, landscapes having green vegetation and white limestone, etc), not least because I didn't see it the first N times I'd seen the pictures, but only after looking hard and presumably thinking about what the colours should have been.
It would have been interesting to have experimented with unfamiliar photographs (maybe even monochrome ones tuned to look right under red light) and to find out if I could see the 'right' colours in them, or if I could have the colours I saw changed by being told what they should have been, but that would rather have required other people to be involved.

Maybe in monochromatic (or near monochromatic) light the colour vision system is so starved of useful real input that hints about what should be being seen are easy for it to adopt?


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## SemiMan (Feb 14, 2013)

MikeAusC said:


> I've missed what a great source of knowledge CPF used to be years ago.
> 
> You can learn SO much more here, than "yeah, the SuperUltraFire is definitely the greatest torch"



We sometimes have some interesting discussions in the automotive forum. There are a few people there who have expertise in the field.


Semiman


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## SemiMan (Feb 14, 2013)

MikeAusC said:


> If you're thinking of buying LEDs of a specific colour close to the edges of the eye's response i.e. close to 400nm or 700nm, be aware that there are two ways of specifying the colour of non-white LEDs by their wavelength in nm (nanometres).
> 
> The numbers here are for Kingbright L-934SRC-G.
> 
> ...




Excellent point ...... unfortunately, I had forgotten just how wide the spectrum was of near IR LEDS :-( I think I am going to look into the cost of sharp 650-660nm filters and see if that makes sense to use.


Semiman


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## SemiMan (Feb 14, 2013)

B0wz3r said:


> Semi, I don't think I am missing the point you're making, I think you're missing mine. I don't have time to give an in-depth response to your latest post because I am going out of town for the next five days on a camping trip with my wife and kids, so a more thorough response will have to wait. As I said before, regardless of issues of threshold sensitivity, the total gain produced by each set of cones is what determines perceived brightness. Yes, this is a physiologically based mechanism, but the problem is you're looking at the wrong level of description. When I can post on this again, I'll make sure to explain how the phenomena of color and brightness constancy work, along with things like lateral inhibition and the opponent process. The opponent process itself is one that takes place at multiple levels in the visual system, not only at the level of the competing inputs to parvo retinal ganglion cells, but also at the level of the cortex in V4 and V5, which actually has a far stronger effect on the determination of perceived brightness than does the retinal response.
> 
> As I indicated before, I think you're missing my point because you're focusing too much on the specifics of neurophysiology in the retina, and aren't looking at it more wholistically, at the level of the primary visual pathway as a whole, or at the level of the perceptual correlate and the conscious experience, which is where my specific training is; the psychophysics of visual perception, attention, color, spatial perception, and motion perception. The example I gave of the purkinje shift is demonstrative of my larger point about how the gain of each set of cones influences perceived brightness, based on the amount of gain each cone channel produces. For now, I won't even bother to try and explain how this gets vastly more complicated once you start looking at individual differences and have to invoke signal detection theory in order to be able to tease out individual bias from actual threshold sensitivity; that's a whole Pandora's box in itself in some ways, and can incredibly muddy the issue of trying to draw causal conclusions about perceptual correlates.
> 
> ...




Would you be surprised to find out (probably not  ) that I am a electronics, lighting, and imaging engineer ... consultant, business manager, etc. ...? I tend not to be the type to just accept expert opinion (not at all directed at you), and hence when I started delving deeply into street lighting and other low level lighting situations including automotive, I took it upon myself to understand within the limits of all the other demands on my time the underlying physiology and some aspects of cognitive psychology associated with vision. It has been an interesting exploration both reinforcing things I knew and had been told, and also revealing flaws in and errors in both things I had been told, and "obvious assumptions" that I had made. 

There has been an ongoing debate, or in scientific terms "Arguments between experts with opposing views and equal sized egos both unwilling to admit years of research may have been wrong." in the street lighting world about the benefits of various spectral power densities for street lighting. There are those that would like to assign higher advantage to lights with high scotopic/photopic ratios, and those that feel the benefit is not warranted. There appears finally be convergence towards and answer of they are both right. High scotopic/photopic lights having advantage at slow speeds (parking lots and low speed urban/residential) and high scotopic/photopic lights having little to no advantage at high speeds where central vision dominates. Out of research on headlight glare, experiments have shown that glare response appears to be dominated by the blue cones, and that melanopsin sensitive ganglion cells do not play into this. In both these fields, "assumptions", even by experts, often do not play out as expected.


I am pushing you on citations as I would like to read articles that are driving you to the conclusions you have made. You obviously know a lot and I would love to draw on that. I apologize for not citing sources. It was not an intended over site merely that I felt I was drawing on common industry knowledge w.r.t. photopic/scotopic response curves, detection thresholds, photo chromatic interval, etc. I can certainly add some citations. Somewhere on CPF (I think in automotive), I have posted citations for melanopsin activity spectrum and papers on melanopsin ganglion cells and pupil response. There is also a citation on the experiments that showed the blue cone weighted cone response for glare. This same experiment using targeted filters ruled out melanopsin ganglion cells as the source of glare response.

I agree with your conclusion that I am looking at the problem from a standpoint of neurophysiology, but I am not convinced that is wrong. Perhaps we need to state the problem:

Problem: What spectrum of light source will allow reading while at the same time having the least impact on the dark adaptation. Dark adaptation being BOTH dark adaptation of the scotopic vision system and photopic vision system.


Writing out the problem, I can see an issue we both may be missing .... maybe I should say you are covering one side of the problem and I the other.


I have been looking at the neurophysiological aspects of what light can I read under that will not impact the dark adaptation of the scotopic vision system. I do believe a neurophysiological viewpoint is primarily correct for this description of the problem or will reach the best result. I am not convinced there is a need to look at the visual pathway as a whole because at a neurophysiological level as I am picking a light source that only barely stimulate the rods while providing enough light to reach mesopic levels. The result will be when that red light is turned off, the rods will still be as close to fully dark adapted as possible.

The light source you have picked will significantly stimulate the rods. A 500nm light that is at mesopic levels will significantly stimulate the rods. To that end, the rods will need to dark adapt after your light turns off. I do not believe there is anything in your argument that refutes that a 680nm source will not stimulate the rods while a 500nm source will and hence the 500nm source will cause a loss of dark adaptation of the rods. Both light sources may require a dark adaptation of the cones after reading, but the cone dark adaptation of the cones is faster than the rod adaptation. 


For the purposes of astronomy though, there is more diversity to the problem. When looking at a whole starfield the rods/scotopic vision are most critical and to that end having a fully dark adapted rods is critical. However, when looking at an object of interest in the center to the telescope eye piece, we are not using scotopic vision but photopic vision. Mesopic would not even come into play due to the lack of rods in the fovea. If this is the type of astronomy work one is doing, then your argument makes complete sense. I completely see where the whole visual pathway comes into play in this instance and how a photopically less intense 500nm source can provide better visual acuity than a similar intensity 680nm source. That less intense 500nm source will also require less dark adaption time for the cones.

I do understand to some degree that light adaptation is not solely a matter of photochemical changes in the receptors. There is a lot of complexity to how the visual system adapts to changing light levels which you alluded to a few posts back when you mentioned inhibition and opponent process. It is my understanding (please correct if wrong) that these changes are fairly rapid compared to the speed of changes in the photochemical response for dark adaption. 


All in all an interesting field and based on a few hours of searching, while there are lots of "related" articles, there is nothing that answers some very specific questions:

a) Spatial resolution of the eye versus wavelength versus light level. There are lots of resolution versus light level, but none I could find that looked at spatial resolution versus lighting level versus a variety of wavelengths across the visible spectrum.

b) Objective studies on the impact of "night vision" with near IR light sources beyond photochromatic interval. I.e. impacts on acuity across the visual field.

c) Impacts on dark adaption and acuity versus various stimulus light sources (wavelengths, levels) sufficient for higher levels of acuity sufficient for reading. Again lots of studies looking at macro effects and hand waving, but not objective acuity.


I am sure within this field there is a good experiment, a good conclusion and hopefully a good patentable concept for a spin off company or licensing 

Have a great vacation and I look forward to picking this up when you get back.


Semiman


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## Dubois (Feb 14, 2013)

I wonder if the OP got his answer in post #4, a day after he posed the question, and has taken off, never to return.:naughty:


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## MikeAusC (Feb 14, 2013)

MikeAusC said:


> OK, you've made it clear what the light should NOT do, but you haven't said what you want to do when it's on. . . .



The OP also has not answered my question of what he wants the light to do when it's ON.


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## MikeAusC (Feb 14, 2013)

SemiMan said:


> . . . . For the purposes of astronomy though, there is more diversity to the problem. When looking at a whole starfield the rods/scotopic vision are most critical and to that end having a fully dark adapted rods is critical. However, when looking at an object of interest in the center to the telescope eye piece, we are not using scotopic vision but photopic vision. . . . .



. . . . but I'm really glad that SemiMan has worked out that for Astronomy, there are in fact two different needs, for which there may be two quite different solutions.

I'd like to add that we use photopic vision for Astronomy not just when looking through the Telescope, but also when observing stars with our unaided eyes. When we see that Mars is red or that stars are distinct points of light, it's clear that they are producing enough light on our Cones to trigger Photopic vision. Those cones where we gaze at the stars are in the Fovea, where there are no Rods to be leached, so we see no "holes" in our nightvision after gazing at a bright star.


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## rocketsurgeon01 (Feb 14, 2013)

Streamlight Sidewinder Compact (Sportsman) or Sidewinder Compact 2


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## SemiMan (Feb 15, 2013)

Dubois said:


> I wonder if the OP got his answer in post #4, a day after he posed the question, and has taken off, never to return.:naughty:



Well if he did disappear, then it has been his/her loss. Fortunately the conversation kept going. Many subtleties to lighting and often you need yours and others preconceived notions to be challenged.

Semiman


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## kzb (Feb 19, 2013)

Wow what a discussion ! I'm not at this level of nirvana, but I'd like to add one thing:

I think there is a big _*learning*_ component to seeing with red light. It takes practice. The first few times you try it, it is is a bit disorienting and you do not think you can see properly. But as you gain experience, it becomes OK.

Maybe it is a combination of brain visual interpretation training, and also could there be some biological adapation going on? Could your retina produce more of certain pigments to enable better vision in red light?


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## wbp (Oct 7, 2013)

Sadly, it looks like ZebraLight has dropped all of the red headlights from their line. I still have an H30R, which has a wide field lens and comes on at the lowest level with a single button push, but they stopped making that a long time ago. The H31R was available the last I looked, along with other red headlights that used different battery types, but the ZebraLight web site no longer lists any of these.

From what I can tell it looks like the next best choice might be the Princeton Tec Remix (3xAAA) or Remix Pro (1xCR123 - my preference). There is also the Princeton Tec Byte (2xAAA) which looks very interesting, it has only a single red LED but I don't have any idea what the illumination is like. One feature I like about the Remix Pro is that you have to hold the button to switch between red and white output, making it hard to get white light by mistake. The Byte does not have this feature, and if the description is correct must be cycled thru the white modes to turn it off, which would rule it out for astronomy (or any dark adapted) use.

I just ordered a Remix Pro and can post back here once I get it if folks are interested.
William


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## beast1210 (Oct 7, 2013)

Worth a quick mention, but as noted above may not be ideal, still a much better light than the energizer offerings. Is the xtar h1/h2


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## yellow (Oct 11, 2013)

I did not follow the whole thread ... so possibly the topic has been already covered:
but as I remember, the actual general agreement is, that it makes no difference if the light is white or red, it is its _brightness_.

with that in mind, think/look for a lowlowlow white light.
Easier to find than just a red and white light is way more useful ...


as to useful ... 
Just 2-3 weeks ago I happened to be invited to a meeting of such astronomy ppl.
They met a some parking lot up on a nearby hill, each of them getting out a man sized "tube", 
shouting at anyone with a light (campers at the site, cars driving by at the street, ...) but who have no clue that they even are there (_*I*_ would have brought some signposts)  
and just one or two of the group of 10 telescopes even had lights - the crappiest ones I might have ever seen...
... even on night hikes I dont see such "lights" ...
the one most used was a red led model, but the guys "needed" it for reading where their stars are and the "map" was a mixture of colors and red - and reading red with a red light is a bit difficult.
They ended up with (one of) my white lights in firefly mode - less bright than the red led one - and were much better of...


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## wbp (Oct 11, 2013)

I received the Princeton Tec Remix Pro yesterday. I ordered the 70 Lumen version but what I got was the 100 Lumen version; I'd guess the 70 Lumen is no longer available. Sadly, even on low it's way too bright for dark adapted astronomy use. I am returning it.

I was thinking of ordering the Princeton Tec Remix (AAA version) but given that the description says "100 Lumens" I'm guessing it's the same light with a different battery case.

The search continues...


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## The Owl (Oct 13, 2013)

One possible headlamp is the Petzl Strix, red 1 lumen, green 1.5 lumen, blue 0.35 lumen, white 40, 15, 0.4 lumen. Looks like Darth Vader.. http://petzl-tactical.com/en/headlamps/headlamps-tactical-operations/headlamps/strix-vl


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## Blue72 (Oct 29, 2013)

I have played around with red lights and different color lights for years to preserve night vision. But they become very frustrating to use especially in a outdoor environment. I know find .4 lumen white light works best. Am I sacrificing night vision???......maybe, but not enough to notice and I don't have anymore banged up toes and it feels more natural


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## wbp (Oct 29, 2013)

@dd61999 - it sounds like you are looking for a light for a different purpose, not astronomy, since you mention "in a outdoor environment". For your purposes red is probably not the best choice.
@TheOwl - wow, that sure is an interesting looking light, I had not found that in my search. "interesting" from an appearance standpoint, though. At 1 lumen red it's too bright for fully dark adapted astronomy use.

I have given up trying to find something that works, and have now built my own headlamp! It was a fun project and I am very happy with the results. I showed it to some fellow astronomers this past weekend and they all want one! I used an RGB light source, so when it comes time to pack up, if I want white light I can switch to that. 

I don't have anything sensitive enough to measure below 1 lumen, but I'd estimate the lowest level red to be about 0.25 lumen, based on a comparison with my ZebraLight H30R on low, which is rated at 0.4 lumen. My new headlamp has one very cool feature which I've never seen before in a headlamp, but you'll have to wait a bit to find out what that is until I've had a chance to do some patent searching...


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## CactusSeed31 (Aug 10, 2014)

Hi to everyone. I know this is an older post. but I find it very interesting and informative. Thanks to everyone for sharing their expertise.

For a few years I have been playing with modifying some Chinese White (copies of Photon) microlights for a friend of mine that loves astronomy. These lights normally take 2 CR2016 batteries. With most 5mm RED leds, you use a single CR2032. The CR2032 has around 230mah @ 3 volts, versus the 80mah in a CR2016. A red LED draws about 25-30ma with a new battery. A White LED draws about 65ma with the 2 CR2016. With a single CR2032, the white leds pulls about 7mah. AND are bright enough to read close up and get around indoors. I can't measure lumens, but both the RED and White Led's are way too bright for astronomy use with just the CR2032. I have NOT tried the deep red 680-700nm peak "deep red" leds; but expect that they too might be too bright for best night vision. 

I recently took an old GREEN Photon 3 that I had, a replaced the 2 X CR2-16 batteries, with 1 CR2032. The Photon 3 has 3 (PWM) output levels, low medium and high. On low with the 1 CR2032, I can use it to walk around indoors, once my eyes get adjusted to the dark for a minute or so. At full power it will light up a normal size room. At full power it only draws 6-7ma, so batteries last a long time. At low, it might be Ok for astronomy. With a RED led, it would probably be better? (Guess it depends who you ask !) For me, the green on low does not seem to affect night vision for reading or looking around a dark home.

I just ordered some 700nm "DEEP RED" leds. And will replace the green led, with that one. (Need to pull out and solder on the new led). Beause they produce less lumens, I expect that low, or even medium may be Ok for Astronomy. (But will use up batteries a bit faster. )

I also ordered a Photon Freedom to play around with. It has a continuously adjustable brightness, with a low that is lower than the Photon 3's... It's a green one, and again, replacing the 2 CR2016 batteries with one CR2032 gives plenty of light for indoor use. Even though it's Green LED is more efficient (brighter on high) than my old green photon 3; At it's lowest setting it is a lot dimmer than the photon 3 on low. The Photon Freedom with a Green LED will have a battery life of at least 32 hrs on high. (And longer on lower and still useful PWM settings). (32hrs = 230mah/7ma) This will be my Emergency EDC/Mini-Survival kit. With a RED Led, expect 8 hrs of battery life as the led pulls 4 X the ma At full power.

I may try out one of the 700nm led's in the Photon Freedom. Those LED's can be replaced without soldering... 

Another recommendation I have, is to roughen the surface of the LED's with 600 grit "wet or dry" silicon carbide sandpaper. You only need about 1 inch square and about 1 minute to roughen up the surface of the LED. This produces a much smoother beam, that is better for area lighting, and for close up reading. (No shadows or streaks).. It also reduces beam "throw", but that's not what I use microlights for... (There are "diffused" LED's available, but the selection is quite limited in beam angle, and color/nm.)

These can be used as a headlamp either using the clip attachment that comes with the photon freedom; OR by using sitck-on Velcro "Dots". One piece goes on the back of the microlight, and the other on your hat's brim, or eyeglasses, etc...


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## Blue72 (Sep 16, 2014)

wbp said:


> @dd61999 - it sounds like you are looking for a light for a different purpose, not astronomy, since you mention "in a outdoor environment"....



I use use white light for astronomy as well. As long as the light is dim enough, you can still maintain scotopic vision


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## survivaledc (Sep 20, 2014)

If you can find one, the h502r by zebralight is perfect for what you are looking for.


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## JAS (Sep 21, 2014)

*Searching For Very Low Brightness, Red LED Headlamp For Astronomy*

A Streamlight ClipMate USB would work. I clip mine to my baseball cap and it works very well for me. 

http://www.streamlight.com/en-us/product/product.html?pid=298

Red LED preserves night vision:

Red LED – High:0 .5 lumens; 9 candela; runs 16 hours
Red LED – Low: 0.2 lumens; 3 candela; runs 65 hours


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## SemiMan (Sep 21, 2014)

That wavelength red does nothing to preserve night vision. Better with the same output in white


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## Nev (May 10, 2017)

survivaledc said:


> If you can find one, the h502r by zebralight is perfect for what you are looking for.



Zebralights now does a darker red ,they call it photo red , would that be better?


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## eh4 (May 11, 2017)

http://midopt.com/filters/bp660/

You'll hear that red doesn't preserve night vision, or that it does, but after hearing contradictory information experiment for yourself. 
With red light I can more easily transition between darkness and illumination, can see more detail in my center of focus, and don't feel eyestrain. 
Dim, warm white is fine too, and that's what I use typically, but it feels and performs differently than red. 
The red light that I have is an old CMG Infinity.


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