Searching for very low brightness, red LED headlamp for astronomy

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?

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.

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!

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.

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.

lampeDépêche said:

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

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+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.

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.

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.

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I was coming here to post this.

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.

Holy crap.

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

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.

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.

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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.

Thank you B0wz3r. Hat's off to you.

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.

stevetaylor199 said:

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

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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

stevetaylor199 said:

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

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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.

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

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.

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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.

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.