# LED Tints/Perceived Brightness/Actual Brightness



## krazy89 (Feb 18, 2009)

Hey Everyone...

i know tints have a BIG impact on the perceived brightness of a particular emitter... what i don't know is how does tint affect the actual brightness of the output?

cool whites are perceived to be brighter than warm whites... but how does that translate to light meter measurements? are they really brighter?

also... how about for penetrating through the elements like rain... fog... is warmer tints better for that?

Anyone shed some light on this?

KraZy


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## DM51 (Feb 18, 2009)

Moving this from LED Flashlights to LED.


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## pipspeak (Feb 18, 2009)

I'm curious, too. 

I would imagine, a photon being a photon whatever its wavelength, that measured output is gonna be the same regardless of tint. But I'm probably wrong. 

Warmer tints are better for penetrating fog etc., hence the fact that most car fog lights are yellow (or should be). Shine a bright, white beam into dense fog and you'll be lucky to see beyond the wall of white at end of your nose.


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## EngrPaul (Feb 18, 2009)

I think you would gain a lot of insight from this article:

Daylight: Is it in the eye of the beholder?

_"Outdoors, ... With the higher light levels the responsitivity of the cones is dominant yet the rods to a much smaller degree are still contributing to the overall response. With this large amount of light entering the eye, 6000K appears white. _
 
_"Moving indoors, the pupil size grows allowing a larger proportion of light to pass. Once again, the iris is not capable of maintaining a constant level of_
_illumination on the retina. Under these reduced lighting conditions, rods with blue sensitivity come more into play and hence the 6000K light that looked white outdoors now appears bluish and 4700K appears white."_

Is it possible that using significantly cool (blueish) light indoors does not cause significant pupillary light reflex, resulting in greater light hitting the retina through the larger pupil opening? This might also help explain why using cool white light indoors can cause eye strain. The more the pupil dilates, the easier it is to focus due to the smaller aperture.

I hear a lot of comments "I love the tint of my flashlight because the color temperature is similar to daylight". The article indicates to me that logic is flawed, since it demonstrates that the lower illumination level of a flashlight requires a warmer tint for proper white balance and visual comfort.


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## blasterman (Feb 18, 2009)

> since it demonstrates that the lower illumination level of a flashlight requires a warmer tint for proper white balance and visual comfort.


 
It's part of the equation, but the explanation is sound.

Another problem is that all LEDs, CFLs/fluorescent tubes have limited CRI because they are only emitting chunks of the visible spectrum, while the reference they are put against is sunlight at 100 CRI. You really need to make comparisons at the same CRI, or it's tough to quantify what's going on perceptually.

Lower light levels only aggravate the issue further. A 6000k CFL or LED with a CRI in the 70's is certainly going to appear more blue/cyan than if they had a CRI in the 90's. Higher CRI values increase production complexity and also tends to reduce efficiency, so this indirectly aggravates the issue as well.


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## asdalton (Feb 18, 2009)

pipspeak said:


> Warmer tints are better for penetrating fog etc., hence the fact that most car fog lights are yellow (or should be). Shine a bright, white beam into dense fog and you'll be lucky to see beyond the wall of white at end of your nose.



This notion keeps popping up on CPF, and it's not true.

Yellow fog lights are a marketing gimmick, made semi-plausible by a misunderstanding of Rayleigh scattering. The fact is that fog particles are much too large to cause the strongly wavelength-dependent scattering that comes from air molecules (over very long distances) or hydrocarbon haze.

And anyway, you cannot make a beam penetrate better by putting a filter in front of it (which is what standard "fog" lights are). A filter merely removes some photons from the beam; the remaining photons continue onward in the same manner as if the filtered-out photons had remained.


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## asdalton (Feb 18, 2009)

krazy89 said:


> i know tints have a BIG impact on the perceived brightness of a particular emitter... what i don't know is how does tint affect the actual brightness of the output?



First of all, measurements in lux (for illuminance) or lumens (for total light output) are already weighted to the human eye's varying sensitivity to different wavelengths. If you are viewing the beam directly, or viewing reflected light from a white surface, then these values are sufficient as far as "perceived brightness" goes.

Things get more complicated if you are trying to view objects that have colors. Typical objects outdoors are disproportionately in the range of green, brown, and yellow--and this is likely the reason why many people prefer warmer tinted beams for outdoor use. You get a larger fraction of light reflected back towards you, for a given lumen output, with a warm tint than with a cool tint.


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## 2xTrinity (Feb 18, 2009)

asdalton said:


> This notion keeps popping up on CPF, and it's not true.
> 
> Yellow fog lights are a marketing gimmick, made semi-plausible by a misunderstanding of Rayleigh scattering. The fact is that fog particles are much too large to cause the strongly wavelength-dependent scattering that comes from air molecules (over very long distances) or hydrocarbon haze.
> 
> And anyway, you cannot make a beam penetrate better by putting a filter in front of it (which is what standard "fog" lights are). A filter merely removes some photons from the beam; the remaining photons continue onward in the same manner as if the filtered-out photons had remained.


The reason this might be true however, in the case of very bluish lights is that under very dark conditions, our eyes are a lot more sensitive to blue back-scattered light, than they are to say green, amber, or red back-scattered light. So the glare from a cool white light in fog is more irritating, even if more energy is not actually being reflected back. This is a very maginal effect in my experience. I believe 90% of the reputation of LEDs sucking for throw in the fog has to do with the fact that they have very bright spill, and weak main beams in general, while most incans concentrate most their lumens into the hotpsot. Comparing cool white LEDs with aspheric lenses, to similarly-bright incans in deep reflections (EG similar lumen output AND similar beam profile) the difference in fog-penetration is minor. A concentrating beam with no spill, held off-axis from your eyes (So backscatter does not hit you as much) is best, regardless of color temperature.

Another benefit for warm vs cool for fog penetration is the following: 

Outdoors most object being viewed do not reflect strongly in the blue portion of the spectrum -- Let's say for an extreme case of argument you shine a blue LED, through fog, at a tree. _I_ shine an amber LED at the same tree. The fog reflects back an equal proportion from both lights, but the _tree_ reflects back very little blue. So the ratio of light reflected from the tree, compared to light reflected from the fog is a lot worse for the blue, meaning the tree is "washed out" by the fog which is still very bright (water reflects all colors equally)

The same argument can be extended to using warm white flashlights outdoors, versus cool white. Materials like trees, grass, and dirt tend to reflect a lot more in the green to red range and very little in blue. I find that lights in about the 4000k range look the most _accurate_ as far as color rendition goes, compared to the same scene viewed under daylight. This is due to the sensitivity shift of our eyes. 

This makes sense if you consider that full moonlight is naturally around 4000k, that this is where our "low ambient light" white balance would work best. Consider that ehen the moon and sun are visible simultaneously, the moon actually looks brown -- but when there is a full moon out, the moon looks almost bluish white. It is our perception that is changing.

Incandescent lamps however at 3000k give me somewhat more contrast -- the red dirt or tree branches tend to "pop out" a lot more compared to the green grass surrounding them. This effect is similar to wearing brown tinted sunglasses during the daytime -- color accuracy is actually worse, but contrast for particular sorts of things is enhanced.

Outight "bug light" colored amber lighting, in my experience, is actually worse than using incan. This is because there really isn't any advantage from the effects I'm describing, but the overall color rendering is a lot worse to begin with.

In this case, you are viewing the tree with the blue light will see a very dim tree, which is being masked by bright blue backscatter (which the eyes are disportionately _more_ sensitive to in low concentrations). In my case the ratio of amber light reflected off the tree, to light reflected off the fog is much greater.


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## pipspeak (Feb 18, 2009)

asdalton said:


> This notion keeps popping up on CPF, and it's not true.
> 
> Yellow fog lights are a marketing gimmick, made semi-plausible by a misunderstanding of Rayleigh scattering. The fact is that fog particles are much too large to cause the strongly wavelength-dependent scattering that comes from air molecules (over very long distances) or hydrocarbon haze.
> 
> And anyway, you cannot make a beam penetrate better by putting a filter in front of it (which is what standard "fog" lights are). A filter merely removes some photons from the beam; the remaining photons continue onward in the same manner as if the filtered-out photons had remained.


 
I yield to those with more knowledge of light behavior then me :laughing: 

I have read about the misinterpretation of Rayleigh's "law" regarding dispersion but it is also true that longer wavelength light *can* penetrate further in some conditions and that our eyes are generally more sensitive to it, hence the fact that yellow or amber is the most used lens color in glasses or goggles for increasing contrast in cloudy, snowy or foggy conditions when contrast is reduced by weather conditions. 

So the principle might be flawed, but the effect can be beneficial.


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## EngrPaul (Feb 18, 2009)

While carrying two equivalent flashlights on a walk in the woods, I have witnessed first hand that a cold emitter flashlight has a lot of spillback and a neutral emitter flashlight has much less. I recognize (see) distant objects much better with the neutral emitter, even though it's a couple flux bins modest.

Indoors, cool white lights have a slight edge at low settings. I.E. a LD01 on low setting is more useful with cool Q5 vs. neutral Q3. I don't know why this is, perhaps LED's aren't as cool at lower current?


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## kuksul08 (Feb 18, 2009)

I personally prefer about 4300K for any conditions. It's a nice white and excellent color, depth, everything.


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## krazy89 (Feb 19, 2009)

thanks guys for all the input... 

i guess this has been a topic that's been discussed to death... as there's tons of theories out there...

and i guess what makes it worse is everyone is color blind to a degree... and 2 people standing side by side... will probably see 2 different things... so i guess in the end.. it's what works best for you...

i'm in the process of trying to decide between 2 same lights... with a lil bit of a tint diff between them... but... i don't have a lux meter... and i don't know of anyway of measuring the color temp of the light coming out of them... so it's so hard to choose with just my naked eyes... 

KraZy


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## bshanahan14rulz (Feb 19, 2009)

if they use the same LED chip, the cooler one will always be brighter. The way manufacturers adjust the tint is usually not (or not only) by using a different phosphor, but using thicker layers of phosphor. Ever notice how on lumiled's tffc LEDs, you can see the surface of the chip, those little circles on it and all. on a warm version, it will be the same chip, but the circles will be less evident because of the thicker phosphor.

Oh, and I'm a believer in the warmer light helps you see better in different conditions argument. I don't think its a physical effect of light of a certain wavelength, though. I think it's more about how our eyes detect contrast between dark and blue and between dark and yellow.


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## BigHonu (Feb 19, 2009)

2xTrinity said:


> The reason this might be true however, in the case of very bluish lights is that under very dark conditions, our eyes are a lot more sensitive to blue back-scattered light, than they are to say green, amber, or red back-scattered light. So the glare from a cool white light in fog is more irritating, even if more energy is not actually being reflected back. This is a very maginal effect in my experience. I believe 90% of the reputation of LEDs sucking for throw in the fog has to do with the fact that they have very bright spill, and weak main beams in general, while most incans concentrate most their lumens into the hotpsot. Comparing cool white LEDs with aspheric lenses, to similarly-bright incans in deep reflections (EG similar lumen output AND similar beam profile) the difference in fog-penetration is minor. A concentrating beam with no spill, held off-axis from your eyes (So backscatter does not hit you as much) is best, regardless of color temperature.
> 
> Another benefit for warm vs cool for fog penetration is the following:
> 
> ...



Great post!


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## ledstein (Feb 20, 2009)

Actually there is another point of the discussion. Incandescent front lights vs xenon front lights. I changed the lights on my car to xenon and in my opinion objects are more visible under xenon (maybe because the light is more intense?).


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## SemiMan (Feb 21, 2009)

EngrPaul said:


> Is it possible that using significantly cool (blueish) light indoors does not cause significant pupillary light reflex, resulting in greater light hitting the retina through the larger pupil opening? This might also help explain why using cool white light indoors can cause eye strain. The more the pupil dilates, the easier it is to focus due to the smaller aperture.
> 
> I hear a lot of comments "I love the tint of my flashlight because the color temperature is similar to daylight". The article indicates to me that logic is flawed, since it demonstrates that the lower illumination level of a flashlight requires a warmer tint for proper white balance and visual comfort.



Actually the first part of your email is completely opposite of what happens. Your eyes actually responds to the blue/blue-green part of the spectrum, not the red. Hence, if you have cool lighting, your pupil will respond properly and close giving better visual acuity versus incandescent light. Using cool-light indoors ABSOLUTELY does not contribute to eye strain. In fact high quality "cool" light is likely to offer much lower eye strain and probably keep you out of glasses longer. Back in the early days of mechanical ballasts, and terribly low CRI flourscents, not to mention many of the attrocious cool-white compact flourescents that have come out recently, yes, you may have gotten irritated by the light. However, if you take a modern electronic ballast and couple that with a high-CRI daylight flourescent bulb, I guarantee, if you are doing anything that requires visual acuity, i.e. reading, working on small parts, etc., that this will cause you much less eye strain then using a halogen reading light. Just turn it off a while before bed time so your seratonin levels can adjust.

I am likely re-iterating much of what was said, but the lumen measurements that are made on LEDs are all PHOTOPIC correctly. I.e. they are measurements that are correlatated against human eye response in a room that is already "bright". So if you took flashlights of various tints in a brightened room and shone them directly into your eyes, they would all appear to be about the same brightness.

Now, if it dark, the relative measurements would be completely different as has been mentioned.

What is not clear to many people is that lumen/lux measurement are "emissive" measurements. They are a measurement of what is coming out of the light source. They are not reflected measurements. Other than CRI, there is no measurement that takes into account "reflected" light.

Low pressure sodium street lights are the most efficient radiant light source, however, in many environments, the light they put out is completely soaked up by the environment. Hence though they may emit 150lumens/watt+, they may not be any more effective than a 70 lumen/watt source that has a different spectrum. This is why LED based streetlights, even with lower lumens, are perceived as being brighter than very low CRI low and high pressure sodium lights that technically emit more lumens. The reality is that in the environment in which they are used, they are brighter. Our perception of the brightness of a light is not how many lumens it puts out, it is how much is reflected back from the objects it hits.

For this reason, any newer flashlight modifications I have been doing are based on Neutral-White LEDs as opposed to cool white (or warm). I find that even though it may not have the raw lumens output of the cool-white, or gives up a few point in CRI to typical warm whites, it provides the best combination of light output and spectrum when I am viewing typical environments I am exposed to.

Semiman


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