What use/purpose for colored LEDs?

I noticed when browsing around the PEAK website (aaaaaggggghhhhh! ... someone really needs to talk to their webmaster - they must have their flashlight engineers doing the web design! - really nice lights, though, I think I want a Baltic) that a couple of the models have colored LED opotions.

RED makes sense, at low levels it will preseve your night vision. Really good for amateur astronomers.
BLUE well, supposedly it makes finding blood trails easier. I've never had to track anything after dark, so I haven't had to worry about this.
UV, yeah, I know, not a color (hmmmm or is it?) but good for mineral prospecting, reading security markings, probably some other uses I have no clue about.

But what is the point (other than 'just because you can') for GREEN or CYAN or VIOLET or other color (ORANGE, YELLOW?) LED's, I'm really quite curious. Not that I need a reason to get a colored LED light, but hey, if I have a need, and just don't know about it, yet, I'm sure you guys can help me out.

Green is quickly gaining on red as the goto color for low light use, your eyes are much more sensitive to green light so lower levels can be used.

Also, have you ever tried to read a topographical map (or most highway maps) under Red light? All the topographical/street data printed in red magically disappears.

BigBluefish said:

But what is the point (other than 'just because you can') for GREEN

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mwaldron said:

Green is quickly gaining on red as the goto color for low light use, your eyes are much more sensitive to green light so lower levels can be used.

Also, have you ever tried to read a topographical map (or most highway maps) under Red light? All the topographical/street data printed in red magically disappears.

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Yes, I've done a little reading, and understand you can see better with low levels of green light as opposed to red light, but I thought the green would not preserve your night vision as well as the red. If that's so, maybe the question is do you need to see as well as you can while just using a little light (walking a dark trail) or do you need to presever your night vision (use the controls on your telescope/camera)?

And yes, trying to read a topo map under red light sucks.

I can't argue for pros/cons of using green, I still use red.

I was just saying that's why for the green. I can't even say 'that's why' for some of the other colors though!

One thing I discovered accidentally was that when camping, I didn't get swarmed by flies, mosquitoes and moths when using a blue LED light. So I always give that as a reason to have a different colored light. It SUCKS trying to get filtered water from a stream at night, with a cloud of hungry insects dive bombing and biting your face!

Omega Man said:

One thing I discovered accidentally was that when camping, I didn't get swarmed by flies, mosquitoes and moths when using a blue LED light. So I always give that as a reason to have a different colored light. It SUCKS trying to get filtered water from a stream at night, with a cloud of hungry insects dive bombing and biting your face!

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That sounds like a good idea. I remember now reading this somewhere before. It would probably work well for fishing after dark, too.

I wonder if orange or yellow LEDs would work as well? I remember all those yellow/orange 'bug light' bulbs the beach cottages had on their porches when I was a kid. They seemed to work.

BigBluefish said:

That sounds like a good idea. I remember now reading this somewhere before. It would probably work well for fishing after dark, too.

I wonder if orange or yellow LEDs would work as well? I remember all those yellow/orange 'bug light' bulbs the beach cottages had on their porches when I was a kid. They seemed to work.

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Yes, yellow colored light works too! In our wild areas here in Brasil we just use yellow colored lamps in the outter areas of the houses, to not attract insects. It works fine!

mwaldron said:

Green is quickly gaining on red as the goto color for low light use, your eyes are much more sensitive to green light so lower levels can be used.

Also, have you ever tried to read a topographical map (or most highway maps) under Red light? All the topographical/street data printed in red magically disappears.

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You have two types of sensors in your eye--rods and cones. Rods are used for low light levels (such as night time vision) and cones are used for higher light levels. The cones give you both color perception as well as much higher visual acuity. A flashlight for night use, such as astronomers use, typically would provide just enough light to activate the cones in your eye at a reasonable distance, thus providing sharp vision.

Depending on the specific wavelength, the cones in your eye are roughly 25% more sensitive to green light than to red (and roughly 150% more sensitive to green than to blue). So it would make sense to use green light, yes?

Unfortunately not. The low light receptors in your eye (rods) work off a chemical called rhodopsin or visual purple. Under bright light (enough to activate the cones in your eye), visual purple becomes transparent (bleached) and your rods stop working. However, it is not just the intensity of the light that affects the bleaching. It is also the color. Red light (depending somewhat on temperature) does not bleach rhodopsin, so you don't lose your dark adaption. This is not the case for green light.

Green light--lets you see things with the list possible lumen output (eye's sensitivity)
Red light--requires slightly more light to gain the same acuity, but disturbs your dark adaptation less than green light

Your point about contour maps is a good one. I hadn't considered that.

It's good to know there is a scientific reason for me disliking the switch to green light.

jwillson said:

Your point about contour maps is a good one. I hadn't considered that.

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That was the only reason I even have a green LED. Very few things that I use have anything actually printed in green ink. Neither SureFire nor Fenix makes green filters though...

Colored light works well for certain applications.

Some examples include:
- Green light: some animals cannot detect green, which is useful for hunters
- Cyan: used in Forensics (with goggles) to identify fingerprints, typically when specific sprays and/or powders are applied.
- UV: look for scorpions or again in Forensics (with goggles) to find serological evidence.

Green lights are used in greenhouses, I think.

Green LEDs are awesome because they are so bright. The distinctive color and unimaginable brightness of your light is sure to impress your friends. It also sets you apart in a crowd of flashlights.

Green is best used with light intensification goggles and scopes.

Sub_Umbra said:

I use blue/green for nearly everything that doesn't require color rendering. In lights with widely variable output like the Covert Photon Freedom in NV green and the Rigel MIL-Starlight Mini (both of which are actually CYAN) dark adapted vision may be easily preserved while still seeing far more detail than red. There are other interesting uses for blue/green lights that may be found if you scratch around a bit.

Blue/green may also be had in very bright lights like the old turquoise ARC-AAA or the CMG Infinity. These two models were brighter than any other lights in the same models offered.

Blue is a great color as it is the easyest color to hide from an audience -- which is why it is used so widely backstage and in other technical spaces by techies in theatres. Blue can also often be had brighter than other LEDs in the same model lights.

Since blue light is not focused correctly in normal eyes it may actually sharpen the vision of abnormal eyes --
there are a number at CPF who need glasses and can read more easily with blue light. I'm one of them.

Amber may be very discrete. My old CMG Infinity in amber is so dim that it is the perfect menu/wine list light in a cafe with subdued lighting -- places where an ARC-AAA might be too bright. Handy.

The biggest problem with colored lights is the fact that we are not used to them. Making the most out of any monochromatic light is a learned experience and it takes time, experimentation and lots of practice to develop the skill set needed to make the most of the advantages they offer.

The same is actually true of getting the most out of white light but in the case of white we're all pretty used to what the sun's light is like. Even with white there are still professions where users learn to see more than ordinary folk. They really do develop a trained eye. This is even more true for people working with monochromatic lights. It takes time and work.

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As far as what color is best for preserving 'dark adapted vision' -- that's a pretty controvercial subject outside of the lab in the real world. There are many, many viable strategies that work very well that are quite counter-intuitive, like using CYAN at extremely low levels, levels so low that only a few lights made may achieve -- like the above mentioned Rigel MIL Starlight or the Covert Photon Freedom. A look around the archives will show a plethora of dark adapted vision approaches that don't involve red that not only work with other colors including white, but the schemes are in use every night by solders, sailors, spooks, hunters and theatre techies. Also, many fail to realize that many lights sold as being red are actually orange-red, in the 630nm range and will still have a negative impact on rods.

Sub_Umbra said:

Sorry to sound pedantic, but we have to be clear - for any vision OTHER than true scotopic (human night vision) - then it's dimmness that matters more than color and almost any color will do....

However for true scotopic - RODS ONLY - vision then only red and above about 650nm would be the correct color which does not affect scotopic vision - and it almost does NOT matter about the brightness/dimmness level (within reason of course) -

This is due to simple physiology of the eye -
Rods which are VERY sensitive - but are not sensitive to light above about 650nm -
this means the Rods canNOT see red light above 650nm -
any other color/shorter wavelength can be seen, and will affect one's night scotopic vision.I agree with you, but, two points:

The vast majority of the times when we refer to Dark Adapted Vision it would be much more accurate to state that we are using Mesopic and not Scotopic vision. Most of my Dark Adapted Vision definately falls into the Mesopic catagory -- which is why I never used the term Scotopic in my post. Dark Adapted Vision does not and usually is not Scotopic vision in the pure text book definition. That changes everything.
The notion that Dark Adapted vision cannot be negatively impacted by bright wavelengths longer than 650 nm is just wrong and couldn't make any sense unless our eyes only contained rods. Yes, you are right that those wavelengths won't affect the rods, but that is not the problem.

Light in that range is so hard to see with that there is a great tendancy to crank up the intensity -- especially if you think it won't hurt your Dark Adapted vision. What can happen is that you can dazzle your red cones to such an extent that an afterimage may be burnt into your red cones that is capable of completely overpowering the very weak output of your rods. Bear in mind that it has not desensitized your rods one iota -- it has only caused their weak output to be lost in the noise of an afterimage in your red cones.

If you don't believe it try this very quick and dirty test: When you have some Dark Adapted vision built up (note that I didn't use the word scotopic) close one eye and shine a light of 650nm wavelength into your open eye for a few seconds. Then turn it out and look around with one eye closed, alternating eyes. You will see that your Dark Adapted vision has been impaired by the bright light -- which had no direct impact on your rods, whatsoever.

So, brightness matters, even in wavelengths that may in no way directly affect the rods.

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But what is the point (other than 'just because you can') for GREEN or CYAN or VIOLET or other color (ORANGE, YELLOW?) LED's, I'm really quite curious. Not that I need a reason to get a colored LED light, but hey, if I have a need, and just don't know about it, yet, I'm sure you guys can help me out.

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Human eye response. The black curve is photoptic "daylight" response, and the green curve is scoptopic "night vision" response. Our eyes can use both curves simulatenously, in a mode called "mesopic" response -- moonlight, dawn/dusk, and most outdoor flashlight use falls into the mesopic catgeory.

The following is based on my own experience with using different colors:

Red - Only choice if you want enough light to actually read with (say, a star chart) without ruining scoptopic adaptation for astronomy. However, you have almost no peripheral vision sensitivity to red, which causes a "tunnel vision" effect. Also, many things like leaves absorb red light completely, so it is a poor choice for general lighting.

Amber - Take a look at 590nm on the chart above. This is the color of low pressure sodium street lights (used where I live). Amber is a good "compromise" wavelength. The night vision sensitivity there is still low, but overall sensitivity is very high. Of all single colors I find amber is the most useful as almost everything reflects amber to some extent (ie, both green leaves, and red dirt will reflect different amounts of amber) -- that's why they can get away with it as a street light color.

Green - Can't think of any advantages compared to a dim neutral-white light (ie Moonlight at ~4000k)

Cyan - Can be used in VERY small amounts to actually utilize night vision, a small amount of cyan light can illuminate a huge area enough to see with, although cool-white will work better in most cases.

One case where I used cyan as opposed to white was when I was developing holographic film that was most sensitive to red light. The eyes have peak dark-adapted sensitivity to cyan, and the film has minimal sensitivity. Thus, the "ratio" between eye-sensitivity and film sensitivity there is maximized, making it the best wavelength to use. (I ran a coin cell light with a half-dead 2032 cell, then added a filter to attenuate it further, brightness was comparable to an "indicator" light).

Blue - Not especially useful, if you're looking to induce fluorescence UV is probably a better option.

Green light--lets you see things with the list possible lumen output (eye's sensitivity)
Red light--requires slightly more light to gain the same acuity, but disturbs your dark adaptation less than green light

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Just wanted to clear up a few things. Lumen output is actually a measure of power, weighted by the black curve in my graph above -- so theoretically, one lumen of green light and one lumen of red light will be the same apparent brightness. The difference is that one lumen of green light requires a lot less radiated power to produce.

In practice though, lumens isn't the whole story, as they are a measure of how bright a color appears in the center of your field of vision, under photopic (cones-only) conditions. That's why a lumen of green might actually be better than a lumen of red in mesopic conditions in a low-light situation where peripheral vision is important.

Sometimes you just want something other than white. Colors are pretty :")

+1 on 2xtrinity's amber/orange being a very useful night vision alternative to red. Better color rendition, better peripheral ... and no bugs!