# Red, blue or yellow filter to keep me "less visible" at distance ??



## villa real (Apr 9, 2011)

Hi, everybody.

In some situations, in the woods or in a cave (dark places), I need some light for some little tasks, but I need to keep me invisible (or less visible possible) to other people.

Considering this and the fact that it's a good idea to keep my adapted night vision, what is the best filter for it, red, blue, yellow or another color ?

Thank you.

PS. My flashlight is an Olight M20 S2.


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## tylernt (Apr 11, 2011)

An interesting question... most people want to use light that the eye can most easily see, so we're kind of working backwards here.

http://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html
http://en.wikipedia.org/wiki/Scotopic_vision
http://en.wikipedia.org/wiki/Photopic_vision
http://en.wikipedia.org/wiki/Color_vision#Physiology_of_color_perception

I'd say that either violet (nearly UV) or deep red (nearly infrared) are probably your best options, but I'd love to hear the opinions of others.

I don't know how well color filters work... you may be better off with a purpose-made mono-chromatic LED than trying to filter white light.


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## MikeAusC (Apr 11, 2011)

Any light of any colour that you can see, can also be seen by others.

Your best option is to have a shield so that light ONLY goes where you need it, and not in the direction of other people.


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## RepProdigious (Apr 11, 2011)

MikeAusC said:


> Any light of any colour that you can see, can also be seen by others.
> 
> Your best option is to have a shield so that light ONLY goes where you need it, and not in the direction of other people.


 
AND use the least amount of light you can do with.... some moonlight mode would be best.


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## villa real (Apr 11, 2011)

I got it.
These are my intentions:

1. as *RepProdigiuous* said, use the least amount of light possible, like a moonlight,
2. not to point the light source to someone eyes. Instead of it, keep myself between him and my flashlight,
3. use the filter to make it even more dificult for him to see me, or the light spill on the ground, while I go away... or while I read my papers or do my tasks.

Again, thank you all.


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## tylernt (Apr 11, 2011)

I agree that using as little of directed light as possible is a good idea, but I still suspect different colors may help/hurt the detectability. Remember our OP is using the light close-up and potential observers will be a lot further away -- we know the perceived intensity of light falls off with distance, and not only does light behave differently at different wavelengths (http://en.wikipedia.org/wiki/Raleigh_scattering), the eye also perceives different colors in interesting ways (http://en.wikipedia.org/wiki/Purkinje_effect). The eye is a pretty complex machine that yields some non-intuitive effects (http://en.wikipedia.org/wiki/Averted_vision). It seems a reasonable hypothesis that different colors will be harder to detect at a distance... all that remains now is someone to test it.


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## Lateck (Apr 11, 2011)

This is very interesting. 
I have a question. Why is GREEN light (filters) used in may outoor lights? 


Lateck,


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## Sub_Umbra (Apr 11, 2011)

I really prefer green or cyan for low light situations in the real world. One may see far more detail with green or cyan than one may with red or orange/red and at FAR lower output levels, to boot.

The most difficult part is usually finding a UI that will allow you to reduce the output of a green or cyan light ENOUGH. Also, go with as tight a beam as possible. As mentioned by others, pay strict attention to your *pointing discipline.*

Try a Photon Freedom in cyan with Covert nose. Depower it by replacing the two stock CR2016s with a single CR2032. At the lower voltage it will shift a bit toward green but it will still have lots of blue in it. (similar to the color in my avatar) The levels available to you in this configuration will be fabulous for everyday dark adapted cave tasks.The depowered lows are *hideously low* if you need them and the runtime will increase to about *12 days straight on the new depowered HIGH.* I predict that it's utility will blow your mind.


This is and has been my most switched on light for years.

I should also mention that using monochromatic lights effectively is a *learned skill.* I have spent decades in various careers honing my skills with them. There *is* such a thing as a *trained eye.* Even though these lights may not work for you right away, I'd advise you to keep your mind open and give it an extended trial -- carry one with you into the caves a few times and keep trying them out. It takes time to learn how to process what you get from them. It is not automatic.

We often forget just how many years experience we've been practicing with what we're used to -- the bright floody white light from the sun.

Take your time. Work at it.


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## Erzengel (Apr 11, 2011)

If Your main goal is to be not seen, take read. In the German army they display the visibility of white, red and green light to recruits. Red is the least visible color, human eyes are more sensitive to green, therefore the displays of night vision equipment are always in green and white.
With green light You can recognize more, therefore modern military maps are optimized for the use with green light.
But if You really don't want to be seen, use a light with a Moonlight/Firefly Mode.


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## Sub_Umbra (Apr 11, 2011)

Erzengel said:


> If Your main goal is to be not seen, take read. In the German army they display the visibility of white, red and green light to recruits. Red is the least visible color, human eyes are more sensitive to green, therefore the displays of night vision equipment are always in green and white.
> With green light You can recognize more, therefore modern military maps are optimized for the use with green light.
> But if You really don't want to be seen, use a light with a Moonlight/Firefly Mode.


The real problem with red in the real world is that it is *so hard to see with.*

Most lights that flashaholics call red *are not red at all* -- but are red/orange at ~630NM. The problem with 630nm is twofold: first the human eye is so insensitive to this color region that there is a propensity to make and use the lights *BRIGHTER* than those of other colors. Aside from greatly increasing your IR signature this may have other negative effects on your dark adapted vision since 630nm *STILL ACTUALLY NEGATIVELY IMPACTS* rod vision.

If you are diligent enough to actually seek out a light which will not negatively impact rod vision you will probably end up with something like the Rigel Systems variable light at 640nm. This color is *even much harder to see with* than what most mistakenly think of as red. With both true red and orange/red there is also the *probability* of increasing their output *so* much (because they are so *useless* to see with) that the image burned into the *cones* may overpower the dim image coming from the *rods*...hmmm.

At this point I will mention that this is not just an academic exercise to me. Unlike most readers of this thread I have *personally* spent weeks, and weeks *and still more weeks* actively avoiding looters, rogue cops and elite military units and I did so successfully by discretely employing cyan lights using the tactics I've described. Many here already know this. Others may search out those threads for nuts and bolts details.

In the real world these issues are not nearly as simple as they may seem.


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## KiwiMark (Apr 11, 2011)

Sub_Umbra said:


> The most difficult part is usually finding a UI that will allow you to reduce the output of a green or cyan light ENOUGH.


 
I have a Quark AA and with an Olight green filter + moonlight mode the output is VERY low. It would be hard for someone at a distance to notice the light if they couldn't see the emitter directly - i.e. don't point it towards them.
Most of the time I am not so worried about others seeing me and I just use my Olight diffuser + moonlight mode to give me low light and no hotspot - this is good for preserving night vision and letting me see without hurting my dark adapted eyes (like when going for a pee in the middle of the night while camping).

It could be worth cupping your hand around the flashlight head or making a shroud that blocks the direct vision of the end of the light from anyone nearby. That way the only light that they can see is what reflects off the ground (or whatever you shine the light on) which will be much dimmer than the light directly from the emitter/reflector.

For the choice of colour:
Red will have less impact on your night vision and allow a flashlights low mode to be much lower. However, as mentioned, you will be less able to see details and if you end up having to use a higher output mode on your light to compensate then you have lost the advantage of using red light.

Green light will allow you to see pretty well even with low light output.

White light will also allow you to see pretty well even with low light output.

The real trick is to get a flashlight with a REALLY LOW output mode like the Quark moonlight mode or equivalent (or even lower) and to ensure that the light never shines towards the person you don't want to me detected by. Also bear in mind that in low light they wont be able to see much and unless they detect movement or light you will be pretty much invisible - if you can see them and which way they are facing then that would help a lot.


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## MikeAusC (Apr 11, 2011)

Erzengel said:


> . . . In the German army they display the visibility of white, red and green light to recruits. Red is the least visible color, human eyes are more sensitive to green, . . . . .



Do they use lights emitting the same amount of energy at white, red and green ? or what is the basis for comparison - putting filters (of different density) over a white light ?

I'm aware that the eye is most sensitive at 555nm.




Erzengel said:


> . . . . . human eyes are more sensitive to green, therefore the displays of night vision equipment are always in green and white. . . .



Early generations of nightvision were green because that was the best phosphor they could get. 




Erzengel said:


> . . . . . therefore modern military maps are optimized for the use with green light. . . .



Red will come out black - what colours are used on military maps ?


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## angelofwar (Apr 11, 2011)

Edit: Delete


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## beerwax (Apr 11, 2011)

the violet or red from the 2nd post. once your eyes adjust to the colour you will see well enough, but other people with unadjusted eyes will not notice the colour as readily as they would white. unless of course they are using the same colour as yourself. still in a dark cave a faint violet or red may go unnoticed. you do need to ensure that your emitter or bulb never seen, too much intensity.


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## Burgess (Apr 11, 2011)

Interesting thread.


:thumbsup:
_


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## enomosiki (Apr 11, 2011)

Grab a light amplifier with IR illuminator. Unless whoever you are trying to avoid happen to be blood-sucking vampires, it will work like charm.

Otherwise, stick with a low-output light with a red filter. It's the least visible part of the spectrum, and you will also be able to preserve and regain your night adapted vision faster than blue or green. (Green is the worst, by the way.)


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## angelofwar (Apr 12, 2011)

MikeAusC said:


> Any light of any colour that you can see, can also be seen by others.
> 
> Your best option is to have a shield so that light ONLY goes where you need it, and not in the direction of other people.


 
Which the lazerbrite black-out tube does. It even has a rough, dull finish on the inside to minimize reflection/spill. Try this with a red or green LED. These things are really neat and have tons of uses. Plus, the MOLLE Case you can get is the perfect size for a B65 Operated SF.


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## MikeAusC (Apr 12, 2011)

villa real said:


> Hi, everybody. In some situations, in the woods or in a cave (dark places), I need some light for some little tasks, but I need to keep me invisible (or less visible possible) to other people. . . . .



If you keep the light at the lowest possible level, so that you can JUST see what you're doing at a distance of several metres, then there's a good chance that someone further away will not see the light. If they're using just their eyes, then they will have the same spectral sensitivity as you have. All colours are attenuated equally over distance.





villa real said:


> . . . Considering this and the fact that it's a good idea to keep my adapted night vision, what is the best filter for it, red, blue, yellow or another color ? . . . .



If you need to have sharp vision, then you will need to enough light to activate your photopic vision - scotopic vision works at lower levels but has much lower resolution and is monochromatic.

If you need photopic vision - but ALSO have the ability to quickly adapt to scotopic (night) vision, then you're best off using red light, because that's the part of the visible spectrum where there's the biggest difference between scotopic and photopic sensitivity i.e. you can see using monochromatic photopic vision, but the light has the minimal leaching of the chemicals in the eye that's needed for scotopic night vision. These chemicals build up very slowly once leached by light.

If you need to recognise all colours e.g. read a coloured map, you will need to use white light.


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## Erzengel (Apr 12, 2011)

MikeAusC said:


> Do they use lights emitting the same amount of energy at white, red and green ? or what is the basis for comparison - putting filters (of different density) over a white light ?
> ...
> Red will come out black - what colours are used on military maps ?


 
Our Army issue light is an old (designed 1929) incan light with a red and a green slide up filter.

The new maps are printed in black and different shades of red. I only used them at daylight and hat no chance to test them at night. I'm a reservist, therefore I can't get them every day. The red maps are so new, that they aren't issued completely today.


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## beerwax (Apr 12, 2011)

sometimes you can still do things like read maps with something like a red light, you just have to have patience and let your brain and eyes learn. because all tho everything will appear in different shades of red they are still different shades. and your red light wont be pure red. sort of like black and white tv (back in the days of valves and real electrics). you mightnt have been sure what colour her swimsuit was, but you knew her twin sister was wearing the same.

im actually unconvinced by the whole red/ green nite vision stuff and prefer dimmest possible white .


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## KiwiMark (Apr 12, 2011)

beerwax said:


> im actually unconvinced by the whole red/ green nite vision stuff and prefer dimmest possible white .


 
That should work just fine, as long as:
It is dim enough (only just barely enough for you)
and
the light is shrouded so that no direct light reaches the person you are trying not to be detected by.

Not all lights are dim enough for this, but there are some that are. A dim enough light of any colour should not have much impact on the night adaption of your eyes.
Some lights have a bright flash of light as they turn on before going to the very low output, if you use such a light then it is important that you cover the front of the light when you turn it on.


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## LowFlux (Apr 12, 2011)

For the red vs. green crowd - has anyone ever seen a traffic light from the distance at night? I have a straight stretch of highway coming into town and at night I can see a light which is almost 3.5 miles away. Ditto for the red flashing railroad crossing lights. Dim is the way to go, followed by shielding your light source. I've seen my moonlight mode light reflected from windows that are 50' away, so where you point your flashlight head is important.

Using a monochromatic filter is advantageous for night adapted vision as you only stimulate one spectra.


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## beerwax (Apr 12, 2011)

KiwiMark said:


> Not all lights are dim enough for this, but there are some that are.


 

i used to carry a maglite solitaire with a flat battery. i see some new lights have been given the thumbs up here for there ultra low modes. 

cheers


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## shao.fu.tzer (Apr 12, 2011)

Based on popular theory, I've always used red filters and leds to help preserve my night vision. There definitely seems to be a difference between red and white light. Now I'm just speaking from personal experience, but even the moonlight mode on most nights will screw up my night vision, while a visibly brighter red light, after being used and turned off, will still keep me seeing like I was beforehand... Just my two cents...

Shao


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## MikeAusC (Apr 12, 2011)

shao.fu.tzer said:


> Based on popular theory, I've always used red filters and leds to help preserve my night vision. There definitely seems to be a difference between red and white light. Now I'm just speaking from personal experience, but even the moonlight mode on most nights will screw up my night vision, while a visibly brighter red light, after being used and turned off, will still keep me seeing like I was beforehand... Just my two cents...Shao


 
That's not just from intuition, that's because the red part of the visible spectrum is where there's the biggest difference between scotopic and photopic sensitivity i.e. you can see at high resolution using monochromatic photopic vision, but compared with other colours, red light has the lowest leaching of the chemicals in the eye that's needed for scotopic night vision. These chemicals in the eye build up very slowly, once leached by light.


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