# Purpose of blue light?



## concept0 (Nov 30, 2008)

I bought a Photon Freedom with a blue LED a year or two or so because it was really cheap compared to the white LED version. I carried that thing for months, but eventually got tired of it and got a white LED. The blue light just never seemed to be that useable and irritated my adjusted eyes almost as much as white light.

Anyway, my question is: what is the purpose of blue light? I know that red lights are used to protect night vision, but why would anyone prefer a blue light?


----------



## Beamshot (Nov 30, 2008)

Im not 100% sure but I think you can use Blue to make it easier to see blood.


----------



## Monocrom (Nov 30, 2008)

Beamshot said:


> Im not 100% sure but I think you can use Blue to make it easier to see blood.


 
I've heard it could be used for other fluids too. 

I have a Surefire FM26 blue filter. But mainly got it to protect the lens on my Surefire M6. 

(Got it at a *huge* discount from a former Surefire Authorized Dealer who lost that status).


----------



## richardcpf (Dec 1, 2008)

incan yellow = not cool
led white = cool
solid blue = win


----------



## concept0 (Dec 1, 2008)

richardcpf said:


> incan yellow = not cool
> led white = cool
> solid blue = win


 
You mean you like blue light better than white? Why?


----------



## Marduke (Dec 1, 2008)

concept0 said:


> You mean you like blue light better than white? Why?




It was sarcasm


----------



## StarHalo (Dec 1, 2008)

Blue light = special
Red light = Roxanne
Dashboard light = Paradise

etc. :nana:


----------



## labrat (Dec 1, 2008)

concept0 said:


> I bought a Photon Freedom with a blue LED a year or two or so because it was really cheap compared to the white LED version. I carried that thing for months, but eventually got tired of it and got a white LED. The blue light just never seemed to be that useable and irritated my adjusted eyes almost as much as white light.
> 
> Anyway, my question is: what is the purpose of blue light? I know that red lights are used to protect night vision, but why would anyone prefer a blue light?



Hunters do use blue light for tracking wounded prey.
Blood shows very good, you can see small splatters on leaves, trunks, on the ground, very easily.


----------



## Kamakazikev24 (Dec 1, 2008)

Is blue light used for map reading as well or is that something else......


----------



## Narcosynthesis (Dec 1, 2008)

White light is for general use
Red light preserves night vision
Blue light is used by hunters for tracking prey, and also I believe projects better in fog or mist than a white light.

Red is what will be used for map reading in the dark usually, as it won't destroy your night vision. As a note, most large scale maps (currently looking at a 1:7500 map) don't use red in the markings, so a red pen is used to make notes without obscuring details - Red is often used in roadmaps and on larger ordnance survey maps (this is in the UK, I have no idea what the US equivalent is...)


----------



## cliff (Dec 1, 2008)

In my experience red light preserves night vision fairly well, and the dimmer it is the better.

As far as tracking blood goes, last year I tried this out in the pitch dark on some fresh gore from a deer kill with both a blue LED and blue incan with a filter. Neither light had any significant effect on the blood that I could see. 

It appears to me that the blood tracking ability of blue light is a hyped-up feature intended to sell more lights to pessimistic hunters. Their best bet would be to stop shooting earlier in the day and take care of business while the sun is up.


----------



## Superdave (Dec 1, 2008)

If you want to track bodily fluids a blue light is a waste... a nice UV led light with a pair of yellow UV glasses is the only way to go. 


Blue lense filters are good for making your kids think they have a light saber on foggy nights. :twothumbs


----------



## defloyd77 (Dec 1, 2008)

The only 2 uses for a blue led alone that I've found are dusting (makes the dust stand out a LOT) and help waking you up (don't shine directly in eyes, use a diffuser). I've tried ketchup on leaves and used a blue led, really doesn't help much, use a blue led and a red led at the same time though and it seriously pops out. Also theoretically speaking blue is bad in fog because of Rayleigh scattering.


----------



## Hooked on Fenix (Dec 1, 2008)

The best use for blue light in my opinion is for fishing or for use in water. Blue light goes straight through the water without reflecting the light back. You can see through the water as if it wasn't there. Don't use it for hiking in the rain though. You won't be able to see puddles until you step in them. Blue light does work well in fog, but it's a fraction as bright as the light from a white l.e.d.. I've found that a tight beam bright white l.e.d. flashlight will do better in fog than a much dimmer blue light. You need a blue light of equal brightness to a white light for it to be better in fog. Cool white works okay in fog and so does warm white. Pure white reflects back the most. In fog, warm white is nice as it also works well in smoke.


----------



## ypsifly (Dec 1, 2008)

I've read somewhere (probably here) that most blue lights on the market are useless for tracking blood trails. It only works when the blue is within a specific wavelength. Last week my GF was running the vacuum cleaner and she ran over some fishing line one of the cats pulled out of the closet. The vacuum seized up and I had to cut the line out with a knife. This left lots of short bits of line all over the carpet. I was using a ROV 1xAA headlamp and the bits of line just about glowed under the blue beam making cleanup a lot easier. I use P-Line and Yozuri copolomers on all my non-fly rods. I don't know if it would work on regular mono.


----------



## Zatoichi (Dec 1, 2008)

ypsifly said:


> I've read somewhere (probably here) that most blue lights on the market are useless for tracking blood trails. It only works when the blue is within a specific wavelength.



UV I think, for blood and some other bodily fluids.


----------



## greenlight (Dec 1, 2008)

Blue LEDs are great for blinding people.


----------



## scottm (Dec 1, 2008)

Blue (high frequency) light scatters most easily, that's why the sky is blue and the sun is yellow. The blue is refracted (scattered) by particulate (dust) in the atmosphere, leaving yellow sunlight. The setting sun goes through more and larger particulate, scattering longer wavelength light, making for deeper reds in the sky and sun. More dust makes more colors, like when there's a forest fire or volcano throwing up more dust.

Blue requires more energy to produce, and carries more energy. I'm working on an LED tester, customer thought I might verify the blue color is installed because it draws more current than the amber. Blue scatters and bounces more in the eye than longer wavelengths, dazzling and irritating. That's why ricers with fake HID headlights are so annoying. Blue scatters readily in fog, so foglights are often filtered to remove blue, leaving amber light. Blue-blocking sunglasses and camera filters take the haze out of hazy scenes, because the haze is scattered blue light. Blue blockers are helpful for older folks with hazy vision.

Dunno about the blood part. Red doesn't reflect blue light well, maybe there will be more contrast in green foliage? Red shows up pretty well with a white light.


----------



## Zatoichi (Dec 1, 2008)

Blood will show up well under UV light because it contains fluorescent molecules.


----------



## Bradlee (Dec 1, 2008)

I've been asking the same question about a blue X5 I've had around for a while and can't find a use for. I tried to fluoresce blood, but nothing happened


----------



## scottm (Dec 1, 2008)

Flourescent materials will emit in visible spectrum when flooded with invisible UV. That makes a nice contrast between the flourescent material and non-flourescent background. The flourescent material also emits when flooded with visible light, but there's no contrast as the background also lights up. It sounds like blood and body fluids have some of that quality of emitting visible light under UV light.


----------



## Hooked on Fenix (Dec 1, 2008)

The lights I have seen that are used for tracking blood use both blue and red l.e.d.s. Blue alone probably won't be that great for tracking blood. With my experience, I know that blue light does work in fog as does warm white. I have hiked in fog many times and have used different colors of light to try in the fog. Basically, any color other than pure white will work in fog. Pure white is is the color of fog and reflects back the most, blinding you, messing up your night vision, and not allowing you to see what's in front of you as easily. You can still punch through the fog with a white light, but only if it's bright with little to no spill light. If the light is a flood light, prepare to be blinded, especially if it's a headlight. A tight beam will scatter some, but much of the light will get through the fog. Amber or warm white light works the best in fog. Blue isn't quite as good but is better than pure white. Green works too. Colors other than white work great in snowy terrain as well for the same reason. I like green light in snow. It is almost as bright as white (Nichia CS brightness in 5mm l.e.d.s) and doesn't reflect light back at you. 

As I said in my first post, using blue light for blood tracking and seeing in fog isn't the best use of the light. Blue light is the most useful for seeing through water.


----------



## f22shift (Dec 1, 2008)

scottm said:


> Flourescent materials will emit in visible spectrum when flooded with invisible UV. That makes a nice contrast between the flourescent material and non-flourescent background. The flourescent material also emits when flooded with visible light, but there's no contrast as the background also lights up. .


 
http://www.hamandcheese.smugmug.com/gallery/3380544_XnUZq#188790504_pyV4c
pic examp of blue, rov 1aa


----------



## 2xTrinity (Dec 1, 2008)

> Also theoretically speaking blue is bad in fog because of Rayleigh scattering.


Nitpick: rayleigh scattering is what happens in clear air. It's the phenomenon where light is scattered from particles smaller than the wavelength of light (eg the air molecules). The water droplets in fog are actually much larger than the wavelength of light, and they tend to backscatter all visible light to roughly the same degree. However, that said, blue is still worse in fog for two reasons:

1) outdoors, nothing you are interested in looking at is actually blue (eg trees, dirt animals) except maybe a handful of flowers. This means even the light that makes it past the fog is not as useful.

2) Your eyes are more senstive to blue in darker conditoins, relative to the other colors. this means that the backscattered light may be apparently brighter or more bothersome.

The major factor in fog is not so much color as beam pattern. A narrow angle light with NO spill, held as far away from the eyes as possible is best.


----------



## darknessemitter (Dec 1, 2008)

ypsifly said:


> I've read somewhere (probably here) that most blue lights on the market are useless for tracking blood trails. It only works when the blue is within a specific wavelength.


 
I have wondered if cyan, or a combination of blue and green might be the more appropriate color for blood contrast, since cyan is the direct opposite of red. The point is to illuminate everything EXCEPT red, and blue doesn't necessarily illuminate some shades of green much better than red. 

However, this still might not be ideal in cases where you're chasing after something; it would be better in cases where you have time to switch back and forth between white light for correct color rendition and cyan or blue and green for contrast. 

Remember, if you use a color that makes blood appear black, then blood won't stand out on a black (or near-black) surface! This is why it would be nice to double check with white light.


----------



## darknessemitter (Dec 1, 2008)

Zatoichi said:


> Blood will show up well under UV light because it contains fluorescent molecules.


 
Um, but don't you need to use some sort of chemical on the blood first, or put it through a centerfuge thing to separate the cells or something?


----------



## darknessemitter (Dec 1, 2008)

f22shift said:


> http://www.hamandcheese.smugmug.com/gallery/3380544_XnUZq#188790504_pyV4c
> pic examp of blue, rov 1aa


 
Also, it can sometimes be usefull wear orange goggles to filter out the visible blue, and make the fluorescence easier to see. Of course, this won't work if the the substance you're looking at fluoresces blue...

One interesting thing I've noticed is that some LED phosphors will only fluoresce under blue light, and not UV, while some other LED phosphors will fluoresce under both blue and UV.


----------



## Zatoichi (Dec 1, 2008)

darknessemitter said:


> Um, but don't you need to use some sort of chemical on the blood first, or put it through a centerfuge thing to separate the cells or something?



No, it will show up under UV light without anything being added.


----------



## KeyGrip (Dec 1, 2008)

Diffused blue light is useful for navigating backstage during a show.


----------



## Lynx_Arc (Dec 1, 2008)

and the main reason for blue Light is that blue LEDs don't have to have the phosphor to turn the light into white and are brighter than the white LEDs of the same batch. It is mostly about hype and cheapness as the first white LEDs were not very bright at all making the blue ones more bright and cheaper = more sellable. 
Now white LEDs are cheap enough that the blue ones are only being used for decoration or RGB type stuff.


----------



## orcinus (Dec 1, 2008)

Weren't there... erm... a hundred identical threads on use of colored light already?


----------



## TMedina (Dec 1, 2008)

concept0 said:


> I bought a Photon Freedom with a blue LED a year or two or so because it was really cheap compared to the white LED version. I carried that thing for months, but eventually got tired of it and got a white LED. The blue light just never seemed to be that useable and irritated my adjusted eyes almost as much as white light.
> 
> Anyway, my question is: what is the purpose of blue light? I know that red lights are used to protect night vision, but why would anyone prefer a blue light?



Blue is a non-white light source that shows blood very well. Green might work as well, but green is more distinctive at night than, say, blue light - at least in my opinion.

The appeal of that particular aspect is probably limited to military medics - most EMTs don't have light discipline restrictions.

Gerber actually makes a mixed light, blue and red, I believe, that is supposed to illuminate blood even better that just blue light - but that's a unique product.

-Trevor


----------



## Zhivago (Dec 1, 2008)

I find that the blue LED's from my Kroma are easiest on my eyes for reading at night. Especially the lowest setting. Produces the least eye strain for me c/w the red or white.


----------



## greenlight (Dec 1, 2008)

2xTrinity said:


> 1) outdoors, nothing you are interested in looking at is actually blue (eg trees, dirt animals) except maybe a handful of flowers. This means even the light that makes it past the fog is not as useful.
> 
> The major factor in fog is not so much color as beam pattern. A narrow angle light with NO spill, held as far away from the eyes as possible is best.


My inova X1.v1 blue works great like this. The beam is bright and sharp, but the spot is noticeably dim, since it is not reflecting off anything blue. It DOES highlight GID safety tape very well. 




KeyGrip said:


> Diffused blue light is useful for navigating backstage during a show.


Nobody sees you if you're backstage.


----------



## concept0 (Dec 1, 2008)

orcinus said:


> Weren't there... erm... a hundred identical threads on use of colored light already?


 
I read stuff like this all the time on forums... People telling other people that a thread already exists.

Don't get me wrong, I don't think orcanis was being rude or anything, but just because a thread exists doesn't mean that it's easy to find. If you search for "blue light" you'll find a bunch of people looking for blue lights or wondering how to build them. I'm sure that the threads that orcanis is referring probably exist, but that doesn't mean anybody can easily find them.

If you search for "purpose blue light" then you'll eventually find a thread that mentions that blue light is "used for blood and general leak detection." But that's not very specific. And by my asking the question again, people presented a bunch more interesting uses, and there was good debate about the actual usefulness of blue light for blood tracking.

Anyway, my point is just that asking a question that may have already been answered somewhere on the forum is not such a bad thing. Of course we don't need 50 threads in a month asking what people should buy for their first 2AA, or whether AA or CR123 are better batteries. But if questions are re-posed every once and a while, it allows people to talk about new developments and evidence.

Plus, people on forums just like talking about stuff. That's why they're on a forum and why they choose to take their time to answer other peoples' questions for free.

So that's my rant. It really has nothing to do with this thread, but I was still frustrated from trying to find the answer to another question, searching this forum, and eventually finding a thread posing my exact question, only to have the thread end prematurely when someone said that the question had already been answered elsewhere on the forum. They didn't give link to the thread, and I searched and searched but never found it.

Sorry for hijacking (and the run-on sentences), but it is my thread...

BTW thanks for all the answers. Unfortunately for my blue photon, I don't hunt or act...


----------



## tzzoooma (Dec 1, 2008)

I always use mine to detect aliens.

http://www.amazon.com/gp/product/B0009K43KE/?tag=cpf0b6-20


----------



## PhotonWrangler (Dec 1, 2008)

tzzoooma said:


> I always use mine to detect aliens.
> 
> http://www.amazon.com/gp/product/B0009K43KE/?tag=cpf0b6-20



Have you found any yet? :huh:

Blue LEds can be used to detect the nearly invisible, uniquely identifying yellow dots that many color laser printers produce.


----------



## angelofwar (Dec 1, 2008)

An Ex-marine over in Iraq with me told me that him and the guys in his unit used blue light to work on the tanks at night, because it was the least visible, from a distance, in a pitch black enviroment...I guess the blue "bounce" blended in with the naturally blue night sky???



TMedina said:


> Gerber actually makes a mixed light, blue and red, I believe, that is supposed to illuminate blood even better that just blue light - but that's a unique product.
> 
> -Trevor


 
Here's a pic of the Carnivore trevor mentioned earlier:


----------



## moldyoldy (Dec 1, 2008)

Deleted


----------



## KeyGrip (Dec 1, 2008)

greenlight said:


> Nobody sees you if you're backstage.



Depends on the theatre. White light has an annoying way of bleeding everywhere.


----------



## Burgess (Dec 1, 2008)

Thank you to PhotonWrangler for
mentioning the (top-secret)

*Yellow Dots of Mystery: Is Your Printer Spying on You?*




_


----------



## tzzoooma (Dec 2, 2008)

PhotonWrangler said:


> Have you found any yet? :huh:



If you saw my wife, the answer would be obvious 

(glad she doesn't read this forum )


----------



## Blue72 (Dec 2, 2008)

Blue light has never worked for me during hunting.

Maybe someone should contact surefire for an answer since they have a lot of blue lights.


----------



## PhotonWrangler (Dec 2, 2008)

tzzoooma said:


> If you saw my wife, the answer would be obvious
> 
> (glad she doesn't read this forum )



...and they said Henny Youngman was dead. :ironic:


----------



## Superdave (Dec 4, 2008)

It's funny that just after i posted in this thread i scored a A2 with blue LED's..

I pretty much tried to find any use for them and have not.. They're getting switched out for either white, or UV.


----------



## Isthereanybodyoutthere (Dec 7, 2008)

Zatoichi said:


> No, it will show up under UV light without anything being added.




Did you actually ever try this yourself ?? 

So lighting a UV flashlight will make the blood wains in the yes light up ??

I know if you spray luminol on blood the UV will make it light up 

I have a black light pen and a black light neon tube and a black light money tester 
Tonight i will in the name of science make my self bleed :mecry:and try it


----------



## Zatoichi (Dec 7, 2008)

Isthereanybodyoutthere said:


> Did you actually ever try this yourself ??
> 
> So lighting a UV flashlight will make the blood wains in the yes light up ??
> 
> ...



Blood and other bodily fluids show up under UV because they already contain fluorescent molecules. Forensic scientist use UV to search for blood (and other stains), though they have more advanced equipment than a simple UV light. How well the blood will show up under UV will depend what it's on.


----------



## Icebreak (Dec 7, 2008)

Well, I've tried it with 395nm, 385nm and 365nm and it didn't work. Using scientific instrumentation some anomalies indicating disease can be detected as fluorescence. I'd be interested in a link to a fact or a link to a pic. If I've been wrong about this I certainly want to know.

On the blue light, yeah I just don't have the inspiration to re-iterate what I've posted about it before. I'll give ya a little gif to check out though. This is a Q5 being invaded by a blue 5mm then by a red 5mm showing what happens when those frequencies are added to and subtracted from a white beam. It helps to get the effect by focusing your eyes on one of the red targets at a time. The blood is left to right, dry, tacky and then fresh.

Blue doesn't highlight blood by my perception. It may for others' persceptions.


----------



## PhotonWrangler (Dec 7, 2008)

Thanks for taking one for the team, Icebreak! Yeah, if blood fluoresced all by itself, Luminol wouldn't exist.


----------



## Zatoichi (Dec 7, 2008)

Here's a couple:

http://chemistry.about.com/cs/howthingswork/f/blblacklight.htm

http://www.enotes.com/forensic-science/ultraviolet-light-analysis


----------



## Zatoichi (Dec 7, 2008)

PhotonWrangler said:


> Thanks for taking one for the team, Icebreak! Yeah, if blood fluoresced all by itself, Luminol wouldn't exist.



From above link:



> Ultraviolet light analysis and other optical examination techniques are recommended by the Federal Bureau of Investigation guidelines as the first choice to examine biologically contaminated evidence. This is because ultraviolet analysis is not destructive. _It allows precise images and preliminary identification of the evidence _*before*_ other analytical methods, such as luminol_ or washing solutions, are applied.


----------



## PhotonWrangler (Dec 7, 2008)

From the next paragraph though - 

"Body fluids such as saliva, semen, vaginal fluids, urine, and perspiration give off fluorescent light when illuminated by a source of ultraviolet light..."

They do not give off "fluorescent light." They give off visible light. Be careful of a technical article that plays fast and loose with facts. While well intended, the writer seems to have only a general grasp of UV.

Blood by itself does not fluoresce. Other evidence in a crime scene does fluoresce, some of it intensely, such as fabrics, dust, oils, etc, but not blood.


----------



## Zatoichi (Dec 7, 2008)

I apologise for my misunderstanding of how it works, but to be fair, I didn't actually say it glows, but that it shows up. I found that article while googling to try and find _how_ it worked. Anyway, here's a pic showing blood showing up after being painted over. The blood actually shows up darker than the surroundings.


----------



## Zatoichi (Dec 7, 2008)

Icebreak, if you have any blood left and still feel adventuous..  try smearing a little on some dark fabric so it doesn't show up well under normal light.


----------



## Icebreak (Dec 7, 2008)

I've tried to detect it on a black leather holster. No glow.

If you were referring to the gif, the red on the black backpack is close the same color of blood. The blue light just makes it harder to see. I think Xenopus's flashing blood tracker is a better way to go. It uses red on white light. If you'd like to actually test these theories yourself I'm going to suggest that a bloody steak is a better choice as a target.

Interesting photos, BTW. Exorcism?


----------



## Zatoichi (Dec 7, 2008)

Icebreak said:


> I've tried to detect it on a black leather holster. No glow.
> 
> If you were referring to the gif, the red on the black backpack is close the same color of blood. The blue light just makes it harder to see. I think Xenopus's flashing blood tracker is a better way to go. It uses red on white light. If you'd like to actually test these theories yourself I'm going to suggest that a bloody steak is a better choice as a target.
> 
> Interesting photos, BTW. Exorcism?



No it doesn't glow, it just constrasts. It will depend on the surface it's on, as I mentioned earlier. I know it shows up (along with other bodily fluids) without anything being added, as I've seen this first hand. My mistake was quickly googling for an explanation of _how_ it works - and believing it! I apologise again for that. 

I'm not sure what you mean about the photos?


----------



## DocD (Dec 7, 2008)

I lived on a farm upto 2000 and when shooting for game i was sucked into the whole blue light thing,but it just didn't work for me
the stuff we see in tv and movie's most of the time they miss out the Luminol spray and so we are left thinking, it's that easy to see blood.

so is there any thing to prove in hard fact to show blue light shows up blood ? or UV without using Luminol? yes some colour's aren't blanked out by blue light, orange being one of the main colours, red is blanked out by blue
just my thought's thanks DocD


----------



## Zatoichi (Dec 7, 2008)

This photo seems to illustrate it better.


----------



## Icebreak (Dec 7, 2008)

Oh, I had misread your album folder.../lexxorcist/blood.

Say, more interesting photos. I think I know what you are getting at now. The frequency of light amplifies the difference in materials. I believe I've heard of something like this in detecting fake antiques or repairs that aren't visible in normal light. The UV doesn't make the glue glow, it just makes it show up by contrast. Does that sound close?


----------



## TMedina (Dec 7, 2008)

Personal note: walking through a darkened hallway using a Fulton light with a red filter - I could not read the rooms marked in red. Could not see the red lettering at all. I knew it was there, but I couldn't see the lettering at all.

I may have to pop a vein to test the theory about which shows blood better, blue or red.

-Trevor


----------



## 2xTrinity (Dec 7, 2008)

TMedina said:


> Personal note: walking through a darkened hallway using a Fulton light with a red filter - I could not read the rooms marked in red. Could not see the red lettering at all. I knew it was there, but I couldn't see the lettering at all.
> 
> I may have to pop a vein to test the theory about which shows blood better, blue or red.
> 
> -Trevor


Niether will be very good. Let me propose an experiment:

Let's say you have a drop of red blood, and a drop of black ink on a white surface in a dark room:

With blue light, blood will appear black against a blue background. It will be impossible to distinguish from black ink. With red light, the blood AND the paper will BOTH appear red, so the black ink will appear black, all else will appear red. With a white light, it will be possible to distinguish all.




> Say, more interesting photos. I think I know what you are getting at now. The frequency of light amplifies the difference in materials. I believe I've heard of something like this in detecting fake antiques or repairs that aren't visible in normal light. The UV doesn't make the glue glow, it just makes it show up by contrast. Does that sound close?


Doesn't make any sense. True UV light (eg, shorter wavelength than 365) will be almost invisible, and near-UV light that IS slighlty visible is often used in conjunction with a yellow filter to eliminate its visibility. The UV light itself then actually hinders contrast.

The key point here is that near UV will fluoresce things like bodily fluids very weakly, not "brilliantly" like a fluorescent highlighter ink. 

Here's another important point: Most "UV" Leds emit in the 380-400nm, which doen't cut it for two reasons:

1) these longer wavelengths are much less effective causing object to fluoresce in the first place.
2) the light itself is much more visible (masking any fluorescence that does occur)

I actually work extensively with a 365nm laser. As the laser is monochromatic, it is much less visible than 365nm UV LEDs or fluorescent lamps which have significant amount of energy in longer portions of the spectrum. I notice all sorts of fluorescence. The light itself is just barely visible, but If I place my hand in front of the beam, my skin fluoresces bright green (as does dust/skin collecting on mirror surfaces etc). 

However, this effect is far less dramatic than putting a piece of white copier paper in front of the beam (which has blue fluorescent dye that reacts to UV)


----------



## Photon Joe (Dec 7, 2008)

Scorpions will glow under UV light, good picture here.
http://www.photonlight.com/Photon-Scorpion-Finder-Specialty-UV-LED-Lights-p/fm-scorpion.htm


----------



## Icebreak (Dec 8, 2008)

2xTrinity said:


> Doesn't make any sense.



The upper ranges of UVA. I'm just being open minded to resolve those pics. Maybe they were using a special lens.

One of three ways to use UV to detect repair is contrast according to this quick Google search on antique rugs.

I've seen some really pretty bacteria using 365nm pass through filter on an incan. May I borrow your laser this weekend?

If I had to use an LED blood tracker I'd try Xenopus's red on white flasher.

If I were needing to track blood I'd use a bad *** HotWire.


----------



## f22shift (Dec 8, 2008)

Photon Joe said:


> Scorpions will glow under UV light, good picture here.
> http://www.photonlight.com/Photon-Scorpion-Finder-Specialty-UV-LED-Lights-p/fm-scorpion.htm


 
sweet always wondered


----------



## Matt7337 (Dec 10, 2008)

I have a question I hope some of you can answer for me... Some time ago (no more than a month or so) I read a topic in the LED flashlights forum about someone having trouble with a light that had several emitters, one primary white emitter and several other coloured ones. I am interested in taking a closer look at this light as I have found the red mode on my recently purchased RA Twisty invaluable whilst out at night... I never knew that you could see so much in the dark without compromising the adjustment your eyes make to low light conditions.

I think the light that I am talking about had the coloured emitters around the outer bezel and had a right angled body (Which rules out the Innova Inforce, which I found after some searching)

Any help would be much appreciated people, many thanks and also for a lot of interesting information in this thread! :twothumbs


----------



## N/Apower (Dec 11, 2008)

labrat said:


> Hunters do use blue light for tracking wounded prey.
> Blood shows very good, you can see small splatters on leaves, trunks, on the ground, very easily.


 
I am a sick sick man and almost accidently nicked myself shaving just to see what it would look like with my blue filter on my SF 6P, but I figured it would be better to ask. Does it just show up as really dark, or does blood show up in some sort of florescent type wierdness in blue light?


----------



## applevision (Dec 11, 2008)

Guys,

I think I just re-read this entire thread and I am still confused--so _is_ there any purpose to the blue light except the possible/in certain situations enhancement of blood or bodily fluids??? :sick2:


----------



## N/Apower (Dec 11, 2008)

applevision said:


> Guys,
> 
> I think I just re-read this entire thread and I am still confused--so _is_ there any purpose to the blue light except the possible/in certain situations enhancement of blood or bodily fluids??? :sick2:


 
Blue-green (not two hues mixed, but one hue, very similar to traffic light green) is the least destructive type of light when it comes to night vision (It destroyes rhodopsin at the slowest/least rate.) Since there are not many filters in this color, we must settle for blue.


----------



## Fallingwater (Dec 11, 2008)

concept0 said:


> You mean you like blue light better than white? Why?


I imagine it's a question of taste


----------



## 2xTrinity (Dec 11, 2008)

N/Apower said:


> Blue-green (not two hues mixed, but one hue, very similar to traffic light green) is the least destructive type of light when it comes to night vision (It destroyes rhodopsin at the slowest/least rate.) Since there are not many filters in this color, we must settle for blue.


That's backwards. Cyan at 500nm will bleach your rhodopsin FASTER than any other wavelength. 

Red at longer than 640nm will not beach rhodopsin at all. However, the consequence of this is that you require a lot more of it to actually see with, and your peripheral vision doesn't work well with it. If you actually have to say read a map in the dark and don't want t lose your night vision, red is your only choice, as any other color bright enough to actually read with, will bleach your rhodopsin. 

If you want to see where you are walking in the dark, IMO a better choice is a VERY dim white light source. The goal in this case is not lots of lumens, but to emulate the lighting levels of say a half-moon, where you can actually use your night vision, but not at a level that's so bright you bleach away your night vision faster than it can be replaced.

IMO the ideal there is very dim neutral white light source (eg, 3600-4200k color temp) -- like the actual moon. A light running 0.2 lumen is sufficient for lighting up a large walkway outdoors in a truly dark environment (ask me how I know).


----------



## applevision (Dec 11, 2008)

2xTrinity said:


> That's backwards. Cyan at 500nm will bleach your rhodopsin FASTER than any other wavelength.
> 
> Red at longer than 640nm will not beach rhodopsin at all. However, the consequence of this is that you require a lot more of it to actually see with, and your peripheral vision doesn't work well with it. If you actually have to say read a map in the dark and don't want t lose your night vision, red is your only choice, as any other color bright enough to actually read with, will bleach your rhodopsin.
> 
> ...


*2xTrinity*: Okay: _How do you know? _

Hee hee!

Also: so this totally puts us back in the same situation: is there really no purpose for blue LEDs except just having a cool color to the light?!


----------



## defloyd77 (Dec 11, 2008)

Oh it can also decrease appetite and make things less appetizing, which you may see on tv (or elsewere) occasionally.


----------



## greenlight (Dec 11, 2008)

They're good for illuminating GID tape in a safety area.


----------



## cliff (Dec 11, 2008)

Matt7337 said:


> a light that had several emitters, one primary white emitter and several other coloured ones.
> 
> I think the light that I am talking about had the coloured emitters around the outer bezel and had a right angled body (Which rules out the Innova Inforce, which I found after some searching)
> 
> :twothumbs


 
It sounds like you are describing the "Firstlight" angle-head. I have one (the LE) and it is the best light I have used so far. Mine has red & blue LEDs around the primary white. The light has a variety of functions/strobes and red LEDs can be used at three different levels. At night this is very useful. 

However, the red-white-blue strobe puts out a serious beacon and shining it out the windshield of a car if you are not the Police could land you in a heap o' trouble.


----------



## Federal LG (Dec 11, 2008)

greenlight said:


> Blue LEDs are great for blinding people.





My sister hates my Fenix strobe mode already...


----------



## darknessemitter (Dec 11, 2008)

applevision said:


> *2xTrinity*: Okay: _How do you know? _
> 
> Hee hee!
> 
> Also: so this totally puts us back in the same situation: is there really no purpose for blue LEDs except just having a cool color to the light?!


 

"How do you know?" 
Is that a joke or a serious question? His explaination is backed up by science. We can get into another one of those discussions about all the less common situations and on several much smaller factors such as how much more degradation 630nm leds will have on your nightvision when compared to 660nm, and how 500nm cyan is the peak wavelenght for the rod cells but if you can see it as cyan instead of grey then it's already degrading your nightvision... But he gave a very good explaination of the basics. 

As for blue leds, YES, there ARE uses for them, but average people might not have a lot of everyday uses for them.


----------



## darknessemitter (Dec 11, 2008)

applevision said:


> *2xTrinity*: Okay: _How do you know? _
> 
> Hee hee!
> 
> Also: so this totally puts us back in the same situation: is there really no purpose for blue LEDs except just having a cool color to the light?!


 
Ok, I'm sorry, I missed the "ask me how I know" comment at the very end of his post. I'm sorry I overreacted.


----------



## greenlight (Dec 11, 2008)

darknessemitter said:


> As for blue leds, YES, there ARE uses for them, but average people might not have a lot of everyday uses for them.


I consider myself 'above average'. I use my blue LED inova X1 a lot, and get comments about it because the beam is so unique.


----------



## Matt7337 (Dec 11, 2008)

cliff said:


> It sounds like you are describing the "Firstlight" angle-head. I have one (the LE) and it is the best light I have used so far. Mine has red & blue LEDs around the primary white. The light has a variety of functions/strobes and red LEDs can be used at three different levels. At night this is very useful.
> 
> However, the red-white-blue strobe puts out a serious beacon and shining it out the windshield of a car if you are not the Police could land you in a heap o' trouble.



Do you work on, or are you ever around boats/ ships? I am honestly not sure if that is the same light... I suppose the chances are high but I remember someone in the thread that i am referring to mentioning that they use the blue light whilst they are out on a boat deck at night. my first thought was that they were military/ naval personell but I suppose they could have been anything from that to a fisherman 

Anyone else know any other right angled lights that use a variety of emitter colours? 

EDIT: Never noticed the word "Tomahawk" in the Firstlight's name... and as soon as I did it rang a bell... thats the very same light, thanks very much for your help. The post I was referring to about the ship was this one. Again thanks for clearing this up!


----------



## N/Apower (Dec 11, 2008)

2xTrinity said:


> That's backwards. Cyan at 500nm will bleach your rhodopsin FASTER than any other wavelength.
> 
> Red at longer than 640nm will not beach rhodopsin at all. However, the consequence of this is that you require a lot more of it to actually see with, and your peripheral vision doesn't work well with it. If you actually have to say read a map in the dark and don't want t lose your night vision, red is your only choice, as any other color bright enough to actually read with, will bleach your rhodopsin.
> 
> ...


 

Right and wrong:

*Retaining Night Vision* 
To retain your night vision, a red light has been the traditional choice since before WWII when 
the military settled on red as the best choice. Recently, there has been a move to green and 
blue-green light, precipitated in large part by the military's change to green, which itself has been primarily motivated by the increased use of night vision equipment. Which is really better? As it turns out, green light offers some advantages over red as a means to retain night vision capability. However, it isn't cut and dried.
Total brightness, or illumination level, of the light has a potentially more significant effect on night vision retention than does the choice of red or green. Because your eyes are more receptive to green light, we gain better visual acuity at lower light levels than when using red light. Green also allows for differentiation between colors that red does not and the magenta used on aviation charts, for example, is readily readable under green light, not always the case with red.
Both reasons contribute to the fact that pilots and many others generally seem to prefer green 
over red, it simply makes it easier to see and read in the dark cockpit. The potential problem is with the actual illumination levels we use, not the color of the light. The brighter the light, the more negative impact on night vision, both in our capacity to see and in how long it takes to gain back optimum night vision. This is true regardless of whether it is red or green.
Ideally, you want to use only as bright a light, red or green, as is necessary to perform your chores and no more. However, if you have a brighter light than you actually need, *a brighter green light will generally have a more negative effect than an equally bright red light.* Green or 
blue-green has a greater capacity to adversely affect night vision because the eyes are about 100 times more sensitive to these colors, so even moderately too bight light can have a serious deleterious effect.
Another complication is that an individual's visual acuity at low light levels varies quite a bit, so what would be perfect for one, might be too bright or too dim for another. In other words, without some means to vary intensity, odds are no light will be perfect. Bottom line is that red or green will both perform adequately, but what you really should be more concerned about is to avoid very high illumination levels, of either color, if retaining night vision acuity is your goal.


http://www.kriana.com/pages/nightvision.html


----------



## 2xTrinity (Dec 12, 2008)

N/Apower said:


> Right and wrong:
> 
> *Retaining Night Vision*
> To retain your night vision, a red light has been the traditional choice since before WWII when
> ...


They were essentially saything the same thing my post did.

One nitpick:

Saying that green allows "better differentiatin of color" is misleading. With a single color light source, one can't differentiate colors at all. For that matter, if the light is bright enough that what you are shining it on actually looks green, it's too bright.

Green light provides better _contrast_ since it is in the middle of the spectrum -- many things reflect it to differing degrees, few things absorb it completely. So you'll see things in lots of shades of gray. Hoewver, with red light, there are tons of things that absorb it completely (such as leaves), and others that reflect it completely (eg the magenta ink in their example). So useful contrast is less. In this case, green is definitely better, for that matter, an extremely dim white light is an even better choice still, in case someone happened to use a light green ink... that is, provided that the light is kept so dim that there's no "color differentiation" even though the light is white.

However, red light CAN be used at much higher illuminances than green. For example, I can flip on a red \lamp inside an observatory to read a star chart, then get back to looking at stars with no difficulty. And star charts aren't printed in magenta ink, so no problem there. On the other hand using a very very dim white or green light, _with enough brightness to read the chart at all _is enough have a detrimental effect on night vision. 

I agree with this article that there must still be a practical limit to the red brightness -- if the light is bright enough to create after-images, those will still interfere with my vision even though my rhodopsin may still be there.



> On the other hand using a very very dim white or green light, _with enough brightness to read the chart at all _is enough have a detrimental effect on night vision.





> *2xTrinity*: Okay: _How do you know? _
> 
> Hee hee!


My liteflux LF2x on its lowest setting on NiMH produces 0.2 lumens. It is currently hosting a 5A neutral white emitter. I have taken it camping and used it for night-hiking successfully. As long as I'm in the dark for about ~15 minutes first, and am careful not to shine the light on anything too reflective or too close to me, it is enough to see clearly where I am going.

If 0.2 lumens isn't quite enough (eg, if some genius thinks its funny to shine his half-dead 3D maglite directly in my dark-adapted eyes) I go to my "fall-back" light kept discretely in my jacket pocket -- a 2C Mag '61 w/ SMO reflector (1000 lumens in a very concentrated beam which nobody ever sees coming...) Then, after nuking everyones night vision, I can ramp my LF2x up to more like ~50 lumens so we can still see where we are going on the way back...


----------



## ruriimasu (Dec 12, 2008)

it is interesting that while i was looking at this thread, there is a news article on the calming effects of blue lights


----------



## darknessemitter (Dec 12, 2008)

N/Apower said:


> As it turns out, green light offers some advantages over red as a means to retain night vision capability.


 
I understand what you're saying later in the post about possible value in sacrificing some of your rhodopsin to see with sharper detail by using green light, but sacrificing rhodipsin is the opposite of "retaining nightvision capability". The rest of your post is clearer, but that first sentence may confuse people. 

EDIT: I guess I should've clicked the link first, I didn't realize that the whole text was excerpt from the article. But regardless of who wrote it, the first sentence is misleading.


----------



## Dead_Nuts (Dec 12, 2008)

I believe there is a model of the Tomahawk that has res and green emitters around the primary reflector. That would give you both colors. (Mine is red/blue)


----------



## defloyd77 (Dec 12, 2008)

ruriimasu said:


> it is interesting that while i was looking at this thread, there is a news article on the calming effects of blue lights



Wow that is very interesting. Oh no, I can now see light makers selling blue led lights as a self defense tool that makes bad guys not wish to harm you.

Kind of makes sense, I think I've read about blue lights helping people with seasonal affective disorder, so I'm sure it works for a pretty similar reason. http://www.news.harvard.edu/gazette/2006/02.09/01-blue.html


----------



## N/Apower (Dec 12, 2008)

defloyd77 said:


> Wow that is very interesting. Oh no, I can now see light makers selling blue led lights as a self defense tool that makes bad guys not wish to harm you.
> 
> Kind of makes sense, I think I've read about blue lights helping people with seasonal affective disorder, so I'm sure it works for a pretty similar reason. http://www.news.harvard.edu/gazette/2006/02.09/01-blue.html


 
No lie? I really dislike holidays. Breaking up last christmas morning did not help. Perhapse I will look into some blue lighting for my bedroom or something.


----------



## electrothump (Dec 13, 2008)

I've had little use for the blue leds. The blood I saw was nothing more than black spots, and you really had to know where they were in order to see them at all. So, no,,, the blue leds on a ROV Xtreme headlight do not show blood any better than the moon. Absolutely no help. One thing I did notice about the blue though is that it doesn't make the reflectors on the back of the car reflect. The red and white do a fine job, but no reflections from the blue. I thought that to be kinda odd. Plus, the blue and red leds will still light the way after the battery gets to weak to power the white led. All that being said, I really like the little headlight. I now have three of them. Car Quest autoparts stills sells them for about 14 bucks.

DN


----------



## defloyd77 (Dec 13, 2008)

Red doesn't reflect with blue, because there isn't blue in red. Have any of you tried blue and red with blood yet?


----------



## bsharke (Jan 2, 2009)

cliff said:


> In my experience red light preserves night vision fairly well, and the dimmer it is the better.
> 
> As far as tracking blood goes, last year I tried this out in the pitch dark on some fresh gore from a deer kill with both a blue LED and blue incan with a filter. Neither light had any significant effect on the blood that I could see.
> 
> It appears to me that the blood tracking ability of blue light is a hyped-up feature intended to sell more lights to pessimistic hunters. Their best bet would be to stop shooting earlier in the day and take care of business while the sun is up.



Cliff....you are right on with this thread. A true blood tracking light must have a combination of red and white (red 2-1/2 times white is ideal i.e. 10 red to 4 white) led to really make the blood glow bright red. A UV LED blacklight will actually make the blood appear a lighter shade of brown.


----------



## ConfederateScott (Jan 2, 2009)

I've said it before and I'll say it again, people who claim that blood "stands out" under blue light have never tried it themselves. More than likely they are repeating something they've heard or read and think it's true. But it's not true. It's a modern day wives tale. In fact (yes, from personal experience), blue light makes blood look black. If blood were on white paper or snow it would make it stand out simply from contrast. But on the leaves, grass, soil, etc. having black blood isn't an advantage over red. It's a disadvantage. A good 'ol incandescent light like a 6P is the best blood tracking light I've tried. The best use I've found for my Inova with blue LED's is to follow a trail of flourescent orange flagging tape to my deer stand early in the morning before daylight. The orange tape lights up under the blue light.


----------



## Guy's Dropper (Jan 2, 2009)

I have a small keychain light with a blue led that I've had for a number of years. It is probably just this particular light, but it seems to make the most shades red, orange or green colors really stand out. If I didn't know any better, I'd sat they were fluorescing. I have a larger UV light that has a similar effect on dust and the same colors as the blue light.


----------



## darknessemitter (Jan 2, 2009)

Guy's Dropper said:


> I have a small keychain light with a blue led that I've had for a number of years. It is probably just this particular light, but it seems to make the most shades red, orange or green colors really stand out. If I didn't know any better, I'd sat they were fluorescing. I have a larger UV light that has a similar effect on dust and the same colors as the blue light.


 
A lot of things do fluoresce under blue light, including a lot of commonly used inks and pigments.


----------



## chiphead (Jan 2, 2009)

StarHalo said:


> Blue light = special
> Red light = Roxanne
> Dashboard light = Paradise
> 
> etc. :nana:


Yea!


----------



## Hitthespot (Jan 2, 2009)

I have a couple of blue lights, the poor things got that way from not using them too much. :nana:

Bill


----------



## LightScene (Jan 2, 2009)

These color threads always wander into the night-vision realm. Astronomers find that low level white light works best for being able to see what you are doing and still retain night vision. 2xTrinity has already pointed this out, and I just wanted to back him up.

You really need a variable intensity light to get to the perfect setting for your own eyes, depending on how much ambient light there is.


----------



## applevision (Jan 3, 2009)

Not to be a pain in the neck, but I'm really interested in this question in general. My sense is that we can summarize what the purpose of blue light is:

1. _May_ be helpful for maintaining night vision (I'm still not totally clear on this and I wish there were a really definitive way to put this to rest)

2. Good for Seasonal Affective Disorder: although we know that this must be used with the proper brightness, in the peripheral vision, at specific times of day for this to actually matter in this regard; there is very good science to back that up.

3. _May_ have a "calming effect"

What am I missing?

Things that it seems it is NOT good for:

1. NOT good for finding blood (I think the folks who argued against this in this post were very convincing, esp. those with first-hand experience and testing).

What else?

It will be nice to have a summary for future reference so we can refer to it! Let's make a CPF Consensus Statement! :naughty:


----------



## defloyd77 (Jan 3, 2009)

My 2 cent summary: bad for night vision (compared to red at a given level), bad for blood by itself, good for blood along with red light, good for dusting, good for finding things that fluoresce, good for waking you up and possible treatment for SAD and bad attitudes.


----------



## darknessemitter (Jan 3, 2009)

applevision said:


> Not to be a pain in the neck, but I'm really interested in this question in general. My sense is that we can summarize what the purpose of blue light is:
> 
> 1. _May_ be helpful for maintaining night vision (I'm still not totally clear on this and I wish there were a really definitive way to put this to rest)


 
Wait, what? Where was that in the thread? Blue does not have any special properties for maintaining scotoscopic nightvision, that's what red is for, because the rod cells aren't bleached out by it, I thought we've already been over this in this thread? :shrug:


----------



## petersmith6 (Jan 3, 2009)

i have a Blue led for night Diving,blue is the last colour to be absorbed by water.As for night night work i prefer blue as its closer to moon light but have been playing with Green LEDs ,Verey low powerd green leds can iluminate a hell of a lot.


----------



## iapyx (Jun 21, 2009)

I have a question for which I don't want to start a new thread, so I try this existing thread. It's in the same area:

Can anybody tell me why Surefire doesn't make (as far as I know) yellow filters? The idea is that yellow light cuts through fog easily. At gas stations one can buy yellow sunglasses for when it's foggy outside. Incans cut through fog better too. (light frequency) 
The question is: if I had a yellow filter for my U2A, would that improve my sight at foggy areas. Or maybe not? 

Do blue or red filters give better results than yellow filters?

thanks and sorry if this question has already been discussed (couldn't find it).


----------



## Scott Packard (Jun 21, 2009)

Many years ago I played with yellow vs. white trying to "cut" through fog so thick that I could only see 1-3 road dots ahead. The power of light in watts makes a difference in how far down the road the light gets; with the amber fog lights less of it seems to blast back into your eyes. Also, if you have a handheld spotlight you get a lot more light coming back into your eyes than if you have bumper-mounted lights.


----------



## darknessemitter (Jun 23, 2009)

iapyx said:


> I have a question for which I don't want to start a new thread, so I try this existing thread. It's in the same area:
> 
> Can anybody tell me why Surefire doesn't make (as far as I know) yellow filters? The idea is that yellow light cuts through fog easily. At gas stations one can buy yellow sunglasses for when it's foggy outside. Incans cut through fog better too. (light frequency)
> The question is: if I had a yellow filter for my U2A, would that improve my sight at foggy areas. Or maybe not?
> ...


 
I think some testing determined that yellow-filtered light doesn't necessarily cut through fog much better than white, but yellow filters reduce some of the blue wavelenghts, making the remaining glare a bit easier on human eyes. It might also make it easier for some people's eyes to focus, but this can vary depending on if someone is near-sighted or far-sighted.


----------



## darknessemitter (Jun 23, 2009)

Scott Packard said:


> Many years ago I played with yellow vs. white trying to "cut" through fog so thick that I could only see 1-3 road dots ahead. The power of light in watts makes a difference in how far down the road the light gets; with the amber fog lights less of it seems to blast back into your eyes. Also, if you have a handheld spotlight you get a lot more light coming back into your eyes than if you have bumper-mounted lights.


 
Were you switching between your normal level white headlights and low-mounted fog lights? One thing to keep in mind in addition to the colored filters is that fog lights (but not some other types of off road lights) are focused differently, usually wide horizontally but very narrow vertically, so it creates kind of "sheet" of light low along the road. Sometimes that can make a bigger difference than the tint.


----------



## chiphead (Jun 23, 2009)

Since we're talking about blue, what about Cyan? I saw a module with a Cree Cyan led, I though it would be great in a new Minimag/AA. But I couldn't get my mine around the cyan.

chiphead


----------



## angelofwar (Jun 23, 2009)

iapyx said:


> I have a question for which I don't want to start a new thread, so I try this existing thread. It's in the same area:
> 
> Can anybody tell me why Surefire doesn't make (as far as I know) yellow filters? The idea is that yellow light cuts through fog easily. At gas stations one can buy yellow sunglasses for when it's foggy outside. Incans cut through fog better too. (light frequency)
> The question is: if I had a yellow filter for my U2A, would that improve my sight at foggy areas. Or maybe not?
> ...


 
They do make "amber" filters for there Hell Fighters...I guess they (rightly, IMHO) figured, as do the auto manufacturers, that this "yellow light to cut through fog" is really only useful in vehicle applications??? I know that fulton included amber filters with they're grey military lights for use aboard navy/coast gaurd vessels (heavy fog conditions on deck?)


----------



## 2xTrinity (Jun 23, 2009)

darknessemitter said:


> Were you switching between your normal level white headlights and low-mounted fog lights? One thing to keep in mind in addition to the colored filters is that fog lights (but not some other types of off road lights) are focused differently, usually wide horizontally but very narrow vertically, so it creates kind of "sheet" of light low along the road. Sometimes that can make a bigger difference than the tint.


The beam angle is the most important by far. Color actually has a negligible effect on beam penetration in my experience.

For one, there's a myth that yellow light will actually penetrate further through fog. This is due to incorrectly attributing Rayleigh Scattering (preferential scattering of short wavlength light off of particles much smaller than the wavelength of light -- responsible for making the sky blue) to scattering in fog, which consists of water droplets that are larger than the wavelength of light, and therefore reflect all colors approximately equally.

Beam pattern is more important, especailly becase spherical droplets are strong retro-reflectors. If you have a light source that is aimed along the same plane as your eyes (eg, a flashlight at close to eye level), a huge amount of light will be scattered back into your eyes. However, if the light source is moved low to the ground, and all the rays out of the light source stay for the most part out of the plane of your eyes, 
(as in fog lights which are low, and spread light horizontally) the light will not tend to reflect off the fog back into your face. 

Spill is also a huge problem. This is a bigger reason why LED lights do poorly in fog than most incandescent lamps -- most LEDs that don't use an apsheric lens or TIR optic distribute a huge percentage of their output as "spill". Almost none of this light actually reaches your target, and most of it will scatter off of fog, hindering your visoin.

What I suspect is the biggest reasion warm colored light appears "better" outdoors is that objects, such as grass, soil, animals etc. they tend to be shades of brown or green, and tend to NOT reflect blue very much. This means a heavily blue tinted light will tend to reflect proportionally less from your target than green and red, but reflect the SAME proportionally from the fog. So your useful contrast of your object against the lit up fog will be lower. But all the colors _penetrate_ the fog to the same extent.

A quick sanity check here is that if you look at a bright light source way off in the distance through haze it will NOT typically appear to be strongly "yellow-shifted" at all, just faint or fuzzy.


----------



## defloyd77 (Jun 24, 2009)

2xTrinity said:


> The beam angle is the most important by far. Color actually has a negligible effect on beam penetration in my experience.
> 
> For one, there's a myth that yellow light will actually penetrate further through fog. This is due to incorrectly attributing Rayleigh Scattering (preferential scattering of short wavlength light off of particles much smaller than the wavelength of light -- responsible for making the sky blue) to scattering in fog, which consists of water droplets that are larger than the wavelength of light, and therefore reflect all colors approximately equally.
> 
> ...



+1 to everything said here.

Strangely enough, I've noticed when it's foggy, my local airport's "spinning lights" the ones that rotate and have a white light on one side and filtered blue on the other, the blue one can be seen much further out than the white. I don't think the blue one's brighter, but I could be mistaken.


----------



## iapyx (Jun 24, 2009)

darknessemitter said:


> I think some testing determined that yellow-filtered light doesn't necessarily cut through fog much better than white, but yellow filters reduce some of the blue wavelenghts, making the remaining glare a bit easier on human eyes. It might also make it easier for some people's eyes to focus, but this can vary depending on if someone is near-sighted or far-sighted.


 
hm, I mentioned yellow since incans (more yellow) are known to cut through fog better than LEDs. 

Many years ago I went skiing every year and I borrowed yellow sunglasses from a friend. My sight improved enormously whether foggy or not. 

As someone else mentioned: blue light being better visible in fog than other colours doesn't say anything about one's sight improving when using a coloured light. 

I don't mean to sound critical. All your help is very welcome and very much appreciated. In the end I would like to buy a filter if it improves my sight during fog. 

thanks.


----------

