Is using red light for "preserving night vision" simply a myth?

Ever since I was a boy all the telescope/astronomy magazines would pitch flashlights with red light claiming that "red light helps preserve human's night vision and keeps your pupils dilated so you can still see well in low light when you turn them off". Up 'til now I've always accepted that but I've never seen any scientific evidence for this other than marketing, testimonials, and various anecdotal stories.

For all I know the real reason red lights work at preserving human's ability to see in low light (by keeping the pupils from contracting) is simply because when you put a color gel or filter over a white light you reduce its output greatly and this reduced output is really what preserves one's night vision; the color has nothing to do with it!

I've been experimenting with attaching my own DIY red filters to some flashlights but I'm wondering if this is really in vain since they have a variable output from dark to light capability in the first place and I simply should be using that instead. Hmm...

Does anyone have any links to scientific articles, ideally in scholarly journals, that have tested if red light truly helps preserve human's night vision as opposed to simply dimmed light or using other colors? Testimonials, anecdotal stories, and marketer's pitch stories I can find thousands of examples of; that's not what I'm looking for.

Thanks.

mzil said:

Ever since I was a boy all the telescope/astronomy magazines would pitch flashlights with red light claiming that "red light helps preserve human's night vision and keeps your pupils dilated so you can still see well in low light when you turn them off". Up 'til now I've always accepted that but I've never seen any scientific evidence for this other than marketing, testimonials, and various anecdotal stories.

For all I know the real reason red lights work at preserving human's ability to see in low light (by keeping the pupils from contracting) is simply because when you put a color gel or filter over a white light you reduce its output greatly and this reduced output is really what preserves one's night vision; the color has nothing to do with it!

I've been experimenting with attaching my own DIY red filters to some flashlights but I'm wondering if this is really in vain since they have a variable output from dark to light capability in the first place and I simply should be using that instead. Hmm...

Does anyone have any links to scientific articles, ideally in scholarly journals, that have tested if red light truly helps preserve human's night vision as opposed to simply dimmed light or using other colors? Testimonials, anecdotal stories, and marketer's pitch stories I can find thousands of examples of; that's not what I'm looking for.

Thanks.

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Interesting question! I've always accepted it at face value. Here's an article on Flashlight Reviews with a little backround on physiology: http://www.flashlightreviews.com/qa/nightvision.htm

The show Mythbusters actually did a piece on night vision. They didn't really touch on using red light but they explained the night vision concept.

The light receptors in your eyes gets desensitized when it's exposed to light, and the brighter the light, the more desensitized it gets. That's why when you go from a lighted room to a dark room, it takes a while for the receptors to calm down and become more sensitive. With that being said, I would think your theory with using a red filter and it reducing the brightness of the light is valid. But to add to that, I think its because red has the lowest frequency in the visible spectrum and thus the has the least energy. So therefore, it is the color that's least likely to over-stimulate the receptors and will help preserve your night vision. But that doesn't mean it preserves it completely. Many times, I've been camping or night hiking and when i use a red light for a prolonged period of time, I do lose a little bit of night vision, but it comes back to me much quicker than if I used a full spectrum light.

sween1911 said:

Interesting question! I've always accepted it at face value. Here's an article on Flashlight Reviews with a little backround on physiology: http://www.flashlightreviews.com/qa/nightvision.htm

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good link, just make sure the red light is around 1 lumen

badtziscool said:

But to add to that, I think its because red has the lowest frequency in the visible spectrum and thus the has the least energy. So therefore, it is the color that's least likely to over-stimulate the receptors and will help preserve your night vision. But that doesn't mean it preserves it completely. Many times, I've been camping or night hiking and when i use a red light for a prolonged period of time, I do lose a little bit of night vision, but it comes back to me much quicker than if I used a full spectrum light.

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The problem with the whole energy idea is that your eyes are very insensitive to red light. A mW of green light will easily be 10x brighter to your eyes than a mW of red light (even though they are exactly the same energy). So by this logic, green should be the best for night vision as it is the lowest energy. However, I don't think this works. There would have to be another reason for red to preserve NV if this were true, and I don't think "it's dimmer" is a valid reason as then you could just use a dim white light.

EDIT: I just read that link, it explains it very well.

Does anyone have any links to scientific articles, ideally in scholarly journals, that have tested if red light truly helps preserve human's night vision as opposed to simply dimmed light or using other colors? Testimonials, anecdotal stories, and marketer's pitch stories I can find thousands of examples of; that's not what I'm looking for.

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Suppose you find that scholarly journals claim red light is a myth. Are you going to believe them blindly and ignore all of your personal experience, assuming that it is automatically right because "scientists" said so? What if you find two articles that contradict each other?

Buy a red and white keychain light, go into a dark room, and test it yourself. If it works or doesnt work for you, go by that. It is pathetic to ask "scientists" if something looks a certain way to your eye.

FWIW, I keep a red photon freedom next to my bed because regular white lights, even on low mode, shock my dark-adapted eyes.

There was an excellent article on how night vision works in an issue of Astronomy magazine way back in the 70's. (Don't ask me for the volume and issue number. The best I can do is that it was probably around 1976 give or take a year.)

The numbers quoted were interesting. Going from a very bright room to a very dark one will cause your pupils to expand fairly quickly, but the increase in light gathering from pupil dilation is only about 10x.

The real increase comes with changes to the sensitivity of the retina. Over a period of at least an hour, the retina's sensitivity can change by up to 100,000X. For a total change in light sensitivity of ~1,000,000X.

Many things can affect how sensitive the retina can become including age, past or present eye deseases, whether you smoke, what drugs are in your system, etc.

Also, if you've spent the day exposed to very bright light, it will take much longer for your eyes to reach their full sensitivity and will limit how sensitive they can become on a given night. For example, a day spent out in the sunlight at a beach without sunglasses might limit your eyes from becoming fully dark adapted for several days. Some astronomers would wear red goggles and stay indoors for the entire day(or two) before an important night of observing.

So, the issue with red light hasn't got anything to do with pupil dilation, it's about retinal sensitivity.

Not a scientific, quantitative answer but I can tell you from actual experience that a relatively high output red damages my night vision more than a low output YG.

mzil said:

Ever since I was a boy all the telescope/astronomy magazines would pitch flashlights with red light claiming that "red light helps preserve human's night vision and keeps your pupils dilated so you can still see well in low light when you turn them off". Up 'til now I've always accepted that but I've never seen any scientific evidence for this other than marketing, testimonials, and various anecdotal stories.

For all I know the real reason red lights work at preserving human's ability to see in low light (by keeping the pupils from contracting) is simply because when you put a color gel or filter over a white light you reduce its output greatly and this reduced output is really what preserves one's night vision; the color has nothing to do with it!

I've been experimenting with attaching my own DIY red filters to some flashlights but I'm wondering if this is really in vain since they have a variable output from dark to light capability in the first place and I simply should be using that instead. Hmm...

Does anyone have any links to scientific articles, ideally in scholarly journals, that have tested if red light truly helps preserve human's night vision as opposed to simply dimmed light or using other colors? Testimonials, anecdotal stories, and marketer's pitch stories I can find thousands of examples of; that's not what I'm looking for.

Thanks.

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Red light is preferable over white (or other colors), but the explanation given is wrong. I give references; you can google them, but I don't know if you can get the original articles. I can, being on a university network. Libraries should be able to get them, though.

The standard explanation already given is wrong, since both cones and rods react the same above wavelengths of 650 nm or so (red), but rods are much more sensitive at lower wavelengths, thus their night vision gets killed [S. Hecht and Y. Hsia, J. opt. Soc. Am. 35 (1945)]. Thus, for the same 'brightness experience' (for lack of a better understandable term) of red and white light, the eyes will recover more quickly if red light is used [E. O. Hulburt, 'Time of Dark Adaptation after Stimulation by Various Brightnesses and Colors', J. opt. Soc. Am. 41 (1951)]. It's not that red will protect night vision, but destroys it much less than white.

However, red light loses its advantage with lower brightness (and I'm talking really low brightness here). If you don't need much brightness, you can get away with low intensity white [S.M. Luria and David A. Kobus, 'The Relative Effectiveness of Red and White Light for Subsequent Dark-Adaptation', Naval Submarine Medical Research Laboratory, Report 1036 (1984)]. White has the advantage of giving you sensitivity of a full color spectrum over any monochromatic source. It essentially makes no sense to restrict your eyes to the sensitivity of just one color, since you will need much higher brightness with the one color to get the same 'brightness experience' as with white.

As always, YMMV. The above articles assume 'normal' 'average' vision. If you're color-blind, for instance, your cones and rods react differently anyway, and it may not work for you.

Chucula said:

Suppose you find that scholarly journals claim red light is a myth. Are you going to believe them blindly and ignore all of your personal experience, assuming that it is automatically right because "scientists" said so? What if you find two articles that contradict each other?

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Ever heard of the placebo effect? Sorry, I trust "scientists" over any of the pathological science a single person will generate lol...

After doing some more reading, it's important to note differences in procedure. The Submarine Report I referenced, for instance, assumes that most people in the submarine just need to work their equipment, while a few need to prepare themselves for night vision. None of them is truly dark adapted, but they want to reduce the time required for getting dark adapted. The authors of the report conclude that for this case, low white is essentially as good as red, without the annoying problems of red.

But for most of us, the problem is different as follows. Your eyes are already dark adapted, and you just need a quick flash of light to locate a piece of equipment or operate the telescope, if you're doing astronomy observations, for instance. From my understanding of the curves, for this case it's much safer to stick with red, as low intensity as possible, to reduce the damages to night-vision as much as possible. IMHO.

Red is still the standard for use for light discipline in the military. A red filter cuts the light output. It REALLY cuts the output of a LED light vs a incan in my experience. I had a lens break in a 5 mode cr123 flashlight and I trimmed a red filter from my minimag kit and it made a huge difference in light output.

Chucula said:

Suppose you find that scholarly journals claim red light is a myth. Are you going to believe them blindly and ignore all of your personal experience, assuming that it is automatically right because "scientists" said so?

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The short answer: YES!

I can't speak for everyone in this forum, but me, myself, I'm a human being, subject to the placebo effect, observer bias, and any number of other foibles.

Remember, always trust your eyes. They never lie:


If the image appears to be in motion, then it must be.
---

My understanding of night vision has little to do with with rods and cones, but rather that once your pupils dilate to 7mm* or so after 5-20 minutes in the dark, some colors of light supposedly re-constrict them less than others.

*[There is actually a very large variance, but that's the average, hence the most common pair of hand-held binoculars for astronomy are 7x50, because their exit pupil, the objective diameter in mm. divided by the magnification, matches the average human's night adapted eye's pupil diameter.]

I'm going in with some of the previous posts. If you live your life relying on scientific articles and not actual life experiences, then you will live a very interesting life indeed. Science, like statistics, can be used to 'prove' almost anything. Experience, now, tells us that people like sailors and soldiers and yes astronomers all use weak red light to preserve their nightsight, and I have too. It works. It works better than other colour lights but they also work to some extent. Better you set out to test that and then to prove why (or why not) than to rely on already published articles. But if you do want to do the authorities thing, I can point you to, for instance, about 10,000 food supplements that were once 'proven' to do something they didn't. And to more than a few that actually made things worse.

Mr Bigglow said:

I'm going in with some of the previous posts. If you live your life relying on scientific articles and not actual life experiences, then you will live a very interesting life indeed. Science, like statistics, can be used to 'prove' almost anything.

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Absolutely 110% completely wrong. The way science is presented can be misleading, Eg "Tasers have been know to cause death" would suggest that tasers are deadly in general. However, if you actually lived on scientific information and didn't take things at face value, you would realize the full statement is something like ".0001% of people tased died" (just as an example). Science has no agenda, and true factual information can not be used to prove anything false, except by human error. The scientific method prevents such a thing from happening. Personal experience is irrelevant next to scientific research.

Its not that hard to get


http://en.wikipedia.org/wiki/Scotopic_vision

http://www.instrumentationguide.com/article/photopic_sctopic.htm

thats why you want red to keep your visoin night adapted

Mr Bigglow said:

I'm going in with some of the previous posts. If you live your life relying on scientific articles and not actual life experiences, then you will live a very interesting life indeed. Science, like statistics, can be used to 'prove' almost anything. Experience, now, tells us that people like sailors and soldiers and yes astronomers all use weak red light to preserve their nightsight, and I have too. It works. It works better than other colour lights but they also work to some extent. Better you set out to test that and then to prove why (or why not) than to rely on already published articles. But if you do want to do the authorities thing, I can point you to, for instance, about 10,000 food supplements that were once 'proven' to do something they didn't. And to more than a few that actually made things worse.

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And if you ignore science and its peer reviewed publications, you end up getting a lot of woo and BS.

If one person says it works, without controls and procedures, it's anecdotal evidence. It's pretty much useless information.

If ten people say it works under controlled conditions with proper procedures and repeated experiments, it becomes science.

Rods and cones...

Cones are for color.

Rods are for black and white.

Rods are not affected by low red light, cones are.

KenAnderson said:

Rods and cones...

Cones are for color.

Rods are for black and white.

Rods are not affected by low red light, cones are.

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Nope. Above ~650 nm light wavelength (red), both rods and cones are affected pretty much the same.

Low red light DOES inhibit dark adaptation. However, it does it MUCH LESS than white, or other colors. A dark adapted eye that gets low red light will recover faster back to dark adaptation than the same eye exposed to white light of comparable brightness. Even so, at really low brightnesses this advantage is pretty slim. If you need to find your way around, low light of any color is about the same. If you need to read something (i.e., need a little more light than just to detect obstacles), but want to disturb your night vision as little as possible, stick with red.

Here are the relevant graphs from the papers I referenced.

First, the usual way the scotopic (rods) and photopic (cones) sensitivity of the human eye is usually depicted:

That's where the original usage of red for night vision came from. It seems to indicate that the rods don't do much in the red. But it ignores that the scotopic rods are a lot more sensitive than the photopic scone -- essentially, both curves should not go up to one on the vertical axis! This graph is normalized, done for illustration purposes! (The curve is also very smooth, and there are no data points. That's usually a pretty good indicator that this is not a 'real' measured curve, but an illustrative curve. In this case it unfortunately does not illustrate what we are interested in.)

In reality, it looks like this:

As one can see, both curves pretty much overlap above 600 nm. So, red light for night vision is just a myth. Or is it? After all, anecdotal evidence (as provided in this thread) seems to show that red IS better for night vision. Let's test it scientifically.

If one tests how well people can actually see in the dark after exposing their eyes to red or white light, one gets this:

The vertical axis shows how bright a test object needs to be to be seen. The horizontal axis shows how much time elapsed after exposure. Some notes:

1) Both red and white affect night vision negatively (the hump at the left).
2) Red is always better than white -- people can see fainter objects, and the time to recover is shorter.
3) At low brightness (the bottom graph), white and red are very close.
[sidenote: One should note that white exposure is always lower than red. That's been done to take into account that people need a little bit more red brightness to see than with white]

If one takes the time it takes to reach perfect night vision (the flat horizontal part in the graphs above), and plots them against the brightness used to expose the eyes, one gets this:

As one can see, at about 10 footcandles (108 lux) of illumination, white starts to perform a lot worse than red. Below that, it's still better to use red, but not by much. To put things in perspective, 10 ft-cd is about the illumination needed to read (as in: make out the letters on a menu. I personally whip out my EDC in restaurants that use such low light levels to read the menu). And before someone asks, it is not possible to recommend a lumen value for a light from this. As one can always move the light farther away to reduce the illumination level, and the lumen value does not take into account the 'spotness' of the particular light.

Hope that resolves things.