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A number of companies have started marketing LED lamps that can be switched to a red mode. The claim is that red light is less harmful to one's night vision. Given that our eyes are less sensitive to red light, though, I'm not convinced that that red light is any better than dim white light. So if "equivalent" luminosity is defined where it's equally easy to, say, read a book in both light types, is there some physiological reason that red light is better than the equivalent white light?

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5 Answers 5

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This is a very good question. Red light is routinely used by scientific laboratories to do low light dissections of retinas, and of course it is used in other low light contexts such as printing plate development.

In both of the above contexts, you have a clear subject: the retina being dissected or the printing plate being developed. In the case of the printing plate the film has been designed to be specifically non-reactive to red light, so red light is used because your eyes can see it, but the film doesn't react to it. Similarly in some scientific settings it makes sense to use red light during dissections. Mice lack a long wavelength opsin, and therefore using a dim red light allows the experimenter to have a relative sight advantage compared to the mouse when keeping the mouse dark adapted.

But in the case you're asking about, there is no film or animal to serve as a second party. So is there any intrinsic advantage to using red light? As it turns out, there is. The fovea, which is in the center of our eye and used for high acuity vision, has no rods and primarily L- or red sensitive cones. Note the high density center area which lacks blue sensitive cones and has 2:1 red to green cones.

retinal mosaic

So by having red light present, you stimulate this area. But red light is present in white light, too, why not just use that? Leonardo's answer comes the closest, but it's a little off. Red light is used because it preferentially stimulates L cones more than rods, but you are definitely not able to preserve night vision by using red light. Why not? Well it may look like it is possible to exclusively stimulate cones from the chromatic sensitivity figure

chromatic sensitivity

But that figure is 1) normalized and 2) not indicative of synaptic signal processing. 1000's of rods can converge onto a single ganglion cell, where cone convergence in the fovea can be on the order of a single cone per ganglion cell. When it comes to perception, in order to compare the black rod line above with the red L-cone line you'd have to magnify it dramatically in size. Practically speaking, it is nearly impossible to stimulate cone pathways without stimulating rod pathways when using a relatively broad spectrum LED that you're powering with a battery. Maybe with a high power infrared laser.

So the purpose of using red light is to attempt to balance the activation of high sensitivity (red insensitive) rods with that of the low sensitivity (but red sensitive) cones in the fovea. While using a similar level of rod activation with blue light, you would perceive a "blind spot" where your fovea is.

Finally, instrinsically photosensitive cells (the melanopsin cells brought up) do not factor into this processing. These cells are activated only with extraordinarily bright levels of light, and the therefore do not enter into conversations dealing with night vision.

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How can I see blue things by the center of my eye? –  Anixx Sep 8 '12 at 0:26
    
You're essentially a dichromat in the center of the eye, so you are blue-yellow color blind there. That means you can distinguish blue from any color, except yellow, over a very small region. You can find a more thorough discussion of the phenomenon by George Wald here –  Salain Sep 8 '12 at 2:39
    
No, I can distinguish blue from yellow well in the center of my sight. –  Anixx Sep 8 '12 at 7:51
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Occam's razor be damned, this must be you. –  Salain Sep 8 '12 at 14:42
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@Anixx, do you have color blindness at the edge of your vision? (Answer, yes, but your brain fills it in) –  Shep Sep 9 '12 at 19:38

Yes red light is useful IF and only if >650 nm. Some of the answers above are close, but they miss the issue because they used the same scale for rods and cones which they are not on. Rods are several orders of magnitude more sensitive than cones and hence why the graphs above make it look like they have no sensitivity out to 700nm, they actually do, just not nearly as much.

Look up photochromatic step. It explains why >650nm works and it is well documented.

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I looked up photochromatic step, but I'm not sure what I'm looking for, do you have some link to whatever you're talking about? –  Shep Jul 16 '13 at 0:41

Another reason why red lights are now sponsorized for night illumination is because they are supposed to be safer in terms of interference on the circadian cycle. This is not related to better vision, but better health.

The mammalian eye senses the light by the conventional rode and cone cells. However, a third light-sensing cell type has been recently identified. This third light-sensor is based on melanopsin-positive cells. While rod and cone cells respond best to white, full spectrum light, melanopsin cells only respond to a specific bandwidth of blue light, in the range of 446-477 nanometers. These cells connect and regulate brain centers responsible for circadian rhythms. Therefore, during the night, blue-light exposure might interfere with circadian rhytms facilitating the onset of depression and other metabolic derangements associated with circadian cycle see another SE question. White light contains also blue light, while red light does not. This is apparently the rationale to claim that red light is less harmful. However, no clinical evidence is available to my knowledge, and the threshold of blue light required to stimulate melanopsin receptors is probably over the common night illumination.

References:

  1. Hattar S, Lucas RJ, Mrosovsky N, Thompson S, Douglas RH, Hankins MW, Lem J, Biel M, Hofmann F, Foster RG, et al.. 2003. Melanopsin and rod-cone photoreceptive systems account for all major accessory visual functions in mice. Nature 424: 76–81.
  2. Holzman DC. 2010. What’s in a color? The unique human health effect of blue light. Environmental health perspectives 118: A22–7.
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Very interesting, so a white light may mess up my sleep patterns? So is the answer to the night-vision question "no", "maybe", or "inconclusive"? –  Shep May 3 '12 at 12:27
    
As I understand it, I would summarise this as "probably - maybe not your sleep pattern but your sleep quality will probably be affected". –  Armatus May 3 '12 at 12:43
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@Armatus, I agree that it will probably affect my sleep patterns, but this answer says nothing about night-vision, that's what I'm curious about. –  Shep May 3 '12 at 12:49
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Thinking about it, since our eyes are closed - wouldn't any light become red-ish anyway because it would go through our (blood-filled) lids? –  Armatus May 3 '12 at 12:54
    
@Shep 1) sorry I'm not aware about consequences on night-vision, are companies selling red lamps claiming improvements on night-vision? 2) Red light is the only one passing through our tissues, but lamps are used after sunset when you are still awake. –  Gianpaolo R May 3 '12 at 14:07

As mentioned in the other answer, red light does not affect the melanopsin receptors in eyes. These receptors not only regulate body rhythm but also regulate the diameters of the eye pupils. Once they are stimulated, the eye thinks that there is enough light around and the pupils get smaller to protect the eye against over-exposition, which reduces sensitivity.

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any references for for melanopsin--pupils link? –  Shep May 7 '12 at 15:06
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In the linked article: "In the mammalian retina, besides the conventional rod-cone system, a melanopsin-associated photoreceptive system exists that conveys photic information for accessory visual functions such as pupillary light reflex and circadian photo-entrainment. " –  Anixx May 7 '12 at 20:33
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There are multiple ways that pupils can be dilated, and this one only comes into play with very very bright light. –  Salain Sep 7 '12 at 15:53

Here is a comparison of the range of wavelength sensitivities for both rod cells (labelled R) to the 3 subtypes of cones cells (labelled S, M and L) from Wikipedia.

Cone and rod wavelength sensitivities.

If one is exposed to red light (above ~650 nm), it would activate the L-type cones mainly (possibly some M-type activation), but no rod activation. Rods are the low light receptor cells in our eyes, and as such, are very sensitive to the photon density, or brightness, entering the eye.

This is just my speculation, but I think it's plausible that if you were in a completely dark environment with just a red light, filtering out the higher frequencies, night vision could be spared in the sense that we don't activate the rod cells.

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so the rods and cones adjust to brightness independently? If so, you may have the answer. –  Shep May 5 '12 at 18:52
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There will certainly be a response by each cell (if they are sensitive to the wavelengths of light), and sensitivity adjustments on a network level. It stands to reason that if you only activate using a frequency the L cones can be activated by, then it would work. –  leonardo May 5 '12 at 18:57
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One such cell-specific mechanism, at least, is the direct result of active rhodopsin depletion: when it reacts to light, the rhodopsin pigment photobleaches and must be recycled to reactivate it. This recycling is quite slow in rod cells; Wikipedia says it takes about half an hour for the rhodopsin level to fully regenerate after exposure to bright light. Thus, to retain night vision, it would certainly seem desirable to avoid exposure to light at frequencies that can trigger rhodopsin transformation. –  Ilmari Karonen May 10 '12 at 12:24
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It doesn't quite work like this because the chart above shows the relative chromatic sensitivity, which is very different from the luminal sensitivity. That means that you can't compare the height of the Rod curve to that of the L-type cone because they've been normalized to peaks with very different sensitivities. –  Salain Sep 7 '12 at 15:50
    
A portion of this post has been incorporated into During night flying, which color lens for a flashlight is most beneficial? on Aviation. –  Michael Kjörling Dec 16 at 8:42

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