Many readers know that during World War II, military aircraft used red internal lighting. Pilots used red flashlights to view maps while in flight. The reason for using red is that if ordinary full-spectrum (white) light had been used, this would spoil the “night vision” (vision using rods instead of cones). Red light does not harm night vision as much as white light. This blog article talks about using night lights around the house that are red instead of white.
Part of the background for this blog article is a reminder that the normal human retina contains two kinds of photoreceptors — cones and rods. The cones perceive color. The rods only see in gray scale. There is a good reason to have both kinds of photoreceptors. It turns out that for a cone to do its job, it needs to be receiving a lot of light. In contrast, the rods are able to do their job with very dim lighting.
If you are in a very dark setting (at night with no artificial light, and with little or no moonlight), and if you allow your eyes to adjust to the dark, you will notice that you are not seeing any color. You are seeing only shades of gray. That means your rods are doing your seeing, and the cones are taking a rest.
If you are in one of these dark settings, and if you allow a bright light to shine into your eyes, what you will notice is that your ability to see well in that dark setting has been ruined. The bright light has overwhelmed your rods. The words that we use to describe this is that the bright light has (fortunately, only temporarily) spoiled your “night vision”. Everyday experience tells us that it can take as long as ten minutes for one’s night vision to return. Another way to say this is that if your rods have been overwhelmed by a bright light, it takes your rods as long as ten minutes to recover.
Having set this background, we can now remind ourselves of what we learned in physics class, which is that the amount of energy contained in a particular photon is determined by the color of the photon. The equation sets forth the relationship. In this equation ν is the Greek letter “nu” which is the frequency of of a particular photon, h is Planck’s constant, and E is the energy contained in that photon. The spectrum of light perceptible to humans runs from red to violet, with red being the lowest frequency in visible light, and violet being the highest frequency in visible light.
We can determine through our own experimentation that if our eyes are adjusted to the dark, and if we are then exposed to red light, this will spoil our night vision much less than white light would. To the extent that exposure to red light does spoil our night vision, the recovery from the exposure is faster. It might only take three or four minutes, instead of ten minutes, for our rods to recover if the exposure was to red light instead of white light.
This now brings us around to the main point of today’s blog article. I recently swapped out the white night lights in my house for red night lights. Now if I am padding around the house at night, the night lights that I encounter are red lights. I did this with a goal of reducing the extent to which the night lights spoil my night vision.
If you click around on the internet, you will find that in recent times it has become possible to get red night lights. The nice thing about the red night lights that you can get these days is that they use red LEDs. This is a lot better than the legacy approach which was to put a red filter in front of an ordinary incandescent bulb. The red-filter approach does not filter out all of the photons that have higher energies than red. But a night light that uses red LEDs simply does not emit photons of higher energies than red. (And it is much more energy-efficient than older night lights.)
The red night lights that I recently placed into service (see photo above) use only about a half a watt each when they are lit. (Amazon link.) Each night light emits about 30 lumens of light. The night light has an ambient-light sensor so that it turns itself off during the day. A typical annual operating cost for one such night light might be 25 cents.
My guess is that I have better night vision nowadays when I am padding around the house at night.