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It is commonly accepted as a fact that red light doesn’t damage your night vision as much as white light. In practical application this may be a little less straightforward than once thought.
Here’s a short summary to the best of my knowledge about which lights you should use while trying to preserve your night vision:
- When looking at dim pinpoints of light use a fully-dimmable red light – or white if you prefer.
- When spotting silhouettes in the dark use a fully-dimmable white light
- When reading use a fully-dimmable white light, though red will do
- When reading color charts use a fully-dimmable white light
Why Do You Care?
The real question is, what are you planning to do (both with the light and in the dark)? If you’re developing photos you’ll need different conditions than if you plan to go stargazing. The Navy did quite a number of studies on dark adaptation in the mid-eighties. It is my impression they were trying to find a solution that suited the officer of the deck – the guy on the periscope – as well as the other staff in the compartment who have varied duties which don’t require dark adaptation.
A quote from the Naval study on lighting choices in submarines at night:
Red light has been used because it is well established that subsequent dark adaptation is faster than with any other color. However, the magnitude of this advantage depends on the intensity of the adapting light. … The measure that has typically been used to show this phenomenon is the time required to detect pinpoints of light at absolute scotopic threshold–that is, the individual’s maximum sensitivity. Although this is undoubtedly of great importance at times, it is probably true that in most practical situations the observer is required to operate at something less than maximum sensitivity and to detect targets quite different from spots of light.
The Navy also considered the additional eye-fatigue caused by red lighting and low-level lighting, as well limitations such as the inability to distinguish color under red lighting.
Dark adaptation is faster after exposure to red light, but not by much. Is it enough to be worthwhile? Probably not, but maybe, depending.
The Naval studies found that the benefit gained by using red lights is entirely lost when people were asked to detect silhouettes of ships against a simulated sky. This is at least partially because you don’t need to be fully dark-adapted to see a silhouette. They found that using a very dim white light was just as good.
In order to detect the dimmest possible point of light though, you do need to be fully dark-adapted. You must admit, stargazing is exactly that: trying to see pinpoints of light. So there is a benefit to using a red – but only if you’re going stargazing in an extremely dark place. If you’re stargazing from within a city, you will likely never be somewhere that dark.
Benefits and Caveats
The other major benefit to using a red light is that usually you (or someone else) has put a red filter on that light. This in an of itself dims the light down. You always need to be using the dimmest possible light to keep your night vision – and this applies to red as well as white (or blue or green or purple). If you’re using a superbright red LED flashlight, unless you need that much light, you’re probably losing more night vision than necessary.
Also – that red color that is beneficial to perfect night vision is that rich ruby-red, not so much an orangey-red or pinky-red (those are highly technical terms).
Keeping whatever light you have as dim as possible is the most important.
Until you can find me a fully-dimmable white flashlight (one that dims smoothly all the way from full-bright to off), I’ll keep using my dying-battery red keychain LED, and single-AA tiny filtered incandescent flashlight. I’ll also probably only use them when I’m digging in my telescope bag or checking the site of my telescope set up to make sure I haven’t left anything behind. Don’t forget – humans have pretty good night vision – use it.
Lastly, assist your fellow stargazers, change any outdoor fixture you can to one that is approved for reducing glare and light pollution by the International Dark-Sky Association.
Where’d I Get My Info?
St. Louis Places – Night Vision is a website that has a thorough and interesting discussion of red light and night vision . He cites good sources, but it is not itself a primary source.
Lurta, S.M. , S. M., and D. A. Kobus. Immediate Visibility after Red and White Adaptation . Submarine Base, Groton, Conn.: Naval Submarine Medical Research Laboratory, 1985. (A Naval study of light adaptation for use in submarines – primary source)
Thanks to Steph Zimmerman for the help in searching out real sources of information on this.
365 Days of Astronomy  is awesome – and they’re hosting this as a podcast. Check it out!
~ A l i c e !