Turn night into day (use bright light), or preserve night vision (use low light)?
Because of the eye’s physiology, it isn’t possible to both use a bright flashlight and preserve your night vision. Our eyes go through chemical changes in response to light, and these changes take time to reverse. It can take as little as several minutes or as long as several days to recover maximum night vision following exposure to bright light. Note that each person’s night vision potential is different, with age, presbyopia, smoking, medications and selection of parents among the many factors that affect how well anybody can potentially see at night.
In very dim (scotopic) conditions, when the cones aren’t contributing to vision, we do not perceive any color or details. We can see objects in black and white only, like on our grandparents’ TV. This is a common experience on starlit or overcast, moonless nights. As mentioned earlier, the eye responds differently to different parts of the wavelength. But it’s even more complicated than that. The cones, which give us our acute and color vision (photopic), are most sensitive at 555nm (yellow-green). This is the reason, by the way, for the newer fire truck and ambulance colors - it’s the color we see most readily during the day - not “fire engine” red. The rods, on which we rely for our night vision, are most sensitive at 505nm (blue-green). One result of this phenomenon is that blue-green light looks brighter at night than red. Unlike the cones, which can see all of what we humans perceive as the visible spectrum, rods essentially cut off all wavelengths greater than about 640nm, the red portion of the visible spectrum. What this means to us is that for all colors other than red, as they are gradually dimmed there comes a point - called the cone threshold - where color is no longer perceived but light still is. At this point only the rods are contributing to vision. With red, however, the cones and rods lose perception simultaneously, and the red light simply winks out.
How can the lightweight backpacker take advantage of this technical gobbledygook?
Most people think they want a flashlight that gives bright, even, white illumination to emulate daylight conditions as best as possible. This approach keeps us in familiar territory perceiving both detail and color after dark. The bright-‘n-white approach, however, is not the most efficient one, either for power management or for making use of our eyes’ natural capabilities.
For example, if you want to see the most detail possible you need to maximize stimulation of the cones because they provide detail along with color. Using yellow-green LEDs in your flashlight accomplishes this, while simultaneously using less current.
Or perhaps you want to use the least amount of light you can get away with for navigating in the dark while, again, using the least power possible. In this case you might want a blue-green light source that maximizes rod sensitivity..
The third approach is to do everything possible to maximize and maintain your night vision potential. This means either using no light at all, or sparingly using a very dim red light. If you must use a light, even briefly, covering one eye will protect that eye’s night sensitivity while sacrificing the other’s to the temporary “flash blindness.” Flashlights sold to astronomers are universally red, despite some thought that green might be more effective. Rigel Systems makes a continuously variable LED flashlight, powered by a 9-volt battery, that’s available in several color variations. The all-red or red/white light is marketed to astronomers, and green versions are sold to folks who use night vision gear. A simple thumbwheel adjusts the light output.
Red or Green-Yellow? The two theories might be summarized like this:
To read a map or star chart you need to use your photopic (cone) vision because your scotopic vision will not allow you to read the details. Because you want to minimize flash blindness, you want to use the least amount of light possible, which means a dimmable yellow-green (555 nm) light source turned just high enough to read by. The downside is that you’re affecting the rods along with the cones at this wavelength.
Because you’re going to experience a limited amount of flash blindness no matter what, you can limit the effect on your cones by using a wavelength (deep red) that your rods can’t see and won’t respond to. Again, turn up the light only as much as needed to read your map. The downside is that the light required to see will be much more intense than if you were using yellow-green. (Editor’s Note: On many non-military maps, important lines are often printed in shades of red and will be invisible under a pure red light).
Astronomy groups generally forbid the use of anything but red lights after dark at star parties, so they’ve cast their vote. If you’re interested in experimenting for yourself, Rigel can make a red/green dimmable light on special order. Note: a green light will not allow you to distinguish the wooded (green) areas on a topographical map.
Daytime Strategies. Extensive research with pilots has ascertained that a day spent in bright sunlight can significantly affect night vision. Studies have shown that ten consecutive days of sunlight exposure cut our nighttime visual acuity, visibility range, and contrast discrimination in half. With enough daily exposures to sunlight, normal rod sensitivity may not be reached at all, something to consider if you’re vying for the job as cabana boy at Club Med, Bora-Bora. Fifteen percent transmission sunglasses with full spectrum (gray) lenses are recommended to best protect night vision. An ironic note to summiteers and alpine fans is that high altitude negatively affects night vision, diminishing it a reported 5% at 3,500 feet, 20% at 10,000 feet and 35% at 13,000 feet. Tying up the package are the facts that advancing age and, for all you backpacking smokers, the carbon monoxide in tobacco smoke also degrade night vision. With all these factors working against us, it’s a wonder we can see at all in the mountains after sundown, so you might want to take a light along on those two a.m. bathroom breaks to keep from wandering off that cliff.
