When buying LED flashlights, many people assume that bright, clear, white light is the best way to view the world at night. For many situations, white light is, indeed, perfect. For instance, LED flashlights with white lights work well for walking to the campground bathrooms or lighting one’s trick-or-treating path.
At the same time, however, white light actually destroys our natural night vision. To understand why many hunters, fishermen and military personnel avoid using white LED flashlights light when navigating in the dark, it’s important to understand the biological processes that make human vision possible.
How the Human Eye Works
The human eye works much like a camera. It controls the amount of light that enters it, in order to have a clear exposure for the incoming image. In a camera, this image is recorded on film. In the human eye, light travels through the cornea, the pupil, the lens and jelly-like vitreous humor tissue to the retina, a layer of tissue at the back of the eyeball.
The retina is made up of millions of light-sensitive cells called rods and cones. When light hits the rods and cones, they send electrical signals to the brain by way of the optic nerve. We see the world after the brain has translated the light signals sent by the rods and cones. In general, cones detect color and detail, mostly in the daytime, while rods facilitate monochromatic vision in low-light conditions.
Rods and Your Night Vision
The chemical within the rods that facilitates night vision is called rhodopsin. Rhodopsin is a biological pigment that is extremely sensitive to light. (As a side note, carrots are good for the eyes because they are rich in carotene, which our bodies synthesize into rhodopsin.) The absorption of light causes rhodopsin molecules to change shape; this is the mechanism within rods that allows the detection of light.
If the eye is exposed to bright light, it usually take about half an hour for the rhodopsin molecules to return to their night vision shape. This is why it takes time for your eyes to be able to see in the dark after exposure to a bright light, such as that from an LED flashlight. (Hitchcock used this fact to dramatic effect when he had Jimmy Stewart’s character use flash bulbs to blind and deter his pursuer in the film Rear Window.)
In addition to what happens when rhodopsin is exposed to light, an important characteristic of cones and rods is the light spectrum where each type of cell is most sensitive. Cones are most sensitive around the yellow-green section of the light spectrum. (This is why newer crosswalk signs are given a bright yellow-green tone – this makes them highly visible to the cones in drivers’ eyes.) On the other hand, rods are most sensitive to blue-green light. This means that colors on the opposite side of the spectrum, such as red, are difficult for the rods to detect. This is why many outdoorsmen who are concerned with preserving their night vision opt for colored LED flashlights instead of white.
Preserving Your Night Vision
All of this biological background is meant to provide insight into choosing the best LED flashlight for your night vision needs. When selecting LED flashlights, many people aim to mimic daylight; they look for clear, bright white light. This approach makes sense – we are most comfortable with daytime conditions in which our cones can detect both detail and color.
However, to fully maximize your eyes’ potential at night, it is more effective to use an LED flashlight that can emit yellow-green, blue-green, or red light. Some LED flashlights contain all three of these light options in addition to white light.
Which color spectrum you should choose for your LED flashlight will depend on your usage. For occasions when you want to use as little light as possible, it makes sense to choose red light. Red light can’t be detected by the eye’s rods and is therefore a good color for preserving night vision. (Indeed, when astronomers gather for star parties, they forbid the use of anything but red lights after dark.)
At other times, such as when you need to read a color map, an LED flashlight emitting a small amount of yellow-green light will allow you to maximize your cones’ color- and detail-detecting potential. (To protect your night vision, you can cover one eye while using the yellow-green light.) Finally, for times when you want to use as little light as possible and don’t need to see detail, blue-green light may be the best LED flashlight setting.