For as long as I can remember, I have been told to use red lights at night to protect my night vision. Red lights have been used by the military since before WW2. Recently, the US military changed from red to green light.
Part of the reason for the military’s change is the use of night vision equipment. However, as it turns out, green light or blue-green offers some advantages over red as a means to retain night vision capability.
A study out of New Zealand shows that low-intensity red light or green light helps preserve your night vision and reduce recovery time, once white light illumination is turned off. Low-intensity red light or green light leaves night vision ready once the high-intensity light is turned off.
Brightness and Night Vision
The biggest factor in protecting your night vision is the brightness or illumination level of the light. Light brightness has a more significant effect on night vision retention than does the choice of red or green.
It turns out that your eyes are more receptive to green light than red. We gain better visual acuity at lower levels of brightness using green light than when using red light. In addition, green lights also help you tell the differences between other colors than red lights.
For example, the magenta used on aviation charts is readily readable under green light, but not always with red light.
Variations in Visual Acuity
Another complicating factor is variations in visual acuity at low light levels; what would be perfect for one, might be too bright or too dim for another.
Chances are that without some means to vary the intensity of the light, no light will be perfect. Red, green or blue-green lights will both help to protect your night vision. If retaining night vision acuity is your goal, the biggest concern is avoiding very high illumination levels, of either color.
When moving in the dark, you want the lowest level of brightness possible. In this case, you might want a green or blue-green light source since your eyes are more receptive to these colors than red. Red requires a high level of brightness.
You can find flashlight filters on the internet or improvise using cellophane paper.