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http://www.1stconnect.com/anozira/SiteTops/light/filters.htm
Many flashlights can accept filters. (Here we are talking about regular incandescent lamp flashlights. The color of the new LED flashlights is set by the particular LED the manufacturer used. The user can not change it.) Some flashlights come with filters; they are sold as accessories for others. So, other than pretty colors, what are they for?
In the wilderness, any light stands out and will be noticed. Color has a lot to do with how easily and how far away the eye will detect the light. There is a spectrum of colors with red at one end, blue at the other, and green in the middle. Red light has the least energy and is the least noticed - there's more of it in nature. Blue light contains higher energy and, since it is uncommon in nature, it is readily noticed. In terms of how little light it takes to be detected, the eye is most sensitive to green.
Use the blue filter for signaling when you WANT to be seen. The orange and especially the red filters are a little less likely to be noticed if that is your wish. The blue filter produces light that is especially noticeable at dusk since the light from the setting sun is very red.
The red filter has another very important function. It allows you to use your flashlight without destroying your natural night vision. Once exposed to bright light, the eye takes about 20 minutes to re-adapt to the dark and become its' most sensitive. To protect your night vision, you want to use the least light possible of any color. Red light will not effect natural night vision nearly as much as the whiter light directly from the flashlight bulb. Thus, red light - and the dimmest possible for the task at hand - will best preserve natural human night vision. The filter does two things. It reduces the total amount of light and allows only red light to get to the eye.
The orange filter is a compromise, more light in exchange for a little loss of dark adaptation. Using the red and the orange together produces the same appearance to the eye but the total light is less and this is sometimes useful.
Red is also preferred for walking or signalling in fog. An incandescent lamp produces white light which is some of all colors including blue. Blue is scattered by fog more than red. Red light travels to an object and back with less scattering, while blue light is scattered and appears as a uniform, featureless glare. Red light performs better in fog than the white light from an unfiltered lamp.
Red is not, however, the best choice for driving in fog. Preserving human night vision is not the major consideration. As a practical matter, an amber filter passes more light from the lamp than a red filter. Also the eye is more sensitive to yellow than red. Amber or yellow produces the maximum light, as judged by the eye, without the glare that the blue in a bare lamp would produce. The amber or yellow filter of a conventional fog light removes the offensive blue wavelengths leaving the most visible colors in the larges quantity.
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