Most people know that a flashlight's shape and power can help to make it applicable for specific uses, but a colored light bulb also can change a flashlight's uses. Compared to a regular, white flashlight bulb, red and green flashlight bulbs offer very specific advantages that make them useful in certain situations.
Regular Flashlight Bulbs
In order to understand why red and green flashlight bulbs are useful, it is important to understand how regular, white flashlight bulbs work. Light produced by a regular bulb, whether from a heated element or light-emitting diode (LED), is not filtered by the bulb's see-through plastic. It is described as "white" light because it contains all wavelengths of light from the visible spectrum, roughly from 400 to 700nm. Objects illuminated by a flashlight equipped with a regular, white bulb display all their regular colors, and the light is simple to perceive even from far away because the combination of all visible spectrum wavelengths produces a very intense light.
Red Flashlight Bulbs
Red flashlight bulbs function similarly to regular, white flashlight bulbs, producing light in the same way. A coating within a red flashlight bulb's shell, however, filters out all light that is not about 650nm, which is the wavelength that corresponds to the color red.
A red flashlight bulb offers several advantages over a regular, white flashlight bulb. Because of how the human eye works, red light provides greater contrast to objects it illuminates, allowing you to see a greater number of details. Another advantage is that a red flashlight bulb is more difficult to notice from a distance. Red light also does not interfere with night vision or night vision technology. So red flashlight bulbs are often used by photographers and the military for night work.
Green Flashlight Bulbs
Green flashlight bulbs also function similarly to regular, white flashlight bulbs, but the green bulbs' coating blocks all light except light with a wavelength of about 510nm, the wavelength that corresponds to the color green.
Green flashlight bulbs are advantageous because green light is very soft. The soft light tends to make the environment react to it; animals, including fish, are not spooked by a flashlight with a green bulb, and insects are not attracted to green light. Because of these advantages, hunters and fisherman often use green flashlight bulbs.
Blue Flashlight Bulbs
Blue flashlight bulbs also are available. They also have a coating that filters out all but a specific wavelength of light, in their case 475nm, which corresponds to the color blue. The blue light produced by blue flashlight bulbs is very mild and doesn't affect night vision. As a result, some people use flashlights with blue bulbs to read maps at night. The blue light also illuminates trace amounts of blood and bodily fluids particularly well, and so law enforcement investigators, hunters, and search and rescue personnel often use flashlights with blue bulbs to track a bleeding subject.