The Surefire FM35 Red Filter produces a smooth beam of red light that will not
degrade night vision as white light would. Hunters use the red filter to avoid
spooking game during predawn hours - deer see poorly in the red color
spectrum. The filter is also useful for close range, non-tactical applications
- such as map reading -- as it reduces the intensity of light output. Applying
thumb pressure to the locking tab releases the cover; a spring mechanism keeps
the open cover out of the way by holding it flat against the flashlight. A
cam-locking lever keeps the entire unit securely attached to the flashlight.