The FM15 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.