an optical filter which blocks a substantial portion of a selected color of the visual light spectrum from entering one eye . the selected portion is the normal color of an object which is to be viewed by both eyes . the other eye is simultaneously allowed to have a substantial portion of the selected color to enter it . when used to aid a hunter in tracking a blood trail , a substantial portion of the red color of the blood is blocked from entering one eye while the other eye is allowed to receive a substantial portion of the red color .