Since the 1970s a variety of lens products have been marketed which take the form of tinted lenses applied monocularly or binocularly with different tints in each eye. The tints are usually a strong red, purple or pink color. Often the tint is put into a contact lenses, making the solution relatively discrete. While such devices can in theory provide additional spectral information, it is understood according to vision science that these devices work on the principle of binocularly rivalry (disruption of depth perception) and do not fundamentally improve color vision. Due to the low risk and possible benefit to some individuals, the FDA has permitted such devices to be sold provided that they are clearly labeled that 1) they are not a cure for color blindness and 2) have minimal therapeutic effectiveness.