The inability to use light intensity to represent negative values limits the potential of optical computing. The protein bacteriorhodopsin, an optically switchable bistable material, was used to represent an image as a local concentration of one of its two states. Light of one wavelength increased this concentration and represented positive intensity, whereas light of a different wavelength decreased the concentration and represented negative intensity. Optical subtraction was demonstrated by performing the mathematical operation of a difference of Gaussians. The electro-optical characteristics of bacteriorhodopsin films portend a variety of practical applications for this system.