Abstract

There is disclosed a system capable of subtracting in real time the intensities of two incoherently illuminated scenes and providing the sign of the subtracted information. The system utilizes two birefringent devices such as liquid crystal light valves, onto which the two images to be subtracted are projected. One valve is analyzed in between crossed polarizers, or equivalent, while the other one is in between parallel polarizers or equivalent. Both polarizer arrangements, for example, may be implemented with a polarizing beam splitter and a quarter wave plate. The common output image plane, onto which both images are superposed, displays an intensity proportional to the difference between the two inputs. This intensity "rides" on a constant background intensity thus displaying the difference signal as well as its polarity. The read-in and read-out beams could be coherent as well as incoherent, the latter being more desirable due to its speckle-free image. No other system is presently known which can perform incoherent subtraction. Experimental results obtained with incoherent illumination are given.