Contrast enhancement in optical systems

A projection microscope with a copper-vapor laser image amplifier was used to demonstrate image contrast enhancement on introduction of an optical feedback which ensured that the object was illuminated by its own image. In the case of a real low-contrast object it was found that the contrast could be improved by a factor of 4. An analysis of the effect was made on the basis of a model of noncoherent addition of images, which showed that the image contrast of the object could be enhanced substantially and, in principle, the maximum (unity) value of the contrast could be achieved.