Image differentiation with the aid of a phase knife

A scheme for optical differentiation of images based on spatial filtering is proposed. A phase knife—a glass plate consisting of two halves, whose thicknesses are chosen so that the optical path difference between the light waves passed through them makes up half the wavelength—serves as a filter. The linearity of the transformation and the sensitivity of the system to noise are investigated theoretically. The scheme was experimentally implemented on the basis of a liquid-crystal light valve. The experimental results confirm that such a setup can be efficiently used for processing images with a narrow-band spatial spectrum and for visualization of phase objects.