A Pockels-effect light valve: Phototitus. Applications to optical image processing

An electrooptic light valve is described, using the Pockels effect in a KD2PO4crystal cooled close to its Curie temperature and addressed by photoconduction in an amorphous selenium layer. The preliminary performance levels obtained are: resolution 40 lp/mm, maximum contrast ratio 70/1, writing time 10 µs, sensitivity <100 erg/cm²for 10/1 contrast, and a maximum modulation emciency of ≃75 percent. The linearity of the Pockels effect allows image addition and subtraction, the latter being obtained by reversing the applied voltage between two exposures. Experiments of first-and second-order image derivation using incoherent light are described, the latter obtained by subtraction of a blurred image from the initial one. The grainless structure of the KD2PO4plate prevents all loss of coherence in coherent optical data processing. Furthermore, suppression of the zero order in the Fourier transform of an image has been obtained by subtracting from that image a uniform one whose amplitude is equal to the average amplitude of the initial image; this results in the suppression of the optical noise introduced by the scattering of the zero-order term in coherent filtering experiments. Character-recognition experiments are described employing a filter hologram previously made using the same device. The high-speed capability of the device would permit the construction of automatic reading systems at least ten times faster than those generally available.