A real-time Fourier-plane filter

An electrically addressed liquid-crystal Fourier-plane filter capable of real-time optical image processing is described. The filter consists of two parts, a wedge filter having 40 9° segments and a ring filter having 20 concentric rings in a 1-in diameter active area. Transmission of the filter in the off (transparent) state exceeds 50 percent. By using polarizing optics, contrast as high as 10
⁴
:1 can be achieved at voltages compatible with FET switching technology. A phenomenological model for the dynamic scattering will be presented for this special case. The filter is designed to be operated from a computer and is addressed by a 7-b binary word that includes an ON or OFF command and selects any one of the 20 rings or 20 wedge pairs. The overall system uses addressable latches so that once an element is in a specified state, it will remain there until a change of state command is received. The drive for the liquid crystal filter is ±30 V peak at 60 Hz. These parameters give a rise time for the scattering of 20 ms and a decay time of 80-100 ms. A complete description of the operational parameters of the device will be given and the results of several filtering experiments discussed.