FRINGE VISIBILITY IMPROVEMENT USING AN ASYNCHRONOUS IMAGE-SUBTRACTINGOPTICALLY ADDRESSED SPATIAL LIGHT-MODULATOR

We demonstrate the application of an asynchronous image-subtraction op tically addressed spatial light modulator to particle image velocimetr y fringe processing. The device comprises an amorphous silicon p-i-n-i -p photosensor and a ferroelelectric liquid-crystal light-modulating l ayer. The images to be subtracted are encoded on two separate waveleng ths. The operation of the device is described, and characterization sh ows a frame rate of 100 Hz, a resolution of 3 line pairs/mm, and a wri te-light sensitivity of approximate to 1 mW/cm(2) at a wavelength of 5 14 nm. The device is read by the use of light with a 633-nm wavelength whereas the subtraction light is at a wavelength of 670 nm. Using thi s device to subtract a nonuniform pedestal from the optically computed power spectral density function (the Young's fringe pattern), we find we can improve the signal-to-clutter ratio of peaks in the image-tran smittance autocorrelation function of particle image velocimetry trans parencies. The device also permits processing of very low-visibility f ringe patterns, generated from doubly exposed images, in which one ima ge has half the transmittance of the other. These could not be process ed with a nonsubtracting, binary, liquid-crystal optically addressed s patial light modulator.