Abstract
An optical technique is introduced for pseudo-color representation of height or deformation of a solid object, a flow velocity distribution of fluid and so on. First, a photograph is taken in which parts of movement, height, etc. are coded into their own center frequency of spatial bandpass processes. For example, a photograph of a flow of fluid carrying aluminium powder on its surface is taken under periodical illumination of a flashing stroboscope. Then the frequency-coded transparency is converted into a colored image by an optical spatial filtering processor being illuminated by spatially coherent white light and having an off-axis slit as the spatial filter.
A simple mathematical procedure is derived for estimating an output color of the processor from the spatial power spectral density of the input transparency. In the derivation the tristimulus values are expressed respectively by the output powers of the three linear systems whose power transfer functions are easily calculated under the given slit configuration. It is shown that the small-signal resolution of the processor is approximately equal to that of an incoherently illuminated optical system of the same configuration and size having an on-axis slit of the same width. It is recommended that the slit width is approximately 1/5 of the slit offset from the optical axis for optimally compromising the excitation purity with the resolution, and that the bandwidth vs. center frequency ratio should be less than 1/5 to make chromaticity determined mostly by the center frequency and little affected by the spectral shape of input signals.
Experimental results are shown in colored figures. The rangeability of input frequency is roughly 1:1.5, but it can be adjusted using a prism if necessary.
