A method of optical pseudo-color representation is introduced for distinguishing height distribution of a solid object, flow velocity distribution of fluid and so on. First, a photograph is taken in a proper way so that each part of it posesses a bandpass spatial power spectrum whose center frequency represents height or flow velocity at the corresponding part of the object. Then the "frequency-coded" transparency is converted to colored image by a coherent optical processor with white light illumination.
Oichi Atoda, Oichi Atoda,
"Optical Pseudo-Color Processing For Moved Objects", Proc. SPIE 0189, 13th Intl Congress on High Speed Photography and Photonics, (27 July 1979); doi: 10.1117/12.957661; https://doi.org/10.1117/12.957661