Experimental demonstration of coherence effects and linearity in microdensitometry.

Microdensitometer measurements of photographic film transmittance made at high spatial frequencies demonstrate that the basic instrument is nonlinear in that its response is a function of the spatial frequency and the mode of illumination. A method developed for measuring the spatial coherence, making use of a Wollaston prism shearing interferometer, has been applied to typical microdensitometers, documenting the existence of the partial coherence. The trace of a phase edge was found to be a graphical means of demonstrating one of the consequences of the partial coherence. Since partial coherence is present in all image-forming instruments, a linear instrument that avoids the imaging step and collects all the transmitted light has been developed. This linear instrument is limited in spatial frequency only by the size of the scanning spot. The system transfer function is the Fourier transform of the scanning spot irradiance. A linear microdensitometer was assembled on an optical bench using a cw laser source and standard microscope objectives, and it was used to trace a series of samples to test its response at high spatial frequencies. The linearity is demonstrated with both edge width measurements and flat response to a phase edge.