Paper Substrate Spread Function and the MTF of Photographic Paper

Although photographic papers have been manufactured for at least as long as film has been made, there is little in the literature on their image microstructure properties. One of the earliest comprehensive experimental investigations of the “sharpness” (really the MTF) of photographic papers is that reported by Ruth Stapleton.1 One of the general conclusions of her work was that the Modulation Transfer Functions of photographic paper, as measured by edge-gradient analysis, were never as high as those produced by the photographic emulsion (film) by itself. The cited cause of this was the “multiple internal reflexions in the gelatine layer.” Although this is, perhaps, a contributing factor, it does not appear to be the one of consequence. Present knowledge suggests that the most significant factor is the spread of light within the paper base, which we now term the paper optical spread function, POSF, or its Fourier Transform, the paper optical MTF. The problem of light spreading within the bulk of a paper image substrate was identified by Yule and Nielsen2 over 50 years ago. Their interest was to explain why the reflectance density of halftone images was higher than that predicted by the ink fractional area of the halftone. They termed this phenomenon “optical dot Paper Substrate Spread Function and the MTF of Photographic Paper