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Nouvelle méthode de mesure de la netteté des émulsions photographiques
The paper describes a new method to measure the acutance of photographic materials. This method is based principally on Fourier analysis of the amplitude of plane wave falling on a periodical object in Fraunhofer diffraction. The field where this Fourier analysis takes place is first defined and an optical diffractometer for power spectrum measurement is described.
The optical diffractometer is checked up with simple signals such as a single narrow and long slit and a square-well amplitude ruling (Grayson type). The photographic application of the previous statements can be made in the following manner : a Grayson type ruling having a long period is imaged on the tested emulsion. The spatial image period must reach about 400 to avoid dependence effects between adjacent periods of the signals. The samples are immersed in a liquid of the same refractive index as that of gelatine to eliminate any phase variation. The amplitude of the photographic signal can be reconstituted from its monochromatic power spectrum. A psychometric study suggests a·good correlation between the number of harmonics contained in a signal and visual impressions of sharpness. From the power spectrum of such photographic rulings, one can also deduce a “statistical mean information” which can be used to define the acutance of a material. In this case the signal-to-noise ratio is taken into account as well as the number of hamonics.
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A propos de : H. Thiry
Université de Liège