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An instrument has been built to measure graininess in terms of the reciprocal of the blending magnification of an image of the sample as viewed by binocular vision from a constant distance. This instrument is essentially an autofocus variable-magnification projector which forms an image of constant size and of a luminance which is proportional to the transmittance of the sample. The image is formed on a rear-projection screen which is part of the front wall of a special observation room. The blending magnification may be determined either by permitting the observer to vary the magnification continuously until graininess is just perceptible, or by the experimenter varying the magnification discontinuously and presenting the enlarged image of the sample to the observer at ten predetermined magnifications and having the observer judge only whether or not graininess is perceptible at each magnification. With the latter method, the image at each of the predetermined magnifications is repeatedly presented to the observer in a random order and the frequency of perception of graininess at each magnification of the image is plotted as a function of magnification. From this psychometric function the blending magnification is determined as that magnification at which the observer has perceived graininess during 50 percent of the observations.
© 1951 Optical Society of America
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