We define a grain modulation transfer function (MTF) using unstructured grain images. This MTF is found to be similar in shape to the sine-wave MTF, but about 1/25 as sensitive. The higher sensitivity of the one-dimensional sine-wave grating is due to its highly structured pattern. In a second experiment, the results show that no simple unichannel linear model of vision can explain the visibility of the grain images. However, the results are consistent with a simple multichannel linear model.
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Robert E. Greenwood (1973) studied this question.
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