This paper describes studies of the threshold modulation curve for photographic film. The threshold modulation curve is an empirically derived relation showing the need at any spatial frequency for modulation in the aerial image of a tribar for resolution to be achieved. The paper deals with the history, raison d’être, theory, method of measurement, and accuracy and precision of the threshold modulation curve. The statistical aspects of the threshold modulation curve are emphasized, and the influence of factors that cause variations in the curve are examined. Examples are given of current and potential applications of the threshold modulation curve.
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