Improving the sensitometric and OECF standards:recognizing the photosensitive exposure range

摘要

This article demonstrates that and how sensitometric and opto-electronic characteristic function (OECF) standards should be changed: The sensitivity S of all photosensitive arrays is—and, in standards, should be—determined by the midtone photosensitive exposure of the array H_(mid) the binary logarithm of which is [log_2(H_(max)) + log_2(H_(min)]/2. The reference exposure H_o = S/H_(sp) of a photosensitive array is-and, in standards, should be-determined by the width of the photosensitive exposure range A, which is determined by the measured minimum and maximum usable photosensitive exposures, H_(max) and H_(min). Knowledge of H_(max) and H_(min) (or H_(mid) and A) makes the reference exposure H_o, the speed point exposure H_(sp) and the safety factor a unnecessary, and these extraneous quantities should be eliminated from the sensitometric standards. In addition, the sensitometric standard for solid-state arrays, ISO 12232-2006, and the OECF standard, ISO 14524-1999, should be changed because they use a photosensitive exposure range which is inappropriate for solid-state arrays. Finally, this article shows that all current standards establish midtone reflectances R_(mid) for standard photosensitive arrays that are much less than oft-touted 18% making most references to 18% inappropriate.