Visibility gauge.

摘要

A visibility gauge in the form of a disc or strip of diffusing material arranged to be interposed in the light path from a subject to an observer. The diffusiveness of the strip varies along its length so that the contrast between the subject and its background can be selectively varied by arranging the strip to transmit progressively greater or smaller proportions of diffused light until a threshold of visibility is reached. The strip may be in the form of a wedge of diffusing material or a clear strip with one surface treated to be progressively more diffusive towards one end.