Fundamental Principles of Meters Used to Measure Visibility

Abstract

Attention is drawn to the difficulty of assessing the visibility of an object when the difference in brightness between the object and its background exceeds the value necessary to produce the best performance under the particular experimental conditions. It is suggested that if the visibility scale is to be extended to brightness differences which exceed this value, then the visibility of the object must be considered in terms of the observer's performance under all conditions of sensitivity, distraction, attention, and unexpectedness which can occur in practice during the carrying out of the visual task.

Meters which have been designed to measure the visibility of an object depend upon making the visual conditions more difficult by producing optically a condition in which the difference in brightness between the object and its background is equal to a threshold value. It is shown that in the case of objects which are presented against backgrounds of different brightness, these objects will be arranged in an order of visibility which depends on the exact manner by which the threshold condition is produced. Inconsistencies which may arise from the use of a meter both in the presence and absence of glare sources are also indicated. The need is stressed for careful consideration of the results given by a meter both in relation to the nature of the visual task and the differences in lighting conditions under which comparisons of visibility are made.

Consideration is also given to visibility meters which have been designed to measure the efficiency of street lighting installations. As these meters take into account many of the factors affecting the visibility of objects in the street, they can usefully be employed to study the effect of these factors. On the other hand, since the meters do not take all the important factors into account, the results given by the meters in different installations should not be taken as measuring the efficiency of these installations in promoting object visibility.