The effect of glare on the brightness difference threshold

Abstract

By the brightness difference threshold is ordinarily understood the least perceptible difference of brightness of two contiguous test-fields when the brightness of one of these has a given value. It is well known, however, that this critical brightness difference depends not only on the given test-field brightness but also on the brightness in the remaining parts of the visual field. A change in the latter is accomplished in general by a change in the threshold. When an unshielded light source is introduced into the field of view, thee field brightness being otherwise unchanged, the threshold is found experimentally to increase. The observed increase in this case may be regarded as one of the things intended when the unshielded light source is described as "glaring." The researches of Bordoni and others have shown that the change in the threshold varies continuously with the intensity of the glare source and with its position in the field of view. The important conclusion has also been arrived at that provided the linear dimensions of the glare source do not exceed a certain limit, the effect on the threshold depends only on the candle-power in the direction of the subject's eyes and not explicitly on the size or brightness of the source. When such is the case, we may speak of a point source of glare and specify it merely by its position in the visual field and its candle-power. In the present paper an account is given of the determination of the threshold in the presence of a point source of glare for various values of the general brightness level in the field, and for different positions and intensities of the glare source. The ranges of the variables have been chosen with a view to the application of the results in practical illumination problems, such as the design of street lighting installations to give best visibility conditions. The method of measurement is chosen to correspond as far as possible to normal vision conditions, the subject having the unrestricted use of both eyes. In a number of the earlier threshold determinations, notably those of König and Brodhun, the chief interest was the retinal sensitivity, and pupillary accommodation and binocular effects were eliminated by using one eye only and an artificial pupil. The threshold appropriate to these conditions will not necessarily be the same as that for normal vision conditions.