The effect of a glaring light source on extrafoveal vision

When an exposed light source is present in the field of view, the visibility of neighbouring objects is impaired owing to what may be called the glare effect of the light source. This glare effect has been studied by several investigators, who have measured the increase in the smallest perceptible brightness difference (brightness difference threshold) when the glare source is introduced in such manner that the brightness distribution in the field remains otherwise unchanged. In all the earlier work attention was confined to the case when the subject viewed the brightness difference to be perceived, by direct or foveal vision. For this case the investigations of Luckiesh and Holladay (1925) and of Stiles (1929) have shown that if the field has a uniform brightness ofB c./sq. ft. and if a point source, locatedθ ° from the test object, produces an illumination ofE ft. c. on the pupillary plane at the subject’s eye, then the brightness difference thresholdT is raised from the valueT 0 appropriate to the field brightnessB , to the valueT G appropriate to a field brightnessβ , whereβ =B +kE /θ n . (1) Thus the effect of the glare source on the brightness difference threshold is reproduced by superposing a uniform brightnessγ =kE /θ n on the field brightnessB .β is termed the equivalent uniform field brightness, andγ =kE /θ n , the equivalent veiling brightness, of the given glare condition. Various values of the constantsk andn have been obtained by different investigators, but the best representative values may be taken asn = approximately 2,k = approximately 10. (See Appendix.) It has (Stiles and Crawford 1932) that the value ofβ for a given glare condition is independent of the size, colour and exposure time of the brightness difference used in the determination of the difference threshold.