The liminal brightness increment for white light for different conditions of the foveal and parafoveal retina

Abstract

We have given in a previous paper (1933) the results of measurements of the liminal brightness increment (l. b. i.) for monochromatic light of different wave-length throughout the spectrum, for various conditions of adaptation of the eye, and for foveal and 5°-parafoveal vision. In the present paper measurements of the l. b. i. will be reported, in which the additional stimulation representing the test spot whose presence or absence is adjudged by the subject, consists of white light or more precisely of radiation with a continuous spectrum corresponding to that of a black body at approximately 2800°K. Conditioning stimulations of the following types have been studied ( a ) uniform background brightness (white), ( b ) uniform surround brightness (white or coloured) with a dark centre field, ( c ) an isolated bright patch or glare source (white or coloured). The test spot was observed either by direct (foveal) or slightly indirect (5°-parafoveal) vision. In Paper 1, attention was directed to the effect of wave-length on the value of the l. b. i., expressed in energy units, when the eye was subjected to one of a few widely different conditioning stimulations. In the present work, the test stimulus colour is fixed and the interest shifts to the effect on the l. b. i. of progressive changes in the conditioning stimulation, either in regard to brightness or colour. White backgrounds and surrounds of brightnesses ranging from zero to 500 c./sq. ft. and white glare sources producing an eye illumination up to 50 ft. c. have been used. In addition the effect of colour of surround or glare source has been studied. Although the colour of the test spot is fixed, its size and flash period have been varied and the data obtained throw light on the operation of these factors under different conditioning stimulations.