The liminal brightness increment as a function of wave-length for different conditions of the foveal and parafoveal retina

Abstract

The eye was brought to a definite condition by viewing a known distribution of brightness in the visual field, made up of a small centre field of uniform brightness B surrounded by an area of uniform brightness B s, together with, in some cases, a bright spot of light 3[degree] above the centre. Concentric with the centre field, a small rectangular patch (0.7[degree] [CHI] 0.16[degree]) of monochromatic light of variable brightness was presented to the subject''s view and the measurements consisted in determining the least brightness (U[lambda]) of this patch such that the subject could just detect its presence. The minimum brightness U[lambda] has been termed the "liminal brightness increment" and in this work its value has been determined in absolute energy units for various wave-lengths [lambda] throughout the spectrum. The value of U[lambda] obtained depends on whether the subject views the test patch by direct (foveal) vision or whether he directs his gaze away from the centre of the field and perceives the test patch by parafoveal or peripheral vision. The cases studied here are foveal and 5[degree]-parafoveal vision. The results are expressed by plotting log (1/U[lambda]) against [lambda] for each condition of the eye studied and for foveal and 5[degree]-parafoveal vision. The foveal and parafoveal curves of log (1/U[lambda]) against [lambda] obtained with the totally dark-adapted eye (B = B s = O), have the same shape as curves obtained under similar conditions by Abney and Watson; for wave-lengths greater than 0.62[MU] foveal and parafoveal values are nearly the same, but as shorter wave-lengths are approached the parafoveal value of log (1/U[lambda]) becomes increasingly greater than the foveal value, i.e., the parafovea becomes much more sensitive than the fovea. For the eye adapted to high brightness levels of white light (e.g., B s = 300 candles/sq. ft.), the curve of log (1/U[lambda]) against [lambda] exhibits a characteristic form with maxima at or near [lambda]=0.44[mu] and [lambda][long dash]0.54[mu] and with a third maximum or bulge in the neighborhood of [lambda]=0.60[mu]. Parafovea and fovea behave similarly in this respect although the parafovea still has a somewhat greater sensitivity in the blue end of the spectrum. With the eye adapted to high brightness of colored light the log (1/U[lambda]) against [lambda] curve is modified by the suppression or partial suppression of one of the three maxima. For example, with B s = 25 candles/sq. ft. of blue light the maximum at 0.44[mu] is absent, the shape of the curve for wave-lengths greater than 0.50[mu] being nearly the same as for B s = 25 candles/sq. ft. of white light. A close connection of the three maxima at 0.44[mu], 0.54[mu] and 0.60[mu], with the 3 mechanisms postulated by the trichromatic theory, is indicated.