A SUPRATHRESHOLD MEASURE OF PERCEIVED BRIGHTNESS

— In response to an energy flux density, E (Λ), the output of a linear detector of radiation with spectral response R (Λ) can be calculated by integrating the product R (Λ) * E (Λ) over the spectral range for which R (Λ) and E (Λ) are both non‐zero. This method is also applied when the eye is used as a detector of radiation although the non‐linearity condition and the associated perception of colour as well as brightness make the validity of this approach difficult to assess. The quantity computed in this way when the CIE luminous efficiency function [i.e. the V (Λ) curvel is used as a measure of the spectral response of the eye is referred to as luminance and is often regarded as the objective correlate of perceived brightness. Judd (1951) showed that the use of the CIE luminous efficiency function in this way underestimates the brightness of short wavelength lights. More recently the inappropriate use of the V (Λ) function has resulted in predictions of luminance levels for light‐sources which were not consistent with direct estimates of visual effectiveness (see Kinney, 1983). In this paper, the perceived brightness of coloured lights is investigated using a modified, delayed, brightness‐matching technique. Psychometric response functions were obtained for some twenty coloured test targets which subtended a visual angle of 2° at the observer's eye, whilst adaptation was maintained to a constant white (3900 K) field. The spectral sensitivity functions derived from these data show little variation for a number of different viewing conditions, but they are significantly different from the CIE luminous efficiency function. Brightness discrimination data for a set of selected broad‐band stimuli are related to the prediction of luminance computed by means of both the CIE V (Λ) function and on the basis of the brightness matching function derived from these experiments.