Use of a reflective neutral-density wedge for the measurement of saturation discrimination

Abstract

The psychophysical measurement of saturation discrimination can be time-consuming, since maintaining constant luminance of a mixture of white plus monochromatic light while varying only its colorimetric purity is difficult without providing separate controls for the independent variation of the luminance of the two components. The necessity of adjusting two controls not only complicates the task for the subject but also extends testing time. We describe an apparatus which provides a relatively rapid means of measuring saturation discrimination. Subsequent to an initial measurement of the photopic luminosity function with heterochromatic flicker photometry, the observer only needs to turn a single control to vary the colorimetric purity of a test field relative to a constant white-light reference field to determine the saturation threshold at each wavelength. This is made possible by the use of a variable beam splitter to combine (mix) the white and monochromatic channels of the test field. The variable beam splitter is a standard reflective neutral-density wedge placed within the optical system at an oblique angle relative to the orthogonal beams being mixed. Preliminary results show that this provides a relatively rapid and reliable means of measuring saturation discrimination in a laboratory or clinical setting and yields results in normal subjects similar to those described by previous researchers.