Defocus-corrected analysis of the foveal Stiles–Crawford effect of the first kind across the visible spectrum

The Stiles–Crawford effect of the first kind describes a gradually diminished visibility of light that enters the eye towards the pupil rim. Although of retinal origin, it is commonly described by a Gaussian pupil apodization whose width is determined by a directionality parameter that depends on retinal eccentricity, wavelength and spatial coherence of the light. As the measurements are done psychophysically they are prone to subjective variations and difficult to obtain across the visible spectrum. In this work, requirements for accurate refractive correction when determining the directionality parameter at any given wavelength are discussed and we show that a current-controlled tunable liquid-polymer lens provides a convenient means to accomplish this without requiring mechanical readjustments. This may be the most convenient way to combat defocus across the visible spectrum in the analysis of the Stiles–Crawford effect as demonstrated through experiments and with a detailed Zemax eye-and-system analysis. The results obtained are discussed in relation to myopia and a reduced directionality for highly myopic eyes. (paper)