FAILURES OF ISOLUMINANCE CAUSED BY OCULAR CHROMATIC ABERRATIONS

Using a simple model eye with a wavelength-dependent diffraction, a wavelength-dependent refractive error (chromatic difference in refractive error), and a wavelength-dependent displacement of the foveal images (transverse chromatic aberration), we have evaluated the luminance modulations in retinal images of isoluminant color gratings. In cases where the chromatic difference in refractive error has been corrected, the retinal image suffers from chromatic parallax, which creates wavelength-dependent displacements of the retinal image that are similar to those caused by transverse chromatic aberration. Our calculations show that all three chromatic aberrations can introduce luminance modulations in the retinal images of isoluminant gratings. These luminance artifacts generally, but not always, increase with increasing spatial frequency. The contrast in the luminance artifact depends critically on the exact refractive error in the uncorrected eye and the precise position of the eye in the corrected case. Wavelength-dependent diffraction has little effect for large pupils (e.g., 5 mm) but can become a significant factor with small pupils. Luminance artifacts created by chromatic aberrations can be more detectable than the original color contrasts at spatial frequencies above 3 cycles/deg.