Photorefraction of the living eye: a model for linear knife edge photoscreening

A computer model is developed to explain colored artifacts in pupil images of the living human eye formed by the linear knife edge photoscreener. An approximate expression for the RGB irradiances recorded by the photoscreener is derived by using the statistical properties of the line spread functions that characterize the source, the aberrations of the eye, and the reflecting properties of the retina. The pupil image for each color is shown to be very much a map of the deviations perpendicular to the knife edge imposed on rays by an equivalent aberration plate, and the overall response of the photoscreener to specific aberrations of the eye is cataloged for errors of accommodation and chromatic aberration, spherical aberration, astigmatism, and tear bands.