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A model is presented for photorefractive testing of the living eye with the circular knife-edge Otago photoscreener. Wavelength-dependent fundal remittance and line spread functions are introduced and used to account for the yellow-orange-red range of colours observed in pupillary images. The response of the photoscreener to errors of focus is described in terms of luminance and dominant wavelength profiles.

Characterisation of the reflection from the fundus of the eye is important for the development of an improved theoretical description of photoscreening. Measurement has been made of the reflected line spread function of the human fundus in vivo using a slit-lamp microscope with video camera and frame-grabber. Line spread functions representative of red, green, and blue light were obtained from fundus sites just temporal to the macula, 10 degrees temporal to the macula, away from the macula temporal, and 30 degrees temporal to the macula. The results show that the lateral spreading of light is greatest for red light, and support the view that red light penetrates further into the fundus before being reflected than green or blue.

An investigation of linear knife-edge photoscreening devices for detecting focusing errors, refractive defects and other optical artifacts in the human eye is described. Pupillary irradiance distributions calculated for a model eye are shown to agree qualitatively with distributions recorded experimentally using video frame grabbing equipment. The form of the pupillary irradiance distribution from a cyclopleged human eye supports a diffuse-like characterization for the reflection/scattering processes at the retina. The extreme sensitivity of the instruments enables real-time detection of refractive effects from tear films on the cornea and real-time tracking of floaters.