Time Variations in Observed Corneal Topography

Measurements of the author's right eye were made with the autocollimating photokeratoscope during working hours for a few days. Using a single number to quantify corneal shape at any given observation, the shape appeared to alter cyclically over the course of a day but the variation was not repeated exactly on succeeding days. Allowing for the temperature coefficient of the photokeratoscope increases the magnitude of the daily changes. The variation results from an additive combination of changes in spherical and aspherical components of corneal topography. Observations of four eyes of three subjects were analysed for changes over times of up to fourteen months but the variations are comparable with the long‐term errors of the measurement system.