Corneal topography: photokeratoscopy including the central region

The central corneal zone is depicted on keratoscope photographs using a small target aperture and a large object distance. Information on the peripheral area is included by employing a hemispherical target with a dense circular and radial pattern. On a 16 mm (R = 8 mm) reference steel sphere the diameter of the innermost reflected mire is 0.20 mm and the distance between mires 0.13 mm. The standard deviation on focusing precision, determined to 0.20 mm employing a fixed object, corresponds to an error of 0.06% in linear measurements on keratographs. The variation in object distance due to the curved image plane is at most 0.55 mm within 60 deg from the optical axis of the system for the reference sphere. Nevertheless, all reflected mires are distinctly reproduced on the keratoscope photograph. For a test object the slopes of light intensity curves on the passage between black and white areas are reduced by only 1.0% with a variation in object distance of 0.55 mm around focus.