314 F. Schaeffel

Corneal radius of curvature

The corneal surface is optically the most powerful surface of the eye; in most vertebrates, it generates more than 60% of the total optical power. Therefore, it is important to measure its radius of curvature. This can be done by infrared photokeratometry in alert mice.15 Mice are placed on a platform (Fig. 5A) and slightly restrained by holding their tails. The platform is positioned at about 15 cm from a metal ring with a diameter of 300 mm that carries eight IR LEDs. The reflections of the IR LEDs on the corneal surface, the first Purkinje images, are also arranged in a circular pattern (Fig. 5B). In a digital video image of the eye, the reflections can be detected by an image processing program and fit by a circle in real time. The diameter of the fitted circle is proportional to the curvature of the cornea — small circles for steep corneas with small radius of curvature, and larger circles for flat corneas.

Although the equations to calculate corneal curvature from camera distance, distance of the IR LEDs of the keratometer, distance of the keratometer to the mouse, and camera magnification have been worked out,44 it is easier to measure a ball bearing with known radius of curvature, compare it to the measured radius of curvature, and find the correction factor. Since the system is almost perfectly linear, one factor is sufficient to derive the true corneal radius of curvature of the animal. The standard

Figure 5. Measurement of corneal radius of curvature in alert mice, using IR photokeratometry.

(A) The mouse (white arrow) is restrained only by holding its tail, but because its eye needs to be positioned in the small field of view of the video camera with a 135 mm lens at a well defined position (depth of field only about 1 mm), the platform is moved back and forth until the first Purkinje images of eight IR LEDs, arranged in a circle, are in good focus. The software continuously fits circles through the eight Purkinje images and provides their radius.