Calibration of photoscreeners for single-subject, contact-induced hyperopic anisometropia

In 1999, the threshold anisometropic hyperopia for photoscreening toddlers and preschoolers was determined to be 1.50 D. We compared crescent size from induced anisometropic hyperopia using three commercially available photoscreeners and compared them with miniature digital video cameras, which have a flash-to-lens dimension similar to two of the three photoscreeners. Photoscreeners were compared with a remote autorefractor.

Anisometropia was induced by placing several known minus contact lenses (-1.00, -1.50, and -2.00 D) in the nondominant eye of a visually healthy, orthophoric emmetrope older than 3 years. Photographs were taken in light and dim conditions with all three cameras with and without the contact lenses. Corneal diameter, pupil diameter, and crescent location were determined. A protocol for flash initiation, focus, and distance was determined for the digital cameras.

Under light and dim ambient light, no camera yielded a significant hyperopic crescent for emmetropia (no contact lens) or induced 1.0-D anisometropia. We propose a simple measure, "delta center crescent" distance from the center of the pupil to the edge of the crescent. Delta center crescent less than 1.5 mm identified most examples of threshold hyperopic anisometropia. The ideal distance for photoscreening using the digital cameras is approximately 1.6 m, whereas the commercially available photoscreeners focus at 1.0 m.

We have developed a protocol for photoscreening and its interpretation with a miniature digital video camera weighing 400 g and costing approximately dollar 1,000. Threshold anisometropic hyperopia can be determined from digital still or audio-labeled video flash images if the pupils are at least 4 mm.