Refractor head, used for quick diagnosis of ophthalmic prescription without using trial lenses and trial frame, c.1950
Refractor heads are used in optometry to prescribe lenses for patients without the need for trial lenses and trial frames. Refraction (change in direction of light rays when they pass from one medium - such as the air to another such as the eye) can be measured. The degree of refraction that a patient's sight differs determines if they require glasses and how strong they should be.
The patient sits behind the instrument looking into the two lower eye pieces. The optometrist adjusts the settings while the patient looks at some text either close up to correct long-sight or far away to correct short-sight. Some optometrist prefer using trial lenses and frames as they allow for a more natural head position than using the refractor head.