A focometer is an instrument for measuring either the visual or the photographic focal length of an objective or of another optical system. It is generally classified as an optometer which measures spherical refractive errors and is intended to provide rural or economically disadvantaged populations spherical prescriptions without the need for complicated protocols, expensive equipment, or electricity. The Focometer is monocular and hand held, which is normally used in natural lighting and allows an individual's refractive power to be read off a linear dioptre scale. Patients rotate the focometer till the best focus is achieved.

History

The focometer was developed by Drs. Ian Berger and Larry Spitzberg at the University of Houston College of Optometry in Houston, Texas, USA to provide a subjective refraction without the need for electricity or complicated protocols. The portable, hand-held instrument is highly appropriate for use in remote and poor areas.

Purpose

The advantages of a focometer over other methods of refraction for use in developing countries are that it is lightweight, compact, relatively inexpensive, fairly quick, and easy to use with minimal training. This clinical trial compared the repeatability, validity, and ease of use of the focometer with an autorefractor.

Usage

A punctual light source is projected by the movable objective lens on the surface of the sample.

The reflected light path is decoupled by a beam splitter. The cylindrical lens splits the focus into a horizontal and a vertical focal line. The detector is positioned midway between the nominal position of the two focal lines. A circle appears on the detector if the surface is exactly in focus. Deviation from the focus position results in an ellipse on the detector. The orientation and eccentricity of the ellipse depends on the magnitude and direction of the defocus.

The shape deviation is evaluated by the different current distributions in the four quadrants of a photodiode and is fed to a control loop (objective lens control). By adjusting the movable objective lens, the control loop causes a tracking movement of the lens on the surface. The track movement of the movable objective is determined by a glass scale and displayed as a measured value.

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