A focometer is an instrument that measures
refractive error
Refractive error, also known as refraction error, is a problem with focusing light accurately on the retina due to the shape of the eye and or cornea. The most common types of refractive error are near-sightedness, far-sightedness, astigmatism ...
s and is intended to provide rural or
economically disadvantaged
The "disadvantaged" is a generic term for individuals or groups of people who:
* Face special problems such as physical or mental disability
* Lack money or economic supportKingdom of Nepal: Economic and Social Inclusion of the Disadvantaged Poo ...
populations spherical
eyeglass prescription
An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a pati ...
s without the need for complicated protocols, expensive equipment, or electricity.
[ The focometer is monocular and hand-held, and is normally used in natural lighting. Patients rotate a collar on the focometer until the best focus is achieved.]dioptre
A dioptre ( British spelling) or diopter ( American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or c ...
scale.
The focometer was developed by Drs. Ian Berger and Larry Spitzberg at the University of Houston College of Optometry in Houston, Texas, to provide a simple, inexpensive means for measuring refractive error in human vision. The portable, hand-held instrument is highly appropriate for use in remote and poor areas.
Focometers measure spherical refractive errors. Astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at ni ...
can also be measured using a "clock target" with the device.[ A study has found, however, that the focometer is less effective for identifying astigmatism than an ]autorefractor
An autorefractor or automated refractor is a computer-controlled machine used during an eye examination to provide an objective measurement of a person's refractive error and prescription for glasses or contact lenses. This is achieved by measuri ...
, and that its axis accuracy is limited to 15°.[
The advantages of a focometer over other methods for use in developing countries are that it is lightweight, compact, relatively inexpensive, fairly quick, and easy to use with minimal training. A clinical trial compared the repeatability, validity, and ease of use of the focometer with an autorefractor.]
References
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Ophthalmic equipment