Focometer

Last updated April 01, 2019

A focometer is an instrument that measures refractive errors and is intended to provide rural or economically disadvantaged populations spherical eyeglass prescriptions without the need for complicated protocols, expensive equipment, or electricity.^[1]^[2] 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.^[2]^[3] The individual's refractive power is then read off a linear dioptre scale.

$Refractive error cause of vision problems$

Refractive error, also known as refraction error, is a problem with focusing light accurately onto the retina due to the shape of the eye. The most common types of refractive error are near-sightedness, far-sightedness, astigmatism, and presbyopia. Near-sightedness results in far away objects being blurry, far-sightedness and presbyopia result in close objects being blurry, astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision, headaches, and eye strain.

An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient. If an examination indicates that corrective lenses are appropriate, the prescriber generally provides the patient with an eyewear prescription at the conclusion of the exam.

A dioptre or diopter is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres. It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at ¹⁄₃ metre. A flat window has an optical power of zero dioptres, and does not cause light to converge or diverge. Dioptres are also sometimes used for other reciprocals of distance, particularly radii of curvature and the vergence of optical beams.

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 can also be measured using a "clock target" with the device.^[2] A study has found, however, that the focometer is less effective for identifying astigmatism than an autorefractor, and that its axis accuracy is limited to 15°.^[4]

$Autorefractor$

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 measuring how light is changed as it enters a person's eye.

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.^[4] It found that the focometer results were within 0.75 dioptres of the autorefractor value 84% of the time. The authors found that repeatability of the measurements improved when patients were allowed to "practice" twice on each eye before taking the real reading.

Related Research Articles

Near-sightedness problem with distance vision

Near-sightedness, also known as short-sightedness and myopia, is an eye disorder where light focuses in front of, instead of on, the retina. This causes distant objects to be blurry while close objects appear normal. Other symptoms may include headaches and eye strain. Severe near-sightedness increases the risk of retinal detachment, cataracts, and glaucoma.

Keratoconus corneal disease characterized by structural changes within the cornea causing it to thin and change, leading to a protruding conical shape

Keratoconus (KC) is a disorder of the eye which results in progressive thinning of the cornea. This may result in blurry vision, double vision, nearsightedness, astigmatism, and light sensitivity. Usually both eyes are affected. In more severe cases a scarring or a circle may be seen within the cornea.

LASIK or Lasik, commonly referred to as laser eye surgery or laser vision correction, is a type of refractive surgery for the correction of myopia, hyperopia, and astigmatism. The LASIK surgery is performed by an ophthalmologist who uses a laser or microkeratome to reshape the eye's cornea in order to improve visual acuity. For most people, LASIK provides a long-lasting alternative to eyeglasses or contact lenses.

Presbyopia is a condition associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Symptoms include difficulty reading small print, having to hold reading material farther away, headaches, and eyestrain. Different people will have different degrees of problems. Other types of refractive errors may exist at the same time as presbyopia.

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture".

Radial keratotomy (RK) is a refractive surgical procedure to correct myopia (nearsightedness) that was developed in 1974, by Svyatoslav Fyodorov, a Russian ophthalmologist. It has been largely supplanted by newer operations, such as photorefractive keratectomy, LASIK, Epi-LASIK and the phakic intraocular lens.

$Refractive surgery medical specialty$

Refractive eye surgery is an eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea (keratomileusis), lens implantation or lens replacement. The most common methods today use excimer lasers to reshape the curvature of the cornea. Successful refractive eye surgery can reduce or cure common vision disorders such as myopia, hyperopia and astigmatism, as well as degenerative disorders like keratoconus.

Intraocular lens (IOL) is a lens implanted in the eye as part of a treatment for cataracts or myopia. The most common type of IOL is the pseudophakic IOL. These are implanted during cataract surgery, after the cloudy eye's natural lens has been removed. The pseudophakic IOL provides the same light focusing function as the natural crystalline lens. The second type of IOL, more commonly known as a phakic intraocular lens (PIOL), is a lens which is placed over the existing natural lens and is used in refractive surgery to change the eye's optical power as a treatment for myopia (nearsightedness).

Convergence insufficiency or convergence disorder is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn towards each other, or sustain convergence.

The amplitude of accommodation is the maximum potential increase in optical power that an eye can achieve in adjusting its focus. It refers to a certain range of object distances for which the retinal image is as sharply focussed as possible.

Corneal topography, also known as photokeratoscopy or videokeratography, is a non-invasive medical imaging technique for mapping the surface curvature of the cornea, the outer structure of the eye. Since the cornea is normally responsible for some 70% of the eye's refractive power, its topography is of critical importance in determining the quality of vision and corneal health.

Astigmatism is a type of refractive error in which the eye does not focus light evenly on the retina. This results in distorted or blurred vision at all distances. Other symptoms can include eyestrain, headaches, and trouble driving at night. If it occurs early in life it can result in amblyopia.

Ocular albinism is a form of albinism which, in contrast to oculocutaneous albinism, presents primarily in the eyes. There are multiple forms of ocular albinism, which are clinically similar.

Adjustable focus eyeglasses are eyeglasses with an adjustable focal length. They compensate for refractive errors by providing variable focusing, allowing users to adjust them for desired distance or prescription, or both.

Laser Blended Vision is a sophisticated laser eye treatment which is used to treat presbyopia or other age-related eye conditions. It can be used to help people that simply need reading glasses, and also those who have started to need bifocal or varifocal spectacle correction due to ageing changes in the eye. It can be used for people who are also short-sighted (myopia) or long-sighted (hyperopia) and who also may have astigmatism.

The Alpins Method, developed by Australian ophthalmologist Noel Alpins, is a system to plan and analyze the results of refractive surgical procedures, such as laser in-situ keratomileus (LASIK). The Alpins Method is also used to plan cataract/toric intraocular lens (IOL) surgical procedures.

The eye, like any other optical system, suffers from a number of specific optical aberrations. The optical quality of the eye is limited by optical aberrations, diffraction and scatter. Correction of spherocylindrical refractive errors has been possible for nearly two centuries following Airy's development of methods to measure and correct ocular astigmatism. It has only recently become possible to measure the aberrations of the eye and with the advent of refractive surgery it might be possible to correct certain types of irregular astigmatism.

$Subjective refraction$

Subjective Refraction is an attempt to determine, by trial and error using the patient’s cooperation, the combination of lenses that will provide the best corrected visual acuity (BCVA). It is a clinical examination used by orthoptists, optometrists and ophthalmologists to determine a patient's need for refractive correction, in the form of glasses or contact lenses. The aim is to improve current unaided vision or vision with current glasses. Glasses must also be comfortable visually. The sharpest final refraction is not always the final script the patient wears comfortably.

References

↑ Berger IB, Spitzberg LA, Nnadozie J (April 1993). "Testing the Focometer—A new refractometer". Optom Vis Sci. 70 (4): 332–8. doi:10.1097/00006324-199304000-00013. ISSN 1040-5488. PMID 8502463.
1 2 3 "The focometer fact sheet". Infocusonline. Retrieved 26 August 2008.
↑ Murthy, GVS; Johnson, G.J. (29 November 1999). "The focometer: use in aphakic correction". Community Eye Health. 12 (31): 43. PMC 1706012  . PMID 17491998.
1 2 du Toit R, Soong K, Brian G, Ramke J (August 2006). "Quantification of refractive error: comparison of autorefractor and focometer". Optom Vis Sci. 83 (8): 582–8. doi:10.1097/01.opx.0000230270.39804.54. PMID 16909083.

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[catinist-1] Berger IB, Spitzberg LA, Nnadozie J (April 1993). "Testing the Focometer—A new refractometer". Optom Vis Sci. 70 (4): 332–8. doi:10.1097/00006324-199304000-00013. ISSN 1040-5488. PMID 8502463.

[Infocus-2] 1 2 3 "The focometer fact sheet". Infocusonline. Retrieved 26 August 2008.

[medcentral-3] Murthy, GVS; Johnson, G.J. (29 November 1999). "The focometer: use in aphakic correction". Community Eye Health. 12 (31): 43. PMC 1706012  . PMID 17491998.

[cat-4] 1 2 du Toit R, Soong K, Brian G, Ramke J (August 2006). "Quantification of refractive error: comparison of autorefractor and focometer". Optom Vis Sci. 83 (8): 582–8. doi:10.1097/01.opx.0000230270.39804.54. PMID 16909083.