A lensmeter or lensometer (sometimes even known as focimeter or vertometer), is an optical instrument used in
ophthalmology
Ophthalmology (, ) is the branch of medicine that deals with the diagnosis, treatment, and surgery of eye diseases and disorders.
An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a ...
. It is mainly used by
optometrists and
opticians to measure the back or front vertex power of a spectacle lens and verify the correct prescription in a pair of
eyeglasses
Glasses, also known as eyeglasses (American English), spectacles (Commonwealth English), or colloquially as specs, are Visual perception, vision eyewear with clear or tinted lens (optics), lenses mounted in a frame that holds them in front ...
, to properly orient and mark uncut lenses, and to confirm the correct mounting of lenses in spectacle frames. Lensmeters can also verify the power of
contact lenses
Contact lenses, or simply contacts, are thin lens (optics), lenses placed directly on the surface of the Human eye, eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct ...
, if a special lens support is used.
The parameters appraised by a lensmeter are the values specified by an
ophthalmologist
Ophthalmology (, ) is the branch of medicine that deals with the diagnosis, treatment, and surgery of eye diseases and disorders.
An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a ...
or
optometrist
Optometry is the healthcare practice concerned with examining the eyes for visual defects, prescribing corrective lenses, and detecting eye abnormalities.
In the United States and Canada, optometrists are those that hold a post-baccalaureate f ...
on the patient's
prescription: sphere, cylinder, axis, add, and in some cases, prism. The lensmeter is also used to check the accuracy of
progressive lens
Progressive lenses are corrective lenses used in glasses, eyeglasses to correct presbyopia and other disorders of accommodation reflex, accommodation. They are characterised by a gradient of increasing optical power, lens power, added to the wear ...
es, and is often capable of marking the lens center and various other measurements critical to proper performance of the lens. It may also be used prior to an
eye examination
An eye examination, commonly known as an eye test, is a series of tests performed to assess Visual acuity, vision and ability to Focus (optics), focus on and discern objects. It also includes other tests and examinations of the human eye, eyes. ...
to obtain the last prescription the patient was given, in order to expedite the subsequent examination.
History
In 1848,
Antoine Claudet
file:Ada Byron daguerreotype by Antoine Claudet 1843 or 1850 - cropped.png, Ada Byron's daguerreotype by Claudet, .
Antoine François Jean Claudet (August 18, 1797 – December 27, 1867) was a French Photography, photographer and artist active i ...
produced the photographometer, an instrument designed to measure the intensity of photogenic rays; and in 1849 he brought out the focimeter, for securing a perfect focus in photographic portraiture.
In 1876,
Hermann Snellen introduced a phakometer which was a similar set up to an
optical bench which could measure the power and find the
optical centre
In Gaussian optics, the cardinal points consist of three pairs of Point (geometry), points located on the optical axis of a Rotational symmetry, rotationally symmetric, focal, optical system. These are the ''Focus (optics), focal points'', the p ...
of a convex lens. Troppman went a step further in 1912, introducing the first direct measuring instrument.
In 1922, a patent was filed for the first projection lensmeter, which has a similar system to the standard lensmeter pictured above, but projects the measuring target onto a screen eliminating the need for correction of the observer's
refractive error
Refractive error is a problem with focus (optics), focusing light accurately on the retina due to the shape of the eye and/or cornea. The most common types of refractive error are myopia, near-sightedness, hyperopia, far-sightedness, astigmatis ...
in the instrument itself and reducing the requirement to peer down a small telescope into the instrument. Despite these advantages the above design is still predominant in the optical world.
[ accessed 20 Jan 2009]
See also
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Lens clock
A lens clock is a mechanical dial indicator that is used to measure the dioptric power of a lens. It is a specialized version of a spherometer. A lens clock measures the curvature of a surface, but gives the result as an optical power in diop ...
*
Optical power
In optics, optical power (also referred to as dioptric power, refractive power, focal power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the ...
*
Diopter
*
Focometer
*
Prism dioptre
*
Corrective lens components
*
Abbe refractometer
References
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External links
Lensometry basicsThe lensmeter
Ophthalmic equipment
Measuring instruments
Articles containing video clips