{{short description|Ophthalmic instrument mainly used by optometrists and opticians}} [[File:Lensmeter.jpg|thumb|right|A lensmeter in an optical shop]] [[File:Lensmeter-2.png|thumb|right|upright|A simple lensmeter cross sectional view.
'''1''' – Adjustable eyepiece '''2''' – Reticle <br />'''3''' – Objective lens '''4''' – Keplerian telescope<br /> '''5''' – Lens holder ''' 6''' – Unknown lens <br />'''7''' – Standard lens '''8''' – Illuminated target<br /> '''9''' – Light source '''10''' – Collimator<br /> '''11''' – Angle adjustment lever <br />'''12''' – Power drum (+20 and -20 diopters) <br />'''13''' – Prism scale knob]]
A '''lensmeter''' or '''lensometer''' (sometimes even known as '''focimeter''' or '''vertometer''')<ref>{{cite book|chapter-url=http://medical-dictionary.thefreedictionary.com/Vertometer|chapter=Vertometer. (n.d.)|title=Millodot: Dictionary of Optometry & Visual Science, 7th edition|year=2009|accessdate=2015-01-04}}</ref><ref>{{cite web|url=http://www.zeiss.de/de/ophthalmic/glossary_e.nsf/frameset.html?ReadForm&Redirect=/C1256E7B002D2A69/0/8BE738D3614A7DD0C1256E8200232F89|archive-url=https://web.archive.org/web/20080512081021/http://www.college-optometrists.org/index.aspx/pcms/site.college.What_We_Do.museyeum.online_exhibitions.optical_instruments.focimeter/|archive-date=12 May 2008|url-status=dead|title=Ophthalmology Glossary on Carl Zeiss Vision GmbH website}}</ref> is an optical instrument used in [[ophthalmology]]. 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 [[glasses|eyeglasses]], 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]] if a special lens support is used.
The parameters appraised by a lensmeter are the values specified by an [[ophthalmologist]] or [[optometrist]] on the patient's [[Eyeglass prescription|prescription]]: sphere, cylinder, axis, add, and in some cases, prism. The lensmeter is also used to check the accuracy of [[progressive lens]]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]] to obtain the last prescription the patient was given, in order to expedite the subsequent examination.
== History == In 1848, [[Antoine Claudet]] 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.<ref>{{cite journal|editor1-last=Solbert|editor1-first=Oscar N.|editor1-link=Oscar Solbert|editor2-last=Newhall|editor2-first=Beaumont|editor2-link=Beaumont Newhall|editor3-last=Card|editor3-first=James G.|editor3-link=James Card|title=The Focimeter|journal=Image|date=February 1952|volume=1|issue=2|pages=1–2|url=http://image.eastmanhouse.org/files/GEH_1952_01_02.pdf|accessdate=June 16, 2014|publisher=[[George Eastman Museum|International Museum of Photography]] at George Eastman House Inc.|location=Rochester, N.Y.|url-status=dead|archiveurl=https://web.archive.org/web/20130312194522/http://image.eastmanhouse.org/files/GEH_1952_01_02.pdf|archivedate=March 12, 2013}}</ref> In 1876, [[Hermann Snellen]] introduced a phakometer which was a similar set up to an [[optical table|optical bench]] which could measure the power and find the [[optical centre]] 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, but projects the measuring target onto a screen eliminating the need for correction of the observer's [[refractive error]] in the instrument itself and reducing the requirement to peer down a small telescope into the instrument.<ref>{{cite web |url=http://www.college-optometrists.org/index.aspx/pcms/site.college.What_We_Do.museyeum.online_exhibitions.optical_instruments.focimeter/ |title=Focimeters |accessdate=2010-01-30 |url-status=dead |archiveurl=https://web.archive.org/web/20080512081021/http://www.college-optometrists.org/index.aspx/pcms/site.college.What_We_Do.museyeum.online_exhibitions.optical_instruments.focimeter/ |archivedate=2008-05-12 }} accessed 20 Jan 2009</ref>
== See also == *[[Lens clock]] *[[Optical power]] *[[Diopter]] *[[Focometer]] *[[Prism dioptre]] *[[Corrective lens#Components of a sphero-cylindrical correction|Corrective lens components]] *[[Abbe refractometer]]
==References== {{reflist}}
==External links== *{{youtube|p=PLQST6JyJD0gwQv2RVZ-b8Uo24VPIHr1B2|title=Lensmeter instructional video series}} *[http://www.2020mag.com/ViewContent/tabid/136/content_id/395/Default.aspx Lensometry basics] {{Webarchive|url=https://web.archive.org/web/20101125164403/http://www.2020mag.com/ViewContent/tabid/136/content_id/395/Default.aspx |date=2010-11-25 }} *[http://spie.org/x32360.xml The lensmeter]
[[Category:Ophthalmic equipment]] [[Category:Measuring instruments]] [[Category:Articles containing video clips]]