A lensmeter or lensometer, also known as a focimeter, is an ophthalmic

Ophthalmology

Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...

 instrument. It is mainly used by optometrists and opticians to verify the correct prescription in a pair of eyeglasses

Glasses

Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

, 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 prescription

Eyeglass prescription

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...

: sphere, cylinder, axis, add, and in some cases, prism. The lensmeter is also used to check the accuracy of progressive lenses

Progressive lenses

Progressive spectacle lenses, also called progressive addition lenses , progressive power lenses, graduated prescription lenses, and varifocal or multifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation...

, 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

Eye examination

An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....

 to obtain the last prescription the patient was given, in order to expedite the subsequent examination.

History

In 1876, Hermann Snellen

Hermann Snellen

Herman Snellen was a Dutch ophthalmologist who introduced the Snellen chart to study visual acuity . He took over directorship of the Netherlands Hospital for Eye Patients after Dr...

 introduced a phakometer which was a similar set up to an optical bench

Optical table

An optical table is platform that is used to support systems used for optics experiments and engineering.-Explanation:In optical systems, especially those involving interferometry, the alignment of each component must be extremely accurate—precise down to a fraction of a wavelength—usually a few...

 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 pictured above, but projects the measuring target onto a screen eliminating the need for correction of the observer's refractive error

Refractive error

A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.-Classification:...

 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.

See also

• Vertometer
  Vertometer
  Vertometer was the trade name of an instrument made by Bausch & Lomb and later by Reichert.Lensometer was the trade name of the nearly similar instrument made by American Optical.Neither the Vertometer or the Lensometer is now being made....
  
  
• Lens clock
  Lens clock
  A lens clock is a mechanical dial indicator that is used to measure 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 diopters, assuming the lens is made of a material with a...
  
  
• Optical power
  Optical power
  Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...
  
  
• Diopter
• Prism dioptre
• Corrective lens components
• Abbe refractometer
  Abbe refractometer
  An Abbe refractometer is a bench-top device for the high-precision measurement of an index of refraction.- Details :Ernst Abbe , working for the Zeiss Company in Jena, Germany in the late 19th century, was the first to develop a laboratory refractometer...