We check the power of your existing spectacles.

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It is one thing to identify a patient's prescription, but quite another to check whether the spectacles supplied to him match the required specification. Within the practice, if a patient has lost his previous prescription, it may be necessary to test his current eyewear to see if it needs replacing following his latest eye examination.

Snellen's phakometer of 1876 was a series of lenses arranged in a row like an 'optical bench' that could measure the degree of diversion of convergent light rays when the lens to be tested was introduced into the middle of the series. It could also help identify the optical centre of a lens but was only any use for convex lenses.

Lens measure
Lens measure

Most opticians used the lens measure (also known as a 'spherometer' and, more recently, by some suppliers as a 'lens clock'), a hand-held device like a pocket watch with a prong to press against the lens, offsetting a pointer that was read against a round scale. As the prescription of a lens depends upon the shape of both front and back surfaces more than one measurement was required. Lens measures were adequate for most bi-convex or bi-concave lenses as used up to the turn of the twentieth century but were neither very accurate nor robust and required frequent calibration. Larger bench-mounted models also existed.

The image shows a Lens measure from the 1950s, by which time such an instrument was rarely the optician's first resort.

In 1912 Troppman introduced the first focimeter - an instrument in which light was passed through a lens in such a way that dioptric values could be read directly off a scale. The illumination source, the measuring device and the microscope through which the practitioner viewed the lens were still static at this stage.

Busch focimeter

In 1915 Busch introduced a classic and long-lasting design based on an astronomical telescope and a test target. The light was mirrored into the device from outside. Later models incorporated their own lamps though before the 1920s the level of illumination achieved was poor. Busch later introduced the closed ring target which was good for use in relation to spherical lenses (not so for cylinders). A green filter was introduced to lessen the amount of chromatic aberration. The object pictured is an early model of Busch focimeter with a closed circle target and light-reflecting mirror, made in Rathenow circa 1925.

Zeiss focimeter 1931

Carl Zeiss Jena introduced its focimeter with a movable microscope in 1915. In the mid 1920s Otto Henker introduced the dotted circle target still used today and around the same time the company introduced a rotatable spectacle support. We think the design of this Zeiss focimeter dates from the early 1920s. It has a prism compensator but still only a crossed-line target.

In 1921 The 'AO Lensometer' was patented by the 40-year old Edgar Derry Tillyer on behalf of the American Optical Company. Somewhat debatably this was claimed to be the first instrument to allow measurement of the effective power of a lens and thus check the accuracy of prescriptions. A company publication, Wellsworth Life (Feb 1922), called it ' of the greatest inventions in ophthalmic science....all the instrument lacks to make it a real human eye is an optic nerve and a mind'.

Did you know?

Some American publications and websites may use the word 'Lensometer' as a generic name for focimeter, but it is in fact just a brand name. The term 'lensmeter' has gained greater currency on both sides of the Atlantic in recent years.


Our picture on the right shows a later version of the Lensometer, probably made in the 1960s.

Circa 1927 Runge & Kaulfus introduced a rotatable measuring device.  

In 1939 Tietze used mirrors to project the measuring scale into the practioner's sight line whilst viewing the upper image of the test target.


In 1946 Keeler introduced a British focimeter, the Keeler Vertex Dioptrescope. This was that company's only attempt at a focimeter. The development costs were immense and they decided not to update it even though it had proved popular. Early models were black and so the grey model in our picture must date from the later 1940s or early 1950s. The Dioptrescope was manufactured until about 1951, continuing to be available from optical suppliers into the 1960s. The focal scale is read through the upper mirror set-up and measures to one eighth of a dioptre.

The British Focimeter

Theodore Hamblin Ltd produced a model that was actually called the 'British Focimeter' which the company could supply in the 1950s alongside imported Zeiss models.

Hamblin focimeter

This 1960s (?) model to the left, made and supplied by Hamblin, was known as the 'Self-Illuminating Focimeter' and is typical of the cream colour then in vogue for ophthalmic instruments. Many practitioners cared little for surface appearance however and stuck their own labels over the instrument bearing, perhaps, a reminder of how to use the device, or a note of when it was last calibrated and serviced.

Today focimetry often precedes a patient's eye examination and, in larger practices, may be carried out by an optical assistant.

West German Zeiss focimeter

This Zeiss focimeter was made in West Germany during the Cold War period and used at the Queen Elizabeth II Hospital, Welwyn Garden City. It has a vertically operated lens marking device. Like the earlier Keeler model it is really quite small and would have taken up very little space. A taller instrument might have been more convenient for some operators.

Topcon focimeter

The Topcon focimeter shown on the right, made in Japan, was not used in a practice but in manufacturing industry. This one is set up for contact lens use and was used on the production line at Madden & Layman Ltd until 1993.

Brochure for AO digital focimeter 1977

The museum also has an example of the American Optical Digital Lensometer, of which this is the promotional brochure. This was an electronic focimeter, originally designed to ensure precision lens manufacture but, by 1977, on offer to retail optometric practices. You may click on the image of the brochure to enlarge it.

A working focimeter in the museum 

Using a focimeter

You'll see that the BOA Museum also uses an old Topcon instrument (Model LM-5) to identify and catalogue the lenses of historical spectacles, many of which were made long before the existence of such an instrument. When one historic frame can look much like another to the untrained eye, a record of the lens power is a very good way of distinguishing between items in an historic collection.

Cut-away focimeter

If you want to know how a focimeter works, we have a cut-away model of a Stearman focimeter that should reveal all. It was previously used for teaching at the City University in London.

Projection Focimeters  

Projection focimeters avoid the practitioner having to peer into the instrument or correct an eyepiece to suit his own refractive error.

The first patent was issued in 1922 but this seems never to have been built. The target image was to have been projected onto a frosted glass plate.

Letchworth projection focimeter

This Mark II Projection Focimeter by the Letchworth Optical Instrument Co is believed to have been used in an optometry practice in Potters Bar, Hertfordshire, from around 1953 until 1994, although the particular model seems to have been marketed around 1965, but it certainly gave around thiert years of good service, possibly more.

SL projection focimeter

The French example to the right, by Société des Lunetiers (c.1955-60), has a hooded screen, an 'intermittent' and 'continuous control' switch and a Rotation test button. A green filter setting helped tackle chromatic aberration. The projecting lens and screen ensured better sighting and increased accuracy with less fatigue to the viewer or, if necessary, several simultaneous viewers. The swivelling screen permitted quick measurement of cylindrical lenses.

The view through a focimeter

The view through an historic focimeter.

Here is one of the images from our extensive collection of photographic slides. These are not catalogued at individual level so it just goes to re-emphasise the importance of getting in touch with us if an image you require cannot be sourced elsewhere. We now have the facility to scan these, though of course the quality of the original transparency is sometimes not the greatest.

Many of the slides were produced for lecture purposes at university optometry departments. This one is a case in point and is part of a set of seventeen slides showing what the practitioner views through the eyepiece of a focimeter. The user has placed a toric lens (as dispensed to people with high degrees of astigmatism) on the holding device within the instrument. The target is as yet unaligned. Photographs like these may one day be important evidence of how optometric equipment worked, not least because health and safety concerns mean that we cannot operate our museum pieces and in most cases the wiring has been cut.