The College of Optometrists

 

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.

 

1876 Snellen's phakometer 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.

 

Most opticians used the lens measure (also known as a 'spherometer'), 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 to the left shows a Lens measure from the 1950s, by which time such an instrument was rarely the optician's first resort.

 

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.

 

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.

 

This object to the right is an early model of Busch focimeter with a closed circle target and light-reflecting mirror, made in Rathenow circa 1925.

 

1915 Carl Zeiss Jena introduced its focimeter with a movable microscope. 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 this Zeiss focimeter on the left dates from the early 1920s. It has a prism compensator but still only a crossed-line target.

 

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 '...one 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'.

 

N.B. 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.

 

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

 

c.1927 Runge & Kaulfus introduced a rotatable measuring device.

  

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

 

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. It was designed in 1946. Early models were black and so the grey model in our picture to the left must date from the later 1940s. 

 

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.

 

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.

 

This 1960s (?) model to the left 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.

 

 

The Zeiss focimeter to the right 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.

 

This Topcon focimeter on the left, 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.

 

On the far right 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.

 

---

 

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 is projected onto a frosted glass plate.

 

This Mark II 1950s example by the Letchworth Optical Instrument Co was used in a practice from around 1953 until 1994. 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.