Look for the letters!
Look for the letters!
Along with the trial frame, test letter charts are perhaps the objects most closely associated in the public's mind with the eye examination...but did you know that the idea of reading from a letter chart dates from as far back as 1862?
Herman Snellen (1834-1908) was a physician attached to the Netherlands Hospital for Eye Patients in Utrecht, a specialist eye hospital where he had worked for four years at the time of his ‘invention’. Various hospitals had already devised their own tests (using passages of readable text). These tests had multiplied in the wake of the invention of the ophthalmoscope in 1851, leading to a lack of standardisation. Snellen may have aimed to end this confusion as the result of a direct request from one of his mentors, the short-lived Albrecht Von Graefe (1828-1870). Whatever the reason, he is generally credited with being the first to introduce charts bearing single letters and numbers (optotypes, 1862) in addition to the more familiar reading tests.
The patient had to identify each alpha-numeric character individually and not just read complete words which might be guessed from the sense of the passage. Snellen described these in an important work Optotypi ad Visum Determinandum which was quickly translated into French, German, Italian and English (Snellen was a noted Anglophile) and so became widely used internationally within a very short timeframe.
The same charts might be used at various distances, but there was no provision for standard illumination, which in any case would have been impossible in the 1860s. They were available in an ‘English’ version from Williams and Norgate by 1864.
Snellen's Optotypes are not identical to the test letters used today! They were printed in an 'Egyptian Paragon' font and were of a serif style: that is to say they included ornamental cross strokes at the ends of each limb.
Ironically, the attempt in subsequent decades to ‘improve’ upon Snellen’s optotypes led to a re-divergence, whereby charts were no longer considered reliable as a comparable basis for estimating visual acuity (VA). This was particularly so of 1920s America. Aside from discrediting the ophthalmic professions, this could have legal implications during hearings for compensation claims etc. Many charts were described as being ‘after Snellen’ without bearing much resemblance to his work at all (for example some Near Vision test types, in which area of work Snellen was never involved!)
The history of ophthalmology... or of optometry?
In 1877 Snellen was appointed Professor Ordinarius in ophthalmology at his old alma mater, the University of Utrecht. Other ophthalmologists were involved in designing new letter-types. These included John Green (1835-1913), Professor of Ophthalmology and Otology in the St Louis College of Physicians and Surgeons, Missouri from 1866. He took up that appointment shortly after returning from an extended stay in Europe, and the design of his test types in 1868 may be seen as one consequence of those travels. Green was the first to experiment with non-serif style letters (of a Gothic style), but this was a false dawn as, stung by criticism over their unfinished appearance, he favoured serif letters once again from 1872.
Ferdinand Monoyer (1836-1912), a specialist in physiological optics, ran an ophthalmic clinic at the hospital in Nancy, France. Monoyer is credited with introducing the first test-types corresponding to a decimal system in 1875. A year later he was appointed to a professorship of medical physics at the University of Lyon. Monoyer selected his font style on an individual letter by letter basis. He felt there was a case for not selecting the same linear dimensions in every instance. For example a letter H that is as wide as it is high can look too drawn out.
Dennett (USA 1885), Blaskovics (Hungary 1923/4), Cowan (USA 1928) and Walker (Great Britain 1942) all produced varieties of test type that found favour at one time or another. This is probably mainly due to the fact that eye hospitals were the main source of research in this period, with training for ophthalmic opticians still being a combination of apprenticeships, evening classes and the odd technical college course.
Attempts at standardisation were the work of ophthalmological congresses (1872, 1904, 1909, 1922). For example the International Congress of Ophthalmology held at Naples in 1909 selected the numerical chart of Carl von Hess (1863-1923), Professor of Ophthalmology at the University of Wurzburg, as the international standard. Its 'Minimum separabile' was to be the minute of arc. In Great Britain, however, a separate optometry profession was growing from the 1890s onwards and the Institute of Ophthalmic Opticians (IOO) did much development work from the 1920s onwards, resulting in Harold Emsley’s IOO Chart of 1933.
In more recent years there has been a move to create electronic charts. Test Chart 2000, designed by Dr David Thomson of the Department of Optometry at The City University, London, was launched in January 2000, and was the first Windows-based computerised test chart in the world. It overcame several hitherto difficult issues such as screen contrast, and offered the chance to vary the letters shown on a frequent basis.
In Britain there are now several university-based optometry departments, and only a minority of patients would have their eyes tested by an ophthalmologist (for whom refraction is an increasingly rare activity), but elsewhere in Europe opticians remain barred from performing refraction and so to them it would not seem strange to refer to Snellen’s chart as a contribution to the ‘history of ophthalmology’. It is fair to say that optometrists in Britain (and some British-influenced countries) and sight-testing ophthalmologists elsewhere are equally the heirs of Snellen.
Reversed letter charts
In the 1940s reversed test-types were filtering through to Great Britain from America, and seemed likely to become the new standard. The patient sat with the chart behind him and viewed the lines through a mirror which had the effect of doubling the distance at which the letters appeared. It was noted, however, that sometimes opticians were invited to refract in ophthalmic hospitals where they could find bigger facilities allowing for a direct chart to be placed at further than six metres from the patient. In 1947 the optometrist L. A. Swann (d.1989) reminded such practitioners that such facilities could be equally scientific, even if less convenient. This points to an interesting factor - that the development of test charts has partly been driven by the amount and nature of space available for consulting rooms.
Astigmatic letters (made up of split vertical, horizontal or oblique lines) can be traced back to the American chart of Dr Pray (sometimes written Prey) in 1869. They were offered for sale in catalogues before the First World War by Groos Ltd, amongst others, but also as late as 1947 by American Optical Company, an example of whose product, of unknown date, is illustrated here.
Many test chart cabinets incorporate 'Snellen's dial', a fan-like arrangement of lines, alongside the more conventional optotypes.
Foreign language charts
The first test charts were Dutch-inspired and so utilised western letter forms and so-called 'Arabic' numerals. As sight-testing spread to other countries, the Snellen type was adapted to fit local circumstances. Examples might include Gothic script, suitable for German-speaking areas, or charts in Hebrew. The motivating force behind the latter type of chart may have been the signficant number of opticians who were Jewish. In the East End of London many of their patients would have been unable to read any other language.
The Emarah Chart designed in 1968 was a pioneering Arabic language test letter chart on a panel of laminated perspex board intended for insertion into a test chart cabinet. That way an optometrist's equipment could be quickly adapted in the event of an influx of Arabic speakers into a local area. The chart was also supplied with two phonetic translation cards in classical Arabic for the benefit of the English-speaking optometrist. It was devised by a professor at the University of Cairo in collaboration with Mr I. D. C Pain of Keeler Ltd. Though drawn from the Arabic alphabet, the letters conform to the Snellen principle of 1-unit line width and 1-unit interspace. (See British Journal of Ophthalmology, Vol 52 (1968) p.489-91). Naturally, some compromise was involved, and the particularly cursive form of the flowing Arabic letters had to be straightened to some extent. An inadvertent result was that to us, viewing the chart forty years on, it appears to contain a smiley face emoticon!
How do you ask a patient to identify the test objects if he cannot read or cannot count?
The Army made effective use of Snellen charts, but found that in practice some recruits could not read. They had to add in their own charts comprised of dots in a line such as that illustrated on the right which dates from 1864. A chart employing arrangements of dots (not unlike dominoes) was used to test Prussian military recruits by the Army physician Max Burchhardt (1831-1897) as early as 1869.
The Landolt ring is the classic illiterate test. The break in the ring (making it look like a letter 'C') can be turned in any direction and the patient asked to point it out.
Similar in concept is the Illiterate 'E' which is really to be considered as a vertical line with three shorter lines stemming from it. Again, an ability to spot the direction in which these face can determine the acuity of the patient. These charts are often made portable, as they have traditionally been applied for domiciliary use.
To find out what to do if you want to ask questions of a patient who cannot hear, see our page on Instructions for the deaf.
Whilst the first transparent test chart was devised in 1893 by Cohn, it was intended for use attached to the surface of a window and required sufficient daylight. Even then it was reflected into a mirror due to the need to avoid glare. The first proper text chart with it own integral illumination was designed by Roth in 1897. Lit by a gas light, it had a short working life as an instrument because the light source bleached the chart and turned other parts of it yellow!
Charts could either be internally or externally illuminated. Ultimately the internally illuminated design won out. It looked neater and was more practical, for example, when a chart was desgined to revolve.
Changing the test targets, by revolving them out of the way, makes it more of a test of visual acuity and less of a memory test!
Another space-saving solution was to project a slide of the test letters onto a viewing surface such as a white wall. The oldest patent for such a device appears to be American, dating from 1922.
Before the Second World War the Fleming Project-O-Chart was heavily advertised though it is unclear how many were ever actually sold. You had to order either the 'A' or the 'B' lens system depending upon whether you intended to project over distances of more than fifteen feet.
The Ferree-Rand projector by Bausch & Lomb, produced during the War, was commercially successful, to a limited degree, outside the States, though like many ophthalmic instruments the extent to which it penetrated the British market was restricted due to high import tariffs. Our museum's example was imported by Frank Dickinson FBOA. It has a rectangular base, tapering tube and a semi-circular lamp unit. An adjustable diaphragm sits at the narrow end of the tube. The charts are inserted to the wider end as transparent slides. They include a vertical red and green 'Chromatest' slide and a horizontal slide bearing test letters and astigmatism diagrams.
Other test chart instruments
The Visometer (unfortunately one of several optical instruments, designed for different purposes, to bear that name) was invented in 1913 by Kirsch. A reduced size test letter target was viewed through lenses as it moved along a scale to replicate the placing of larger letters at a distance. S. R. Stearman designed a version which was described in the Optician (24.2.1933, p.30). The J & R Fleming catalogue for 1936 states that the instrument is 'the result of extensive research into the construction of subjective test charts. The test letters have been carefully selected and grouped to reduce errors due to guessing'.
Did you know?
Test letter charts are classed as a 'subjective' test. That is because their use requires some input from the patient, whose response might be offered as a matter of opinion. e.g. a question 'What is the last line you can read clearly?' may not have a definitive answer. A full eye examination should therefore contain a good balance of subjective and objective tests.
Some useful sources for historians of test letters:
All these titles are available in the College Library.