Ferdinand Monoyer: the eye test and the dioptre

Where it all started

On 9 May 1836, Ferdinand Monoyer was born in the historic city of Lyon in France. His father, a military doctor, could not have known that he would go on to become one of the most influential ophthalmologists in European history. Today, we at Concept celebrate Monoyer’s life on the occasion of his 181st birthday. Although you may not think you know much about Monoyer, it’s impossible to be unfamiliar with his most well-known invention: the Monoyer chart.

The Monoyer chart – or an adapted version of it – hangs in every optician’s practice the world over. It is intended to test what is known as “visual acuity” – that is, the clarity of one’s vision. The chart uses a gradually shrinking typeface, which allows opticians to assess how clear this vision is. Each of the rows of characters is set at a specific unit of measurement – the dioptre – which Monoyer also invented.

A dioptre is a unit of measurement of the optical power of a lens or a curved mirror, equal to the reciprocal of the focal length measured in meters. This may sound complicated at first, but it makes a little more sense when we look at the numbers. Essentially, a dioptre can be worked out by with this equation: 1/metres. In other words, a 3-dioptre lens brings parallel rays of light to focus at 1/3 metres. The Monoyer chart measures this by having its text set at different dioptres. If you begin to struggle at a particular line, an optician will be able to evaluate what dioptre you require in your lenses.

Monoyer himself had an enormously successful life. He began as an associate professor of Medical Physics at the Faculty of Medicine at the University of Strasbourg in 1871. Later, he went on to become the director of the Ophthalmic Clinic of the Faculty of Medicine, University of Nancy from 1872 to 1877 and Professor of medical physics at the Faculty of Medicine, University of Lyon from 1877 to 1909. Upon the occasion of his death in 1912, the President of the Société nationale de Médecine de Lyon concluded a session of the Société with a commemoration to Monoyer, stating that ‘To the memory of this scholar, the Medical Society bows with respect and sadness; she has lost a friend who was also her counsellor who knew to think and to reflect.’ Concept would like to offer our sincere regard to this giant of optometry.

We’ll leave you with a final fact to remember Monoyer by. The Monoyer chart contains its inventor’s full name, hidden amongst the other letters it contains. Why not see if you can find it?