Test-types with rear illumination, for testing visual acuity, English, probably c.1950
Back lit for a clearer view, this display is a chart of letters used by an optician to test a person’s sight at various distances. Such charts are perhaps the most iconic objects in sight testing. A patient with good vision should be able to read the letters at a distance of six metres. The original sight charts were developed in the 1850s by Dutch ophthalmologist Hermann Snellen (1834-1908). The letters, from largest to smallest, are all a precise shape and size. Such charts are still commonly used.