Abstract
Visual acuity, near-distance visual acuity, and reading ability all depend on the minimal angle of resolution. Visual acuity is defined as the inverse of the minimal angle of resolution, measured at a distance of 4 – 6 m, and expressed in units of (min arc)−1. Near visual acuity and reading ability are measured at reading distance. The preferred reading distance is not the same for every subject. A practical way to combine character size and reading distance is to use the ‘M-unit’ for text size and ‘Dioptre’ for the reading distance. For example, a character size of 1 M at a viewing distance of 1 m corresponds to a visual acuity of 1. A character size of 0.20 M at a viewing distance of 25 cm (=4 D) corresponds to a visual acuity of 1/(0.2 × 4)=1.25.
In the rehabilitation institute we had a need for reading charts with different text blocks for repeated testing (OD, OS, OU, and at various illumination levels). Furthermore we wanted text blocks with a comparable layout and reading difficulty. Such reading charts exist in other languages and we preferred to adopt a similar design, together with the M-unit and Dioptre notation. We composed the text and tested the equivalence of text blocks by measuring the reading speed of young (aged 18 – 27 years) and old (aged 60 – 69 years) subjects. We present the resulting set of nine reading charts with a plea for general use of the M-unit and Dioptre notation for near-distance visual acuity and reading ability assessment in visually impaired people.
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