Background: The objective of this article was an analytical review of reading charts with regard to the historical background, printing techniques, print quality and print size.
Methods: For this study original historical reading charts (Jaeger, Snellen, Nieden and Birkhäuser) were investigated microscopically by measuring the lower case letter sizes (iNexis VMA 2520, Nikon, Tokyo). Calculations were made according to EN ISO 8596 and the recommendations of the International Research Council.
Results: In the mid-nineteenth century various reading cards were published; however, at that time it was not possible to print lower case letters according to exactly defined standards. Thus, these reading cards were not comparable to each other or between different language versions. At a reading distance of 32 cm Jaeger No. 1 represented in the original edition (1856) a visual acuity of 0.72 and 0.63 in the version from 1945 and the smallest print size of the Snellen reading test (1862) represented 0.55. Nieden No. 1 (1882) corresponded to a visual acuity of 0.59 and the smallest print size of the Birkhäuser cards (1911) 1.5. In the case of all reading cards except the original Birkhäuser cards, there was no logarithmic progression of the print sizes and the sizes of numbers were also not in accordance with the corresponding letter sizes.
Conclusion: Print sizes of historical reading charts do not adhere to current standards and were usually not logarithmically scaled. These deficits and the lack of comparability between the historical reading charts may have caused reading cards to be currently underestimated as an accurate diagnostic tool. Nevertheless, the historical reading charts were the successful forerunners of developments for modern reading charts.
Keywords: Measurement microscope; Near visual acuity; Reading charts; Reading tests; Visual acuity.