Toward an internationally accepted standard for reading charts.

Patients who suffer from sight-threatening eye diseases share a desire to regain a comfortable reading ability. In light of the modern advances achieved in ophthalmic diagnosis and therapy, and because a significant lack of comparability between reading charts still exists, there is an increasing need for a worldwide standard in the form of a norm for diagnostic reading charts. Already, applied advancements such as digital print, which allow a calibration of the print sizes of reading charts in correctly progressing geometric proportions by using the actual height of a lower case "x" in millimeters (x-height), and psychophysically standardizing reading charts and their test items by applying modern statistical methods have significantly contributed to establishing a norm for reading charts. In 2020, a proposal of the British delegation was accepted by the International Organization for Standardization (ISO) group "Visual Optics and Optical Instruments," and a working group was established. Bearing in mind the efforts of the ISO with regard to an international norm, this review article is intended to (a) give an overview of the historical background and related normative approaches for diagnostic reading tests used in ophthalmology and optometry, (b) explain psychophysical and technical concerns, and (c) discuss the possibilities and limits of concepts that seem relevant to developing a modern standard for reading charts.