Chart construction and letter legibility/readability

Variation in legibility/readability between charts letters may be due to differences in chart construction and/or examination protocols and/or subject selection. An examination of legibility/readability of letters that are common to two chart constructions was undertaken. Identical examination and subject selection protocols for both charts were employed. It was found that ranking of error frequencies for some letters varied between the Bausch & Lomb Compact Acuity Projector and Bailey-Lovie charts. Despite many similarities in ranking, the differences found for the letters D and E indicate that caution should be observed before assuming that legibility/readability determined with one letter chart would apply when using another. Lack of standardisation for chart construction, as well as measurement methods and conditions of measurement, invalidate inter-practice comparisons for visual acuity.