ANSI Z80.21-2020 applies to displays of optotypes for all clinical visual acuity measurement systems that use recognition of optotypes and that are designed for general use including optotypes printed on opaque media, those intended for transillumination, electronically generated or projected displays. It does not apply to special testing of visual acuity, e.g., low-vision, or reverse polarity (e.g., white optotype on a dark background).