ANSI Z80.21-2010 pdf download Instruments – General-Purpose Clinical Visual Acuity Charts
3 Clinical optotype calibration
3.1 The reference optotype shall be the four-position Landolt Ring, or Landolt “C”, which is an interrupted circle whose stroke width and gap width are one-fifth of its outer diameter.
3.2 The reference optotype may be used for clinical visual acuity charts, but usually other optotypes,such as letters, numbers or symbols correlated with the reference optotypes, are used as clinical optotypes.
3.3 The size of each Landolt Ring is specified by the distance (in meters or feet) at which the gap width of that ring subtends one minute of arc.
3.4 Clinical optotypes shall give the same performance as the reference optotypes when presented as specified in this standard. Performance equivalence between the reference optotype and the clinical optotype shall be verified using the method given in Annex A of this standard and the requirement for equivalence is set at a difference in acuity averages of no more than 0.05 log units.
4 System requirements
4.1 Clinical optotypes are to be specified in terms of the distance at which the gap of the equivalent Landolt Ring subtends one minute of arc. For all charts, the manufacturer shall specify the conditions under which the optotypes meet the standard. For projected charts, it is appropriate to specify optotype size as an angular measure, usually by a Snellen fraction or decimal acuity based upon a specific test distance, which may be different from the actual test distance. For charts that are fixed in size and may be used at various test distances, it is appropriate to specify optotype size by the distance in meters (M units) or in feet, at which the gap of the equivalent Landolt Ring subtends one minute of arc.
4.2 The progression of optotype sizes shall be logarithmic and in the progression of 1 :1 0 0.1 , or 1 :1 .2589. The optotype series in table 1 follows this progression. In these acuity sequences, numbers have been rounded. Charts should be prepared so that the 20/20 row (6/6 or 1 .0) is precisely