This Part of BS4274 specifies minimum requirements for test charts and test types employing upright sans-serif capital letters for use in the clinical determination of distance visual acuity.
NOTE Unless otherwise indicated by the context, the word \'letter\' includes, in any orientation, the single letter C (Landolt or broken ring) and the single letter E (illiterate E).
This Part of BS4274 is primarily intended for clinical measurements but may be used for other purposes such as screening, certification and licensing. It is not applicable to chart projectors, charts or types designed for specialized use (e.g. crowding symbols, lower case letters, isolated letters, low vision evaluation and contrast sensitivity testing).
NOTE This British Standard makes no reference to surrounding luminance because this relates to the conditions in the consulting room or clinic itself and this is outside the Scope of this British Standard. Nevertheless, the level of surrounding luminance can seriously affect the visual acuity scores obtained, for example, by producing glare or excessive contrast. Attention, therefore, should be paid to the level of surrounding luminance when installing and utilizing these test charts.