This International Standard specifies a range of Landolt ring optotypes and describes a method for measuring distance visual acuity under daylight conditions for the purposes of certification or licensing. It is neither intended as a standard for clinical measurements nor for the certification of blindness or partial sight. For the purposes of measuring visual acuity, the standard optotype should be used. For clinical use, see the recommendation prepared by the Visual Functions Committee of the International Council of Ophthalmology[1].