Landolt C

The Landolt C, also known as a Landolt ring or Landolt broken ring, is a standardized optotype used in ophthalmology and optometry as a measure of visual acuity. It is similar to the Snellen E, but instead of an "E" shape, it is a ring with a gap. The ring can be oriented in one of four (or eight in some versions) different directions, and the subject must indicate the direction of the gap.

The Landolt C is considered more accurate than some other optotypes because it minimizes the effect of letter recognition on the test result. The subject must simply perceive the location of the gap, reducing the influence of cognitive interpretation. The size of the ring, and therefore the size of the gap, is reduced until the subject can no longer reliably identify the gap's orientation. This determines the visual acuity.

The distance at which a person can correctly identify the orientation of the gap in a Landolt C ring of a specific size is used to calculate their visual acuity. Visual acuity is often expressed as a fraction (e.g., 20/20), and the Landolt C can be used to determine this fraction.

Different standards exist for the exact dimensions of the Landolt C, but generally, the width of the ring, the width of the gap, and the thickness of the ring are all equal to one-fifth of the outer diameter of the ring.

The Landolt C is used extensively in international visual acuity testing, particularly in studies comparing visual acuity across different populations or when language independence is important. It avoids potential literacy bias present in alphabet-based optotypes.