A Landolt C, also known as a Japanese Vision Test, Landolt ring or Landolt broken ring, is an optotype

Optotype

An optotype is a standardized symbol for testing vision. Optotypes can be specially shaped letters, numbers, or geometric symbols. For instance, to determine visual acuity, optotypes of different sizes are presented to a person and the smallest size is determined at which the person can reliably...

, i.e. a standardized symbol used for testing vision. It was developed by the Swiss-born ophthalmologist

Ophthalmology

Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...

 Edmund Landolt

Edmund Landolt

Landolt, Edmund, was a Swiss ophthalmologist stationed in Paris, mostly known for a wide range of publications and his research in the field of ophthalmology.- First years :...

.

The Landolt C consists of a ring that has a gap, thus looking similar to the letter C. The gap can be at various positions (usually left, right, bottom, top and the 45° positions in between) and the task of the tested person is to decide on which side the gap is. The size of the C and its gap are reduced until the subject makes a specified rate of errors. The minimum perceivable angle of the gap is taken as measure of the visual acuity. It is generally practiced in the laboratory.

The stroke width is 1/5 of the diameter, and the gap width is the same. This is identical to the letter C from a Snellen chart

Snellen chart

A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart during 1862...

. The Landolt C is the standard optotype for acuity measurement in most European countries. It was standardized, together with measurement procedures, by the German DIN http://www.din.de, as DIN 58220 (now EN ISO 8596).