An optotype is a vision assessment tool with a series of letters or symbols presented in a standardized format. Medical providers like optometrists and pediatricians ask a patient to look at the optotype and report what they see. This information can be used to determine visual acuity to see if the patient needs to wear corrective lenses. The Snellen chart, using a series of letters that progressively decrease in size, is a well-known version, but other types of charts are available as well.
Standardization can be important for vision assessment because it allows the medical professional to compare with a known baseline. Each optotype is optimized for visibility, with letters or shapes that should be visually crisp when the patient has good vision and sits at the recommended distance. If the patient has trouble discerning between or identifying shapes, this is an indicator that the patient’s vision needs correction.
In young children who cannot read, an optotype uses shapes. The child is asked to describe or identify the shapes in a vision test. Early intervention for children with suspected vision problems can be important. Likewise, adults who are not literate may be tested with a shape-based chart instead of one using letters, which they might find difficult. Several variations of shape-based designs are available for use in these settings.
Administration of a vision test with an optotype must be performed consistently for valid results. The room should have appropriate light levels, and the subject must be seated at the recommended distance. Testers should not bias subjects by isolating specific letters in an attempt to help them see better, and may not prompt them with suggestions about which letters they might be seeing. As the subject goes through the chart or a series of presented flashcards, the tester can take notes about the subject’s visual acuity to use later.
Optotypes also provide a quick tool for assessing visual acuity in environments like a department of motor vehicles. Someone applying for a driver’s license may be asked to pass a quick eye examination administered by an employee of the agency. The applicant views a chart and reads off the letters or reports the symbols, allowing the employee to determine if the applicant can drive safely. In cases where people need their glasses to see comfortably, they may receive a license with a corrective lens restriction, indicating they must wear glasses or contacts to drive.