In the Lighthouse method, the working distance is expressed in which unit?

In the Lighthouse method, the working distance is expressed in which unit?

• A. Meters
• B. Diopters
• C. Centimeters
• D. Inches

Answer: (B)

In this method, the distance from the eye to the instrument is tied directly to refractive power, so the working distance is expressed as diopters. Diopters are the reciprocal of distance in meters, which lets you convert the viewing distance into a lens power that you then use to determine the refraction. For example, a common 50 cm working distance equals 0.50 m, which is 2 diopters. Thus, the unit used is diopters, because the measurement serves to indicate refractive power rather than just a physical distance.

In this method, the distance from the eye to the instrument is tied directly to refractive power, so the working distance is expressed as diopters. Diopters are the reciprocal of distance in meters, which lets you convert the viewing distance into a lens power that you then use to determine the refraction. For example, a common 50 cm working distance equals 0.50 m, which is 2 diopters. Thus, the unit used is diopters, because the measurement serves to indicate refractive power rather than just a physical distance.