Certified Paraoptometric (CPO) Practice Exam 2025

What unit of measurement is used for ophthalmic lenses?

1. Millimeter

2. Diopter

3. Centimeter

4. Degree

The correct answer is: Diopter

The diopter is the correct unit of measurement for ophthalmic lenses. It quantifies the optical power of the lens, which is determined by the curvature and thickness of the lens. The larger the absolute value of the diopter, the stronger the lens, whether for corrective purposes like nearsightedness, farsightedness, or astigmatism. This measurement is critical in optometry and optics, as it indicates how effectively a lens can converge or diverge light rays to improve vision for the patient. While millimeters and centimeters are used to measure various physical dimensions, such as the distance between the lenses or the size of the frame, they do not describe the optical power of the lenses themselves. Degrees, on the other hand, are often associated with angular measurements, such as the angle of strabismus, but they do not pertain to the measurement of lens power. Thus, diopter is the essential unit used in the context of ophthalmic lenses.