How do you measure lens thickness?

• A. Use a micrometer
• B. Measure with a ruler
• C. Use calipers
• D. Use a measuring tape

Answer: (C)

Measuring lens thickness requires an instrument that can accurately span the distance between the front and back surfaces of a lens with flat measurement faces. A Vernier caliper fits this need well: you place the lens between the jaws and read the distance directly, keeping the jaws perpendicular to the lens surfaces to avoid distortion. This gives a precise center or edge thickness in millimeters, depending on where you place the jaws. Rulers or tapes don’t offer the needed precision for ophthalmic lenses, and a micrometer is more challenging to use on a curved object and may not accommodate the lens shape as easily. So, calipers provide the practical precision and versatility needed for measuring lens thickness in usual practice.