Which device is used to verify the accuracy of a finished spectacle lens by reading its prescription?

• A. Lensometer
• B. Refractometer
• C. Tonometer
• D. Phoropter

Answer: (A)

To verify that a finished spectacle lens matches the prescription, you use a lensometer. This instrument specifically reads the lens’s optical power, including sphere and cylinder values, and the axis for astigmatism. By projecting light through the lens and measuring how it focuses, the lensometer tells you the exact powers and alignment of the lens. It can also check where the optical center sits and detect any decentration or unwanted prism. Compare these measured values to the intended prescription; when they line up, the lens is correct.

The other devices serve different roles: a refractometer measures properties like refractive index rather than the prescribed power of a finished lens; a tonometer assesses intraocular pressure; a phoropter is used during an eye exam to determine the patient’s refraction, not to verify a completed lens.