Which instrument can be used to verify that finished spectacles match the prescribed lens power and alignment?

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

Answer: (C)

A lensometer is the device used to verify that finished spectacles match the prescribed lens power and alignment. It works by projecting a known target through each lens and allowing you to read the actual sphere and cylinder powers, as well as the axis of any cylindrical correction. By centering the lens and checking the optical centers, you can confirm the lenses are positioned correctly within the frame and that the axis and power match the prescription. If there’s any prescribed prism, a lensometer can help verify its presence by showing how the lens redirects the beam. Other instruments measure different things—refractometers assess refractive index or patient refraction, tonometers measure intraocular pressure, and perimeters map visual fields—so they don’t provide the exact verification of lens power and alignment that a lensometer does.