What instrument is used to measure glasses prescriptions?

The lensmeter is the instrument specifically designed to measure the prescription of glasses. It can determine the sphere, cylinder, and axis of a lens, which are essential components for creating corrective eyewear tailored to an individual's refractive error. The lensmeter works by projecting light through the lens and measuring the way the light is refracted, allowing the operator to read the prescription directly from the instrument.

In contrast, other instruments listed serve different purposes in eye care. A keratometer measures the curvature of the cornea and is primarily used for fitting contact lenses or assessing conditions like astigmatism. The Maddox rod is a device used to assess binocular vision and can help evaluate misalignments in the eyes but does not provide any information about lens prescriptions. A refractor is used during the subjective refraction process to help determine an individual's glasses prescription by allowing them to experience different lens combinations, but it does not specifically measure existing lens prescriptions like a lensmeter does.