What instrument measures the power of the spectacle lens?

The instrument that measures the power of the spectacle lens is the lensmeter. It is specifically designed to determine the lens prescription by measuring the focal length and curvature of the lenses, allowing for accurate calculations of their optical power.

A lensmeter works by projecting light through the lens and analyzing how the light is refracted. This enables optometrists and opticians to assess both the spherical and cylindrical power of corrective lenses. The precision of the lensmeter makes it an essential tool in both optical laboratories and clinics, facilitating the proper fitting of glasses for optimal vision correction.

In contrast, the other instruments mentioned serve different purposes. An ophthalmoscope is used to inspect the interior structures of the eye, particularly the retina, rather than measuring lens power. A keratometer measures the curvature of the cornea to help fit contact lenses and assess conditions like astigmatism but does not evaluate spectacle lenses. A retinoscope is utilized to estimate the refractive error of the eye by observing the reflection of light off the retina, thus it is focused more on eye examination rather than directly measuring lens power.