Which device is used to measure the back vertex power of a lens?

The lensometer is the appropriate device for measuring the back vertex power of a lens due to its specific design and purpose in optometry. This instrument allows practitioners to accurately determine the optical power of prescription lenses by projecting light through the lens and evaluating the location of the focal point.

When using a lensometer, the technician places the lens in the instrument and aligns it so that the light passes through the optical center. The scale on the lensometer provides a clear reading of the power of the lens, which includes both the front and back surface curvatures. This measurement is critical for ensuring that the lenses will provide the correct vision correction for the patient.

Other options do not serve this function: a biomicroscope is primarily used for examining the anterior segment of the eye, a keratometer is designed to measure the curvature of the cornea, and a radioscope is not related to measuring optical power in lenses. Thus, the lensometer stands out as the specialized tool for determining the back vertex power of corrective lenses.