Which instrument is used to measure the power of spectacle lenses?

The lensometer, sometimes referred to as a vertometer or lensmeter, is the correct instrument used to measure the power of spectacle lenses. This instrument allows the user to ascertain the prescription of a lens by determining the focal length, which is vital for ensuring that the lenses provide the intended corrective power for the patient's vision.

While the autorefractor is primarily used to measure the refractive error of the eye and help determine the initial prescription needed, it does not measure the power of spectacle lenses directly. The keratometer measures the curvature of the cornea, which helps in fitting contact lenses but is not used for spectacle lenses. The pachymeter measures the thickness of the cornea, typically used in assessing conditions such as glaucoma and post-surgical evaluations, but again, it does not relate to measuring lens power. Thus, the lensometer stands out as the specific tool designed for this particular purpose.