Which instrument is used to accurately measure pupillary distance?

The instrument used to accurately measure pupillary distance is the corneal reflex pupillometer. This device employs infrared light to locate the center of each pupil and calculate the distance between them with precision. This measurement is essential for ensuring that optical prescriptions are centered correctly, which contributes to the comfort and effectiveness of eyeglasses or lenses.

The other instruments mentioned serve different purposes. A lensometer is primarily used to measure the prescription of existing lenses, an optometer is concerned with assessing visual acuity, and a pupil gauge is a simpler tool that lacks the advanced measuring capabilities of a pupillometer. While a pupil gauge can provide a rough estimate of pupillary distance, it does not match the accuracy and reliability of the corneal reflex pupillometer, thus making it the preferred choice for precise measurements.