Which optical instrument is used to measure interpupillary distance (PD)?

The correct instrument used to measure interpupillary distance (PD) is a pupillometer. This device specifically measures the distance between the center of one pupil to the center of the other pupil, which is crucial for fitting glasses and ensuring proper alignment of lenses in optical devices.

A pupillometer can provide precise measurements, which are essential for optical prescriptions, especially in cases where the fitting may affect the patient's vision quality. The accuracy of PD measurements directly influences the effectiveness of corrective lenses, as they need to be positioned correctly relative to the individual's pupils.

Other instruments mentioned have different functions. A refractometer is designed to measure the refractive index of glasses and solve vision problems; a tonometer is used to measure intraocular pressure for glaucoma screening; and an ophthalmoscope is used for examining the interior of the eye, particularly the retina. None of these tools are utilized to measure interpupillary distance, highlighting the specialized use of the pupillometer in the realm of optics and vision care.