Which instrument is used specifically to measure the posterior optical zone diameter?

The instrument that is specifically used to measure the posterior optical zone diameter is the measuring magnifier. This device is designed to provide an accurate assessment of the lens's geometry, specifically the curvature and diameter of the posterior optical zone. In optics, understanding these measurements is crucial for fitting contact lenses properly and for evaluating the lens design's performance.

The measuring magnifier typically features lenses that allow for precise observation and measurement, making it an essential tool for opticians and lens manufacturers. This ensures that lenses will provide optimal vision correction and comfort by matching the contours of the eye.

In contrast, some other instruments do serve different purposes. The radioscope is used for measuring refractive errors, while a profile analyzer can evaluate the profile and surface of lenses but not specifically measure the posterior optical zone diameter. A lensometer is primarily used for determining the prescription of finished lenses and verifying power but does not provide the detailed measurements of the zone diameters. Therefore, the measuring magnifier stands out as the specific tool needed for this measurement, highlighting its importance in lens fitting and assessment.