Which measuring tool is best for determining segment height accurately?

The best tool for determining segment height accurately is the actual frame to be worn. When fitting progressive or bifocal lenses, the segment height is crucial for achieving optimal vision and comfort. Measuring segment height requires the specific frame that the lenses will be placed in because the position and shape of the frame can significantly affect how and where the segment should be positioned.

Using the actual frame allows the optician to accurately assess how the lenses will align with the wearer's eyes in their natural position. This ensures that the segment of the multifocal lens aligns appropriately with the pupil height during normal use of the glasses. Factors such as frame style, shape, and how the frame sits on the patient's nose and ears can only be accurately captured by using the frame itself.

While the other tools have their specific uses—like a pupilometer for measuring pupil distance or a lens clock for determining the curvature of lens surfaces—they do not provide the necessary context for segment height as effectively as the actual frame does. The segment height measurement is ultimately tailored to the individual patient's frame for optimal functionality and comfort.