What tool is used to measure the diameter of a rigid lens?

The appropriate tool for measuring the diameter of a rigid lens is a slot gauge. This instrument is specifically designed to accommodate the physical structure of lenses, allowing for precise measurement of their diameter. Slot gauges often feature a slot that can fit around or into the lens, providing an accurate reading of its size.

In contrast, other measurement tools like calipers, micrometers, and optical comparators serve different purposes or may not provide the same level of accuracy or convenience for lens diameter measurements. For instance, calipers are versatile measuring tools but may not fit as snugly around the contour of a lens without risking inaccurate readings. Micrometers provide precision for small dimensions but are typically used in applications requiring thickness measurements rather than diameter. Optical comparators are mainly used for visual inspection and measurement of part profiles to compare them against standards rather than direct diameter readings. Thus, the design and function of the slot gauge make it the most suitable choice for measuring the diameter of rigid lenses.