What does the slot gauge measure?

The slot gauge is specifically designed to measure the diameter of rigid contact lenses. This measurement is crucial for ensuring that the lens fits properly on the eye and aligns with the cornea's geometry. Accurate diameter measurement helps to avoid complications, such as displacement or decentration of the lens, which can lead to discomfort and decreased vision quality for the wearer.

In contrast, the other options pertain to different aspects of lens assessment. While curvature, thickness, and astigmatism are all important factors in lens design and fit, they are measured by different specialized tools and methods, such as a radiuscope for curvature or a caliper for thickness. The slot gauge's primary function is to provide a quick and accurate way to determine the diameter of a rigid lens, making it especially valuable in contact lens fitting and manufacturing.