Spherometers are used to measure the radius or curvature of spherical surfaces such as optical lenses, spherical mirrors, and balls. These small, high-precision optical test instruments are also used to measure the thickness of microscope slides, or the depth of slide depressions.
Spherometers consist of a micrometer screw, a vertical scale or graduated post, and a tripod or table. The screw’s head is fitted with a graduated disk for measuring fractional turns. With divisions marked in millimeters (mm) or fractions of millimeters, the vertical scale is used to measure the height or depth of a surface’s curvature.
The tripod, typically a plastic or metal base with three legs, can be adjusted to accommodate lenses of various sizes. The tripod’s central leg is mechanical and pressure-sensitive, and can be raised or lowered as necessary. Spherometers use special rings and precision flats to provide a platform for optical testing. Spherometer rings are made of heat-treated stainless steel or other high-quality metal materials, and are then ground and fine-lapped.
Often, the measuring accuracy of a spherometer ring is traceable to a standard from the National Institute of Standards and Technology (NIST). When a precision flat is put on the spherometer ring, a reference or zero position is created. When the sample is measured, the spherometer calculates the saggital height of the curved surface. Using the relationship between sag and radius, the radius of curvature can be determined. Although all spherometers are precise, products differ in terms of accuracy, which is typically expressed as a percentage (%).
Spherometers carry product specifications for both measurement range and accuracy. Measurement range parameters include radius convex, radius concave, and the diameter of the part under test. Spherometers that are designed for use with linear encoders may also list specifications such as linear encoder travel, a measurement expressed typically in millimeters (mm). As their name suggests, linear encoders sense and digitize linear position change for positional measurement and feedback to control systems. Accuracy parameters for spherometers describe the resolution and accuracy of the linear encoder, as well as the accuracy of the measurement of the sample’s radius of curvature.