Spherometers

A spherometer is a precision instrument used for measuring the radius of curvature of a spherical surface, such as that of a lens or a mirror. It can also be used to measure the thickness of a thin flat plate.

The most common type of spherometer consists of a central screw with a fine thread, mounted on a tripod frame. The central screw is equipped with a graduated head that allows for precise measurements. Each leg of the tripod is also pointed. When the instrument is placed on a flat surface, the three legs rest on the surface. The central screw is then lowered until its tip also touches the surface. The reading on the graduated head of the screw at this point is recorded.

To measure the radius of curvature of a spherical surface, the spherometer is placed on the surface such that the three legs are in contact with it. The central screw is then adjusted until its tip also makes contact with the surface. The reading on the central screw is noted. The spherometer is then moved to a flat surface, and the central screw is adjusted until its tip touches the flat surface. The reading is again noted. The difference between these two readings gives the height of the spherical cap that is in contact with the instrument. Using this height and the distance between the legs of the tripod (which is a known value), the radius of curvature of the spherical surface can be calculated using a specific formula.

Spherometers are crucial in optical manufacturing, research, and calibration where precise measurements of curvature are essential for creating and verifying optical components. They are also employed in various scientific fields requiring accurate surface profiling.