3. Form deviations

Two types of shape are essentially tested in optics and mechanics: planes on the one hand, and spheres and cylinders of revolution or buffers and rings on the other. These shapes give rise to two types of measurement: straightness and flatness on the one hand, and circularity on the other.

The smooth buffers and rings used as mechanical references give rise to circularity measurements around their axes and straightness measurements along their generatrices.