What is a Datum?

Commonly used reference datums and their intended purpose

A datum is a plane, a straight line, or a point that is used as a reference when processing a material or measuring the dimensions of a target.

Datum references are critical in optical metrology because it helps establish a coordinate system for assessment of UUT alignment prior to taking a measurement.

• Datum features of an optic are used to define a reference geometry and specify a tolerance (i.e. parallelism, perpendicularity, concentricity) relative to it.
• A datum should be large enough to be physically located for use in measurement of these tolerances.
• Well-defined datum features are related to how the optic is mounted during manufacture and/or final application.

Examples of commonly used datum references for optics are shown below: