Freeform surfaces may be classified as complex geometrical features which have no invariance degree. The term 'freeform' is generally used for surfaces without any rotational symmetry. A variety of methods have been suggested for diffusely reflecting freeform surfaces metrology; however, it is much more difficult to measure the shape of specular freeform surfaces, since the surfaces themselves are not visible. We only see their effect from the reflection of incoming light. The measurement of specular surfaces can be done by CMM (Coordinate Measuring Machine) or interferometric means, as shown in Figure 1.
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