Measurement of aspherical and free-form mirror using ultrahigh accurate 3-D profilometer (UA3P)

CMMs (Coordinate Measuring Machines) can measure the geometric tolerances of various mechanical components, but even the most accurate CMM have submicron accuracy, which makes it difficult to use them for evaluating optical components. On the other hand, more accurate μ-CMMs that can measure small optical components have also been developed. Our μ-CMM (UA3P) has three high-precision plane mirrors to construct three-dimensional orthogonal XYZ coordinates and uses a frequency-stabilized HeNe laser as the length scale. By scanning the surface of the object with a tactile-type probe, it is possible to measure the profile of optical components such as aspherical or free-form mirrors and lenses with an accuracy of less than 50nm. In this paper, by using a high accurate flat bar mirror whose profile has been measured by National Institute of Advanced Industrial Science and Technology (AIST) and making error table to correct the profile of the Z reference mirror installed in the machine, we could reduce the profile error of Z reference mirror within 24nm on the 400mm square in the XY plane. For tactile-type measurement machine, if the measurement range is large, the measurement time may be over an hour. Even in such the case of long-time measurements, we verified a method of configuring the profile of measured surfaces by referring to base axis data measured in a short period of time, and confirmed that profile waviness can be detected with 10nm precision by comparison with an interferometer. The advantage of a mechanical tactile measuring machine is that it can evaluate the profile without being influenced by the profile of the object. This means that our μ-CMM can perform highly accurate absolute profile measurement and evaluation for large-area free-form samples that are difficult to measure with an interferometer.