Freeform Metrology Using 2D Contact Profilometry and Specialized Fixturing

Advancements in the design and fabrication of optics are pushing the limits of metrology. Freeform surfaces are becoming more common through the provision of non-symmetric corrections to a system’s wavefront. This may result in improved performance with less optical elements in a system’s design. Fabrication tools have been developed, yet the measurement techniques and drawing specifications of these surfaces are not well defined. There are several approaches to characterize a freeform lens. Profilometry is a common method to measure the profile of an optical surface by using a calibrated stylus. 2D profilometer instruments are popular due to their affordability and versatility. An investment is required to increase a contact profilometer’s capability for 3D aspheric surfaces. This paper will focus on expanding a 2D contact profilometer’s capability by using a specialized fixture with references to map multiple tracings in software. In addition, the fixture will also characterize the centration of the optic. This paper compares the results found with various metrology instruments. The specialized fixture was measured with a 2D contact profilometer to provide full surface characterization of the freeform optic. An advanced 3D profilometer was used to provide measurements for evaluation. Included will be a discussion comparing the freeform data gathered from the different instruments to expected performance using a spherical lens.