Radius of curvature is a critical parameter to measure in the manufacturing of micro-refractive elements. It defines the power of the surface and provides important information about the stability and uniformity of the manufacturing process. The radius of curvature of an optical surface can be measured using an interferometer and radius slide where the distance is measured as the surface is moved between the confocal and cat’s eye positions. However, the radius of curvature for micro-refractive elements can be on the order of a few hundred microns and the uncertainty in the measurement due to stage error motions can become a significant portion of the tolerance. Typically the radius slide is calibrated using an artifact, but the radius of the artifact must be traceable to the base unit of length and the calibration is subject to misalignment errors. Alternatively, the stage error motions can be measured with standard machine tool metrology techniques and used to correct the errors in the radius of curvature measurement. This paper details the implementation of a directly traceable radius of curvature measurement on a micro-interferometer, including alignment procedures, measurement of stage error motions, displacement gauge calibration, and data analysis strategies.
We have focused on measurement needs for micro-refractive lenses and have developed a flexible and compact micro-interferometer that can be used to measure lens radius of curvature and form errors. Transmitted wavefront and back focal length measurements can be easily added to the instrument. This instrument addresses measurement needs for micro refractive lenses. The interferometer is based on a Mitutoyo metallurgical microscope and operates with a 633 nm helium neon source. The radius of curvature measurement is directly traceable, meaning an external artifact is not required for calibration. This requires a careful mechanical design, a detailed alignment procedure with estimates of alignment uncertainties, and stage error motion characterization with estimates of uncertainties. The instrument can also be used to measure some diffractive components and mold form errors. We describe the instrument in this paper and discuss design goals and measurement specifications.