Optical interferometers use a helium-neon laser with transmission spheres to compare a reflected wavefront from an asphere surface to a reference spherical wavefront. A CMM is able to move in three dimensions while collecting data points along the asphere surface. Coordinate measuring machines (CMM) apply a similar concept by touching the optic with a ruby or silicon carbine sphere. A profilometer uses a fine stylus to drag a diamond or ruby tip over the surface, resulting in a high resolution curved profile. A contact profilometer is one of the most common measurement technologies used in asphere manufacturing. Many different techniques and devices are used to qualify an asphere surface during fabrication. Surface metrology serves as feedback into deterministic grinding and polishing platforms. Metrology of asphere surfaces is critical in the precision optics industry.
In addition, accuracy, repeatability, and machine qualification will be discussed. Form and figure maps of a flat, a sphere, and an asphere using UltraSurf will be presented with comparisons to interferometry. Validation of the system accuracy, repeatability, and methodology must be performed to trust the measurement data. There are multiple probe technologies available on UltraSurf, and each probe has strengths and weaknesses relative to the material properties, surface finish, and figure error of an optical component. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset.
It incorporates five air-bearing axes, linear motors, high-resolution feedback, and a non-contact probe. UltraSurf is a computer controlled, non-contact coordinate measuring machine.
OptiPro has developed UltraSurf to qualify the form, figure, and thickness of steep aspheric and freeform optics. Measuring these surfaces with common interferometers or profilometers can be difficult due to large surface slopes or unpolished surface texture. Accurate metrology during the grinding and polishing stages of asphere manufacturing will reduce time and cost. Advancements in optical manufacturing technology allow optical designers to implement steep aspheric or high departure surfaces into their systems.
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