During design and manufacturing, optical systems and lenses are toleranced and tested to ensure the smallest possible performance error. Matching most optics manufacturing companies’ capabilities, lenses are traditionally toleranced with individual surface specifications — surface power and irregularity, or form, error. These tolerances and the associated tests control performance of a single surface, not the entire lens. Because lens designs are built around transmission characteristics such as spot size and RMS wavefront error, the performance of the entire lens, not the individual surfaces, is the true target. Transmitted wavefront error (TWE), which is the error in transmission of light through a lens, is the true target. For aspheric surfaces, traditional single-surface, three-dimensional surface form metrology is not easy. Aside from testing the true target, it may be easier and faster to make use of TWE for aspheric lenses. Using innovative metrology and developing a feed-forward manufacturing strategy, tolerancing and testing TWE can reduce risk in optical designs, improve performance and reduce cost and lead time.
As aspheric lens precision increases, metrology must also become more accurate. For molded and fire polished aspheres, with surface form accuracies ranging from 50-200 microns, two-dimensional profiles are sufficient. Two-dimension profilometry is quick, uncomplicated and can be used to verify surface form accuracies as tight as one micron (as with commercial grade, ground and polished aspheres). However, aspheric lenses are commonly used for imaging applications and are typically toleranced with surface form errors less than 0.633 microns. Two-dimensional surface profiles cannot be used to certify to this accuracy. In order to test a surface’s form and prove it accurate to less than one micron, three-dimensional surface metrology is required.
Reflected wavefront three-dimensional surface form testing can be accurate to less than 0.633 microns, but it often requires expensive, long lead time holograms or diffractive optical elements, a powerful interferometer, and labor intensive setup by a skilled technician. The test setups are often lock-and-key, meaning a very expensive hologram or diffractive element will work only for the one aspheric form it was designed to test. More versatile three-dimensional asphere metrology solutions are available, but they are limited to measuring certain groups of aspheres based on aspheric departure or slope changes.
A step beyond reflected wavefront three-dimensional surface form metrology is TWE testing. Taking into consideration material defects, wedge, thickness variation, irregularity and power, TWE is the sum of all transmission errors. Virtually all errors can be checked and with a feed-forward process, corrected at the same time. Testing and correcting aspheric lenses as they are used, in transmission, tests the true performance of the lens and can eliminate the need for expensive and long lead time holograms or diffractive elements. The test can also be quicker and easier than a reflected wavefront test.