It seems sometimes that there are few challenges left in Fizeau interferometry. Within the span of a lifetime, we moved from fringe counting, to fringe tracing software, to phase shifting. There were beam expanders and families of products with interchangeable accessories. Radius of curvature measurements have come a long way. Then there were instantaneous measurements, and—with 4D’s dynamic interferometry® breakthrough—there are now single frame, single-image phase-shifted measurements. But one challenge remained.
Since the early days of laser interferometry, optical manufacturers have depended on Fizeau interferometers as the workhorse of the polishing shop. They’re accurate and extremely flexible. One of the key advantages of the Fizeau configuration is that the errors introduced by the internal optics are minimized because the test and reference beams are common path through the imaging optics. That ensures you’re measuring just the flaws of the surface under test. Fizeaus are flexible, because their structure makes it possible to make many different kinds of measurements: flat surfaces, spheres, corner cubes, and wavefront transmission, to name a few.
The biggest disappointment
With the proliferation of zoom lenses last century, there was an expectation that optical systems could easily integrate them. Putting a zoom function inside a Fizeau would produce obvious benefits, as the photo here illustrates: measuring a smaller-than-aperture optic means you’re only getting surface data off however many pixels intersect the image of the lens. It means you have fewer data points per measurement.
Particularly when measuring spheres, having the additional pixels on the surface means higher tolerance of slopes: you can measure steeper spheres, and more reliably.
But there was a significant drawback in the initial off-the-shelf zoom lenses integrated in early Fizeau designs and used in an incoherent imaging configuration. These lenses were adequate for their use with video cameras, but in a Fizeau, they degraded the optical resolution of the imaging path of the Fizeau instrument. The result was a blurred image, uselessly sampled by more pixels!
When the available pixel density of the implemented cameras was SVGA or less, these zoom lenses were a good compromise. However, with the advent of megapixel and larger detectors, the situation changed. The early implemented zoom lenses were even less acceptable.
As alternatives, manufacturers sometimes offered Fizeaus with a few fixed-focal-length (FFL) positions, where the interferometer was acceptably precise at discrete magnifications. Others offered “digital zoom”, a technique of enlarging the sensor image to fill the field of view, but without changing the focal angle of the optics. It’s a bigger image, but not actually magnified optically onto the sensor—called “empty magnification”. These alternatives’ imaging quality and ability to measure mid-spatial frequency features, exceeded the capability of systems with optical zoom. But you couldn’t get a true continuous-zoom Fizeau, varying the focal length infinitely along a range. Too hard. “Impossible,” it was said.
Impossible, until now.
The first ever continuous optical zoom Fizeau phase-shifting interferometer (without degrading the image) is now in commercial production at 4D Technology. Optical zoom means the image magnification is changed before it reaches the sensor. You can get great measurements, using magnifications from 1x to 6x. By using the internal optical zoom capability to more fully frame your optics, you maximize your datapoint resolution. Or, use the zoom to see features in greater detail.
The AccuFiz systems with zoom provide 3X of real zoom without imaging degradation! Here’s a short video clip of the system zooming. With the added internal optics, it’s a handful of inches longer than our regular AccuFiz model.
Impossible? Not anymore. We’re proud to be at the leading edge of metrology improvement and innovation.