Freeform Metrology Using Deflectometry (2019)

Precision freeform optics are finding increased usage in new optical systems. Deflectometry is a non-null optical metrology method that has great application potential to be a measurement method for freeform and standard optics, offering a wide slope dynamic range and excellent accuracy and precision. The technique utilizes a known source which emits rays of light that are deflected by the unit under test (UUT) and are captured by a recording camera. By knowing the precise location of the source, the ray intercepts at the UUT, and the camera, the local surface slopes of the UUT can be determined and integrated to obtain a reconstructed surface. This study investigates three major topics to improve deflectometry and identify new Deflectometric based techniques. The first topic explored is a software-based method for an iterative surface reconstruction process. Deflectometry relies on determining the local surface slopes of the UUT by precisely knowing the ray intercept locations at the surface. Any error in the assumed surface model therefore directly reduces the reconstructed surface accuracy. A new processing method was developed called model-free deflectometry, which requires no optical surface model, and instead iteratively reconstructs the optical surface leading to improved final reconstruction accuracy. The method was used to reduce departure of a freeform optic from interferometric results from 15.80 𝜇m root-mean-square (RMS) using model-based deflectometry down to 5.20 𝜇m RMS with the model-free method developed. Further, most of the 5.20 𝜇m RMS residual departure was explained using a simulation to model the inherent noise present from hardware limitations. The second topic is a deflectometry system configuration which generates a virtual 2π steradian measurement volume, enabling full aperture deflectometry measurements of previously unmeasurable flat and convex freeform optics. The technique utilizes a source tilted over the UUT, enabling at least a partial aperture test. However, by clocking the UUT, a series of virtual sources are generated, which when considered as a whole, create a virtual source enclosure around the UUT allowing for a full aperture test. The method was shown to have accuracy similar to an interferometric test for a fast F/1.26 convex sphere and successfully tested a highly freeform Alvarez lens. Lastly, a power scalable, time-modulated high stability infrared source is explored for infrared deflectometry. The new source is an integrating box design with a precision emission area machine cut into an aluminum box. The light sources are modular high-efficiency resistive alloy membranes held in small caps. The design allows for power scaling by adding or removing caps from the source design. The caps are powered in parallel and are modulated at approximately 1 Hz to allow for signal isolation, thereby greatly improving signal to noise ratio. The new source was compared with a traditional tungsten source, both run at the same power output, and the source stability and geometry compared. Several common optical surfaces were tested with both sources to compare the accuracy and precision of the sources. It was found that the integrating box features a significant improvement in performance.