Analysis and New Developments Towards Reliable and Portable Measurements in Deflectometry

摘要

Deflectometry is a powerful metrology technique that uses off-the-shelf equipment to achieve nm-level accuracy measurement. This process is typically made by scanning lines of pixels or encoding the surface slopes information with phase using sinusoidal waves. Various measurement techniques exist, centroiding and phase-shifting being the most accepted, but their sensitivities vary with experimental conditions. We demonstrate solutions based on various parameters such as uncertainty or efficiency. The results are presented in a decision matrix and merit function. The parameters can be varied to represent various conditions. In particular, we are interested in using deflectometry in a context of fast, affordable and robust. Since none of the existing methods perform well under those parameters, we introduce a new method using binary patterns. Binary Pattern Deflectometry allows almost instant, simple and accurate slope retrieval, which is required for applications using mobile devices. We detail the theory of this new deflectometry method and the challenges of its implementation. Furthermore, the binary pattern method can also be combined with a classic phase-shifting method to eliminate the need of a complex unwrapping algorithm and retrieve the absolute phase, especially in cases like segmented optics where spatial algorithms have difficulties. Finally, whether it is used as a standalone or combined with phase-shifting, the binary patterns can, within seconds, calculate the slopes of any specular reflective surface. However there is no portable device to quickly measure eyeglasses, lenses, or mirrors. To complete the study, we present an entirely portable new deflectometry technique that runs on any Android™ smartphone with a front-facing camera. Our technique overcomes some specific issues of portable devices, like screen non-linearity or automatic gain control (AGC). We demonstrate our application by measuring an amateur telescope mirror and simulating the faulty Hubble Space Telescope primary mirror. In both cases, the application found the same amount of aberrations that were measured with an interferometer. Our technique can, in less than a minute, measure the sag errors of curved surfaces smaller than 50 nm RMS, simply using a smartphone.