Model-free deflectometry for freeform optics measurement using an iterative reconstruction technique.

We present a novel model-free iterative data-processing approach that improves surface reconstruction accuracy for deflectometry tests of unknown surfaces. This new processing method iteratively reconstructs the surface, leading to reduced error in the final reconstructed surface. The method was implemented in a deflectometry system, and a freeform surface was tested and compared to interferometric test results. The reconstructed departure from interferometric results was reduced from 15.80 µm RMS with model-based deflectometry down to 5.20 µm RMS with the iterative technique reported here.

Instrument transfer function of slope measuring deflectometry systems.

Slope measuring deflectometry (SMD) systems are developing rapidly in testing freeform optics. They measure the surface slope using a camera and an incoherent source. The principle of the test is mainly discussed in geometric optic domain. The system response as a function of spatial frequency or instrument transfer function (ITF) has yet to be studied thoroughly. Through mathematical modeling, simulation, and experiment we show that the ITF of an SMD system is very close to the modulation transfer function of the camera used. Furthermore, the ITF can be enhanced using a deconvolution filter. This study will lead to more accurate measurements in SMD and will show the physical optics nature of these tests.

Measuring rough optical surfaces using scanning long-wave optical test system. 1. Principle and implementation.

Current metrology tools have limitations when measuring rough aspherical surfaces with 1-2 µm root mean square roughness; thus, the surface cannot be shaped accurately by grinding. To improve the accuracy of grinding, the scanning long-wave optical test system (SLOTS) has been developed to measure rough aspherical surfaces quickly and accurately with high spatial resolution and low cost. It is a long-wave infrared deflectometry device consisting of a heated metal ribbon and an uncooled thermal imaging camera. A slope repeatability of 13.6 µrad and a root-mean-square surface accuracy of 31 nm have been achieved in the measurements of two 4 inch spherical surfaces. The shape of a rough surface ground with 44 µm grits was also measured, and the result matches that from a laser tracker measurement. With further calibration, SLOTS promises to provide robust guidance through the grinding of aspherics.