RESEARCH ON KEY TECHNOLOGIES FOR IMPROVEMENT OF MEASUREMENT ACCURACY OF STEREO DEFLECTOMETRY

Obtaining three-dimensional (3D) shape data of specular surfaces plays an increasingly important role in the quality control and function evaluation of high value-added industry, such as space, automobile, Photovoltaics, integrated circuits and so on.

In recent years, stereo deflectometry has been widely studied and applied for obtaining form information of freeform specular surfaces. Theoretically, the global form measurement accuracy of stereo deflectometry can be up to nanometre. However, the sources of errors limit the measurement accuracy of the current stereo deflectometry application at the scale of submicron.

To this end, this thesis documents the design and development of the calibration methods, error analysis and compensation in the field of stereo deflectometry. To limit the influence of system distortion, a novel holistic calibration technique utilising iterative distortion compensation algorithm has been designed and developed. A search algorithm with an objective function has been developed to solve the low-accuracy initial value problem caused by image distortion and imaging model error. With the intention of decreasing the impact of the phase error in stereo deflectometry, a novel imaging model has been explored the nexus between phase inaccuracy and gradient error. The period of fringe displayed on displaying screen and pixel size of the screen has been studied to augment measurement accuracy through taking into account their impact on sampling phase inaccuracy and gradient miscalculation. In addition, four geometric parameters of a stereo deflectometry system are analysed and evaluated. These are the distance between the main camera and the measured object surface, the angle between main camera ray and surface normal, the distance between the fringe-displaying screen and object and the angle between the main camera and the reference camera. The influence of the geometric parameters on the measurement accuracy is evaluated.

A stereo deflectometry system is designed, optimised and calibrated based on the investigation of this thesis. Two evaluation experiments have been conducted and experimental results indicate the system’s measurement accuracy can achieve tens of nanometres.