Improving the calibration of phase measuring deflectometry by a polynomial representation of the display shape

Jonas Bartsch,Ralf B Bergmann,Michael Kalms

doi:10.1186/s41476-019-0116-1

Abstract

BackgroundPhase measuring deflectometry is a highly precise and full field metrology technique for specular surfaces based on the distortion of known reference patterns observed as a reflection at the surface under test. Typically, liquid crystal displays are employed to provide the required patterns. Due to a lack of research, these displays are used without sufficient calibration.MethodsIn this work, we present an enhanced calibration for phase measuring deflectometry, taking flatness deviations of the display surface into account. The display shape is modelled as a polynomial surface whose coefficients are determined by minimizing the retrace error in a global optimization procedure during calibration. This approach does not require any additional measurements or hardware. Improvements due to the enhanced calibration model are qualified experimentally using a flat and a spherical concave mirror.Results and conclusionThe model-based parameterization of the display surface yields significant improvement on both samples. The peak to valley (PV) of measured deviations on the plane mirror are reduced by 67% to 0.55 μm. Measuring the spherical sample without the display parameterization leads to a rather large shape deviation of 33.40 μm PV which is reduced by 94% to 1.98 μm. The viability of our approach confirms the dominant role of flatness deviations of the display surface as an error source in absolute shape measurement using phase measuring deflectometry.

Highlights

The precise, contact-free and full-field measurement of reflecting surfaces still poses a challenge to modern metrology which is in most cases answered by employing interferometric techniques
The NEC MD211G5, is a grayscale-monitor designed for applications in medical diagnostics, providing better contrast and less angle-dependencies of emissions compared to common consumer displays
Because display shape and location and orientation of the components of the setup are determined in a global optimization procedure, we yield overall different results for the setup geometry by calibrating with both model assumptions

Summary

Introduction

The precise, contact-free and full-field measurement of reflecting surfaces still poses a challenge to modern metrology which is in most cases answered by employing interferometric techniques. Coherencebased techniques usually suffer from some fundamental difficulties, for example a high sensitivity to vibration, rendering their application within an industrial environment a difficult task. The need for sophisticated components and specific optics leads to high costs. Deflectometry, in contrast, provides a robust and low-cost alternative. Deflectometry measures the shape of specular surfaces by tracing the reflection of a structured, incoherent. Phase measuring deflectometry is a highly precise and full field metrology technique for specular surfaces based on the distortion of known reference patterns observed as a reflection at the surface under test. Liquid crystal displays are employed to provide the required patterns. Due to a lack of research, these displays are used without sufficient calibration

Methods

Results

Conclusion