Near optical coaxial phase measuring deflectometry for measuring structured specular surfaces

Phase measuring deflectometry (PMD) is an important technique for the form measurement of specular surfaces. However, the existing stereo-PMD techniques have noticeable weaknesses for structured specular surfaces measurement due to the optical axis of the imaging system must have a notable intersection angle with the optical axis of the display system according to the law of reflection. This leads to the imaging sensor and the fringe display screen must be located on the opposite sides of the normal of the surface under test (SUT), which results in large system volume and measurement shadows when measuring discontinuous specular surfaces. In this paper, we propose a novel near optical coaxial PMD (NCPMD) by utilizing a plate beamsplitter. With the assistance of plate beamsplitter, the optical axis of display screen can be configured much closer to the optical axis of the imaging system which makes the system more compact and has significantly reduced volume compared with the conventional PMD configuration. Moreover, imaging sensors in the proposed configuration can perpendicularly capture the SUT, which can drastically decrease measurement shadows caused by discontinuous structures on the SUT and increases measurement efficiency. A comparison between the proposed NCPMD and the conventional PDM is studied by measuring a specular step to show the advantage of the proposed configuration in reducing measurement error caused by structure shadows. A portable NCPMD prototype with stereo imaging sensors is developed and verified through experiments. Experimental results show the portable prototype has comparable measurement accuracy with the existing PMD techniques while has obviously advanced performances for portable and embedded form measurement, such as small system volume, and light weight.