3D Measurement of Structured Specular Surfaces Using Stereo Direct Phase Measurement Deflectometry

With the rapid development of modern manufacturing processes, ultra-precision structured freeform surfaces are being widely explored for components with special surface functioning. Measurement of the 3D surface form of structured specular objects remains a challenge because of the complexity of the surface form. Benefiting from a high dynamic range and large measuring area, phase measurement deflectometry (PMD) exhibits great potential in the inspection of the specular surfaces. However, the PMD is insensitive to object height, which leads to the PMD only being used for smooth specular surface measurement. Direct phase measurement deflectometry (DPMD) has been introduced to measure structured specular surfaces, but the surface form measurement resolution and accuracy are limited. This paper presents a method named stereo-DPMD for measuring structured specular objects by introducing a stereo deflectometor into DPMD, so that it combines the advantages of slope integration of the stereo deflectometry and discontinuous height measurement from DPMD. The measured object is separated into individual continuous regions, so the surface form of each region can be recovered precisely by slope integration. Then, the relative positions between different regions are evaluated by DPMD system to reconstruct the final 3D shape of the object. Experimental results show that the structured specular surfaces can be measured accurately by the proposed stereo-DPMD method.

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IMPROVING 3D SURFACE MEASUREMENT OF MECHANICAL DETAILS BY STRUCTURED LIGHT USING HIGH DYNAMIC RANGE

Journal of Military Science and Technology ◽

10.54939/1859-1043.j.mst.74.2021.145-153 ◽

2021 ◽

pp. 145-153

Author(s):

Keyword(s):

Dynamic Range ◽

Structured Light ◽

High Surface ◽

High Dynamic Range ◽

Gray Code ◽

Surface Measurement ◽

3D Surface ◽

Projection Pattern ◽

High Dynamic ◽

Surface Gloss

The measurement methods using structured light have the advantage of being fast, accurate, and noncontact with the surface of the object. However, these methods have reached its limitation when measuring mechanical details with high surface gloss, due to the unpredictable reflection of incident rays after reaching to object’s surface that, consequently, leads to the simultaneous appearance of several regions with different brightness. To address this problem, we proposed a method of synthesizing extended dynamic range images based on changing the exposure time of the camera and adjusting the illumination of the projector light source so that 3D point coordinates in both bright and dark areas could be obtained through the process. The dual-camera structured light experimental model and the lightcrafter 4500 projector are synchronized through the trigger, using the gray code in combination with the line-shift projection pattern. Experimental results show that the proposed method can precisely reconstruct the 3D surface of mechanical details, while providing higher performance than the state-of-the-art methods.

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High-precision width measurement method of laser profile sensor

Sensor Review ◽

10.1108/sr-06-2019-0154 ◽

2020 ◽

Vol 40 (6) ◽

pp. 699-707

Author(s):

Yu Feng ◽

Wei Tao ◽

Yiyang Feng ◽

Xiaoqia Yin ◽

Na Lv ◽

...

Keyword(s):

Dynamic Range ◽

High Dynamic Range ◽

Logistic Function ◽

Imaging Method ◽

Height Measurement ◽

Content Type ◽

Accuracy And Precision ◽

Width Measurements ◽

The Stability ◽

Laser Profile

Purpose Although a laser profile sensor (LPS) can be used to measure dimensions, the “shadow region” generally degrades the accuracy and precision of width measurements. The accuracy and precision of such measurements should be improved. Design/methodology/approach In this paper, the authors propose herein a technique that combines high dynamic range (HDR) imaging with logistic fitting. First, a HDR image is composed of several images acquired with different exposure times, which augments the grayscale of the object profile and significantly reduces overexposure. Next, the profile is fit to a logistic function, which provides accurate and precise edge coordinates. Finally, given the edge coordinates, the object width is calculated. Findings To verify the stability of this logistic algorithm, the authors simulate different noise conditions and different degrees of incomplete edge data. In addition, the progressiveness of the algorithm is demonstrated by comparing the results with those of other algorithms and with the height measurement. Furthermore, the suitability of the system is verified experimentally. Research limitations/implications Because of the limitation of the condition of laboratory, in the experimental section, this paper cannot represent perfectly the industrial situation. It makes this section limited in demonstration. Originality/value In this paper, the results show that the measurement accuracy and precision of the width is improved and exceeds that of the height measurement. The proposed HDR imaging method with logistic fitting may be applied to LPS width measurements, which should significantly aid the development of LPSs.

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