Elsevier

Measurement

Volume 186, December 2021, 110140
Measurement

A novel chromatic confocal one-shot 3D measurement system based on DMD

https://doi.org/10.1016/j.measurement.2021.110140Get rights and content

Highlights

  • DMD is used as a beam splitter in the chromatic confocal system.

  • The full-field measurement can be realized by one-shot 3D photography.

  • A mathematic model was established to correlate the height of each point on the target surface with the color information.

Abstract

Aiming at the problem of low measurement efficiency of conventional single-point chromatic confocal technology, a one-shot chromatic confocal measurement system based on DMD (Digital Micromirror Device) is presented in this paper. This system was set up by applying a polychromatic light source, a color camera, a dispersive lens, and DMD. Firstly, the DMD was used to modulate the polychromatic light source to obtain a point light array. Secondly, a series of focal points were generated by the dispersive lens, and the corresponding relationship between color information and height was established. Lastly, by using an optimized HSI model, the height information of the sample could be obtained, and a 3D profile could be reconstructed accordingly. The experimental data showed that the proposed system can measure and restore the 3D topography of the measured surface by one-shot photography, and the measurement accuracy can reach the micron level.

Introduction

With the rapid development of the intelligent manufacturing industry, precision three-dimensional (3D) surface topography measurement has attracted more research interests. In general, there are two kinds of methods in 3D surface topography measurement, contact [1], [2] and non-contact [3], [4] methods. As a non-contact method, laser scanning confocal microscopy (LSCM) has become a powerful detecting method in various microscopic imaging applications, especially when high precision is required. By effectively blocking the scattered lights with pinholes, LSCM can achieve superior resolution for 3D surface topography measurement [5], [6], [7]. However, precise vertical scanning of each measurement point is needed for 3D surface topography measurements with LSCM, which has severely restricted the measurement efficiency.

The chromatic confocal microscopy (CCM) technique was proposed and evolved in recent years to overcome this limitation [8], [9], which is widely used in roughness measurement[10], displacement and thickness measurement[11], [12], biological medical treatment[13], [14], industrial manufacturing[15], [16] and other aspects. Instead of mechanical vertical scanning, a series of focal points with different wavelengths along the axial direction was produced in a CCM system. Therefore, the axial position or displacement of a specimen can be measured by detecting the wavelength. Boettcher [17] combined the chromatic confocal microscope and the spectral measurement technology to design a chromatic confocal spectral coherence topography system, which can achieve high-resolution single-lens topography measurement. Zhao [18] proposed a new method based on the chromatic confocal technique, to fuse the composite standard of multi-heterogeneous sensors in high precision coordinate measuring machines (CMM), which made up for the deficiency of reference balls. Chen [19] proposed a broadband differential confocal method that exploited a novel double-slit chromatic confocal microscopy for one-shot microscopic 3D surface measurement.

Many efforts have been made to improve the performance of CCM, including the optimization of chromatic dispersion devices, and the improvement of the focal point detection algorithm. Cui [20] proposed a dispersion-focus separation design scheme to enable the modulation of measurement range and resolution. Bai [21] introduced a super-resolving pupil filtering element and a modified peak-extraction algorithm to improve the positioning capabilities of a chromatic confocal displacement measurement system. Other efforts were made to improve the measurement efficiency of CCM. Chun [22] presented a chromatic confocal microscope with transverse point-beam scanning to measure the three-dimensional surface without the need for longitudinal mechanical translation. Taphanel [23] used a monochrome line scan camera with six rows as a detector to improve the measurement efficiency.

However, because most available CCM systems were developed based on single-measuring points, measurement efficiency was still restricted by its lateral mechanical scanning. Therefore, full-field measurement by optical beam splitter has become the key technique of rapid CCM measurement. Chen [24] integrated the DMD technique into a CCM system to eliminate the necessity of both vertical and lateral mechanical scanning structures. Hillenbrand [25] built a CCM system based on a pinhole array and analyzed its performance for parallel measurement systems.

Instead of relying on highly complicated devices and assembly processes, another approach was developed by the authors’ team. In this study, DMD was used as an optical beam modulating device, and the vertical and lateral information of the measured object was captured by the system. Through one-shot imaging, the system can reconstruct the 3D topography of the measured surface.

Section snippets

Principle and construction of a chromatic confocal system

The core principle of the chromatic confocal technique is the spectral coding technique, as shown in Fig. 1.

According to the principle of axial dispersion, the polychromatic light beam will be dispersed along the optical axis direction by a dispersion system. The lights with different wavelengths will be focused on the different axial positions. This wavelength distribution can be expressed as a function of λ(z), where z is the axial position, and the output result of the light intensity in

Principle of the color conversion algorithm

In this study, a color conversion algorithm was developed, which mainly involves the RGB color space and HSI color space, as shown in Fig. 5.

The HSI color model starts from the human visual system. The three parameters which are H (hue), S (saturation), and I (intensity) are used to describe colors. The H value, which is a wavelength-dependent parameter, can describe the spectral color and its range is [0, 2π]. Each spectral color has one angle for itself. For example, the angle of spectral red

Construction of the measurement setup

The chromatic confocal measurement experimental platform based on DMD was established, as shown in Fig. 6. The computer was used to connect the color camera to capture the image and control the point array of the DMD. The other components used in this setup are listed in Table 1.

Calibration experiment

In the calibration experiment, a high-reflectivity plane mirror was used as the measured object placed on the platform, which is controlled by the drive motor. As the mirror gradually moves away from the objective along

Conclusion

In this study, a novel one-shot full-field chromatic confocal measurement system based on DMD was developed. To improve the measurement efficiency by eliminating mechanical scanning, a dispersive lens was developed for axial measurement and a DMD was applied to modulate the illumination light beam as an alternative of lateral scanning. In this way, the full-field measurement can be realized by one-shot 3D photography, and the feasibility was proven by theoretical analysis. The calibration test

CRediT authorship contribution statement

Qing Yu: Conceptualization, Methodology, Data curation. Yali Zhang: Writing – original draft, Writing – review & editing. Yi Zhang: Visualization, Investigation. Fang Cheng: Supervision. Wenjian Shang: Validation. Yin Wang: Software.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

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