Digital holographic microscopy (DHM) is a topographic measurement technique that permits full-field, nondestructive, dynamic, quantitative amplitude, and phase-contrast imaging. The technique may realize the lateral resolution with submicron scale and the longitudinal resolution with subnanometer scale, respectively. Improving imaging quality has always been the research focus in DHM since it has a direct effect on the precise topographic measurement. In this paper, the recent progress on phase aberration compensation and coherent noise suppression is reviewed. Included in this review are the hologram spectrum’s centering judgment methods of side band in tilt phase error compensation, the physical and numerical compensation methods in phase aberration compensation, and the single-shot digital process methods in coherent noise suppression. The summaries and analyses for these approaches can contribute to improving the imaging quality and reducing the measurement error of DHM, which will further promote the wider applications of DHM in the topographic measurement fields, such as biology and micro-electro mechanical systems.

Collinear calibration is a typical and common method for a laser (heterodyne) interferometer, but it usually suffers from the influence of the tilt of the target retroreflectors and the dissymmetry of the optical paths during the calibration. This paper mainly analyzes and models the calibration error caused by the tilt error of the target retroreflectors and reveals the error source that is the disturbance from the rotary error of the guideway slider pair. Experimental results prove the validity of the analysis and model of the calibration error. The calibration error is up to 0.5 μm when the tilt error is 0.36°, which is large enough to equal the maximum tolerance of laser interferometer (0.5 μm) in use.

Inter-reflection removal is vital for complex-scene reconstruction. However, most methods assume that the tested surface is a diffuse, and are limited to removal of inter-reflection caused only by diffuse reflections. For all kinds of inter-reflections caused by diffuse and specular reflections, a micro-frequency shifting (MFS) projection technique is presented. Because the modulation variation with frequency in inter-reflection regions is larger than that of other regions, we use the MFS technique to detect inter-reflections, where patterns with specifically designed frequency-shifts and base frequencies are projected. Inter-reflections are detected through large variations in modulation, and removed using a regional-projection technique. Experimental results validate the effectiveness for diffuse and specular inter-reflection removal.

As a fundamental step in fringe projection profilometry, absolute phase unwrapping via single-frequency fringe patterns is still a challenging ill-posed problem, which attracts lots of interest in the research area. To solve the problem above, additional constraints were constructed, such as spatial smoothness constraint (SSC) in spatial phase unwrapping algorithm and viewpoint consistency constraint (VCC) in multi-view systems (e.g., stereo and light-field cameras). However, there still exists phase ambiguity in the unwrapping result based on SSC. Moreover, VCC-based methods rely on additional cameras or light-field cameras, which makes the system complicated and expensive. In this paper, we propose to construct a novel constraint directly from photometric information in captured image intensity, which has never been fully exploited in phase unwrapping. The proposed constraint, named photometric constraint (PC), provides a prospective constraint for absolute phase unwrapping from single-frequency fringe patterns without any additional cameras. Extensive experiments have been conducted for the validation of the proposed method, which achieved comparable performance with the state-of-the-art method, given a traditional camera-projector setup and single high-frequency fringe patterns.

Deflectometry has been widely used to detect defects on specular surfaces. However, it is still very challenging to detect defects on semispecular or diffuse surfaces because of the low contrast and low signal-to-noise ratio. To address this challenge, we proposed a phase-modulation combined method for accurate defect detection. Based on the phase and modulation of captured fringes, a dual-branch convolutional neural network is employed to simultaneously extract geometric and photometric features from the phase-shifting pattern sequence and modulation, which improves the defect detection performance significantly. Compared to state-of-the-art methods, we believe the results demonstrated the proposed method’s effectiveness and capability to reduce false positives.