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Today, there are strict requirements for the quality inspection of mobile phone cameras, as the design tolerance is getting critically tighter. In order to avoid unnecessary disposal of lens components when testing and assembling the complete cameras, testing the quality of each single lens group in advance before the final assembly is effective. However, as part of a whole camera, a single lens group cannot generate a sharp image independently; it needs to be combined with other elements in the testing system and assembled precisely. In order to address this challenge, we propose a fast testing method based on spatial light modulators (SLMs). By taking advantage of the programmable feature of the SLM, the assembly misalignments caused by fixing the lens group to be tested into the testing system are dynamically scanned and compensated at a fast speed. A design criterion of the phase map pattern to be loaded on the SLM is also verified by simulation and is applied on the testing system. In this way, the proposed method significantly reduces the positioning requirement of the lens under test, and thus improves efficiency. The passed yield of tested lens groups reaches 92.6%.

The specification and characterization of mid-spatial-frequency (MSF) ripples for the large-square-aperture optical elements, typically used in high-power laser systems, have received considerable critical attention. It is necessary to resort to a simple and robust way to characterize error surfaces for facilitating prediction of performance degradation and guiding the fabrication and tolerance settings. In this paper, we characterize residual periodic surface undulations called ripple errors for the large square aperture generated from modern subapertures and deterministic optical fabrication techniques through two methods, taking a step from qualitative judgment to quantitative analysis. The cross artifact reduction technology, instead of traditional windowed preprocessing, is introduced into power spectral density to suppress spectrum leakage while retaining the information about the part. An efficient algorithm to generate Legendre moments for two-dimensional Legendre polynomials is proposed to quantify ripple errors. This work contributes to understanding the optical degradation caused by MSF errors and associating the design and performance index with surface parametric description.

Stereo cameras are widely used in wearable visually impaired assistance devices (VIADs). However, the inevitable vibration, shock, and mechanical stress may make the camera pair become misaligned and cause a sharp decline in the quality of the acquired depth map, which significantly influences the performance of VIADs. In this paper, we propose an epipolar-constraint-based unconstrained self-calibration method that requires neither user involvement nor specific environment, while achieving a rotation accuracy of 0.83 mrad and a translation accuracy of 0.42 mm. Several approaches are proposed to address the image matching issues, including blurred images removal, mismatched key points removal, etc. Based on correctly matched key point pairs, a planar quadric-distribution approach is proposed to ensure the quality and consistency of the final key point group. These collection approaches ensure the reliability of key point pairs, which is the most important factor to realize high accuracy with minimum constraint. A comprehensive set of experiments demonstrates the high robustness of the proposed methods, which are suitable for VIADs. We also present a field test with blindfolded users to validate the flexibility and applicability of the approach.

In the automatic detection for surface defects of optical components, the digs and dust particles exhibit similar features: point-like shape and variable intensity reflectivity. On this condition, these two types with entirely different damages are easily confused so that misjudgments will be induced. To solve this problem, a polarization-characteristics-based classification method of digs and dust particles (PCCDD) is proposed based on the polarimetric imaging technique and dark-field imaging technique. First, a dark-field imaging system equipped with a polarization state generator (PSG) and a polarization state analyzer (PSA) is employed to measure and establish normalized Mueller matrices' datasets of digs and dust particles. And by a nonlinear global search combined with a separability evaluation method, the optimal number of acquisitions and corresponding polarization measurement states of the PSG and the PSA are obtained, as well as the parameters of classification function. Then, multiple polarization images are acquired under the optimal states to extract a multidimensional feature description that relates only to the polarization characteristics of the defect; this subsequently acts as the input vector of the classifier to finally achieve the classification. This method takes full advantage of both the difference in polarization properties between digs and dust particles and the characteristic that the polarization properties of digs are relatively invariant while those of dust particles have a large variability. The classification process involves only simple matrix operations. Compared to the traditional discrimination method based on intensity images, the features obtained by this method have a higher separability. Experiments show that the classification accuracy reaches over 90%. This method can be further applied to the recognition and discrimination of other defects in the field of surface defects' detection.

The introduction of RGB-depth (RGB-D) sensors harbors a revolutionary power in the field of navigational assistance for the visually impaired. However, RGB-D sensors are limited by a minimum detectable distance of about 800 mm. This paper proposes an effective approach to decrease the minimum range for navigational assistance based on a RGB-D sensor of RealSense R200. A large-scale stereo matching between two infrared (IR) images and a cross-modal stereo matching between one IR image and RGB image are incorporated for short-range depth acquisition. The minimum range reduction is critical not only for avoiding obstacles up close, but also in the enhancement of traversability awareness. Overall, the minimum detectable distance of RealSense is reduced from 650 mm to 60 mm with qualified accuracy. A traversable line is created to give feedback to visually impaired individuals through stereo sound. The approach is proved to have usefulness and reliability by a comprehensive set of experiments and field tests in real-world scenarios involving real visually impaired participants.

For state-of-the-art optical elements in laser fusion applications, optical surfaces, especially those with residual fabrication error, are generally complex in symmetry, shape of aperture, and spatial frequency distribution. It is critical to represent the optical surface with high accuracy, efficiency, and flexibility during the stages of design, fabrication, and testing. For this purpose, in this paper, we propose that adaptive radial basis functions (ARBF) can be applied to represent the complex optical surface. As more degrees of freedom are harnessed by an adaptive algorithm, the proposed approach presents better accuracy and needs fewer basis functions than that of the classical radial basis functions. Both surfaces, with global and local variation, can be well represented by the proposed method. Furthermore, the fitting ability of ARBF is verified with the measured data from an element polished by magnetorheological finishing technology. Optimization frameworks of the shape parameters for practical use are also discussed. For good measure, spatial relevance between surface height and nodes location shows that ARBF is fairly flexible in representing complex optical surfaces.

Semantic segmentation represents a promising means to unify different detection tasks, especially pixelwise traversability perception as the fundamental enabler in robotic vision systems aiding upper-level navigational applications. However, major research efforts are being put into earning marginal accuracy increments on semantic segmentation benchmarks, without assuring the robustness of real-time segmenters to be deployed in assistive cognition systems for the visually impaired. In this paper, we explore in a comparative study across four perception systems, including a pair of commercial smart glasses, a customized wearable prototype and two portable RGB-Depth (RGB-D) cameras that are being integrated in the next generation of navigation assistance devices. More concretely, we analyze the gap between the concepts of “accuracy” and “robustness” on the critical traversability-related semantic scene understanding. A cluster of efficient deep architectures is proposed, which are built using spatial factorizations, hierarchical dilations and pyramidal representations. Based on these architectures, this research demonstrates the augmented robustness of semantically traversable area parsing against the variations of environmental conditions in diverse RGB-D observations, and sensorial factors such as illumination, imaging quality, field of view and detectable depth range., Ministerio de Economía y Competitividad, Comunidad de Madrid, Dirección General de Tráfico

Depth sensing is a basic issue in three-dimensional computer vision, and structured light is one of the most prevailing methods for it. However, complex surroundings and strong ambient illumination are fairly unfavorable to depth sensing based on structured light. Complex surroundings increase computation overhead and require extra effort to be separated from the target object. Strong ambient illumination is unfavorable to the signal-noise ratio of structured light and, thus, increases the difficulty of decoding. In this paper, we propose that the polarization-coded structured light is capable of target enhanced depth sensing in ambient illumination. We present the polarimetric principle, an improved algorithm of polarization-coded structured light, and signal-noise-ratio analysis in ambient illumination. Experimental results show that polarization-coded structured light is efficient and robust for target depth sensing of a complicated environment. The polarization-coded structured light is promising to the target depth sensing in an outdoor scenario and industrial inspection.

In this paper, a new panoramic annular lens (PAL) with a super wide angle of (30°-120°)×360° is introduced that consists of two parts: a panoramic front unit (PFU) and a relay lens. First, two PFU arrangements referring to the block structured panoramic front unit (BSPFU) and the shell structured panoramic front unit are compared and analyzed in terms of their field curvature correction ability and structural feasibility for obtaining a wide angle PAL. From the comparison, the BSPFU arrangement turns out to be more promising for its good field curvature correction, structure compactness, and convenience for manufacture and assembly. Second, a relay lens composed of six lenses with three Qsubbfs/subaspheres has been designed to match the BSPFU. Finally, image performance analysis of the optimized super wide angle PAL is presented. The results show that the designed PAL equipped with a BSPFU and a Qsubbfs/subaspherized relay lens achieves good performance under a super wide field of view of (30°-120°)×360°.

By analyzing the spectral domain's phase information, one can use spectrally resolved white-light interferometry (SRWLI) to obtain the profile with a single frame of an interferogram. We present here a two-dimensional (2D) SRWLI method that can be applied to measure narrow rectangle areas. A frequency comb is produced by using a Fabry-Perot (F-P) etalon to filter the broadband source. With the filtered frequency comb illumination, the interference patterns under adjacent wavelengths would be separated by a little distance, which enables us to obtain a 2D profile with a small width. The experimental details of measurement on a step sample are discussed in this paper.

Light beam scanning using a dispersive element and wavelength tuning is coupled with fiber-optic interferometry to realize a new surface measurement instrument. The instrument is capable of measuring nanoscale surface structures and form deviations. It features active vibration compensation and a small optical probe size that may be placed remotely from the main apparatus. Active vibration compensation is provided by the multiplexing of two interferometers with near common paths. Closed loop control of a mirror mounted on a piezoelectric transducer is used to keep the path length stable. Experiments were carried out to deduce the effectiveness of the vibration compensation and the ability to carry out a real measurement in the face of large environmental disturbance.

This paper discusses the pulse signal of a highly sensitive lateral deformable optical microelectromechanical systems (MEMS) displacement sensor based on Wood's anomalies and its corresponding tolerance. The optical reflection amplitude of the device changes with the displacement of the nanostructured grating elements. Unexpectedly, the device's original sinusoidal signal develops into a new signal form (i.e., a pulse signal), when the air gap between the two layers of gratings decreases. Since the slope of the pulse signal, namely 2.5%/nm (i.e., 0.65 dB/nm), is eight times higher than that of the original signal form, namely 0.3%/nm (i.e., 0.03 dB/nm), the sensitivity of the structure improves by eight times. However, this device is very sensitive to parameters such as its wavelength, period, duty ratio, and air gap. In this paper we used rigorous coupled wavelength analysis (RCWA) to analyze and optimize the respective influence of each parameter on the device's performance. We have introduced two methods to search for the optimal setting and have demonstrated the optimal settings of different incident lights. The simulation results indicate that it is close to 85% possible to achieve an actual device with the highest slope superior to 0.5%/nm and it is 64% possible that the highest slope of an actual device falls in the interval ranging from 1.0%/nm to 2.0%/nm. All the simulated data helped us better understand the tolerance of the pulse signal and guide us toward the development of an actual device.

A subnanometer resolution displacement sensor based on a grating interferometric cavity with intensity compensation and phase modulation is proposed and experimentally demonstrated in this paper. The grating interferometric cavity is composed of a frequency-stabilized laser source, a diffraction grating, and a mirror. To realize a subnanometer resolution, the intensity compensation and phase modulation technique are introduced, which are achieved by an intensity compensation light path, three closed placed photodetectors, a processing circuit and a piezoelectric ceramic transducer, and a lock-in amplifier. The intensity compensation technique can improve the stability of the output intensity signal greatly while the phase modulation technique can increase the signal-to-noise ratio dramatically. The detected signal is intensity modulated and processed by a particular arithmetic circuit. Experimental results indicate that the sensitivity of this displacement sensor is 44.75 mV/nm and the highest resolution can reach 0.017 nm, which is 27 times better than the one without intensity compensation and phase modulation. As a high-performance sensor with immunity to electromagnetic interference, this displacement sensor has potential to be used in nanoscience and technology.

A common-path and compact wavefront diagnosis system for both continuous and transient wavefronts measurement is proposed based on cross grating lateral shearing interferometer (CGLSI). Derived from the basic CGLSI configuration, this system employs an aplanatic lens to convert the wavefront under test into a convergent beam, which makes it possible for CGLSI to test the wavefront of collimated beams. A geometrical optics model for grating pitch determination and a Fresnel diffraction model for order selection mask design are presented. Then a detailed analysis about the influence of the grating pitch, the distance from the cross grating to the order selection mask and the numerical aperture of the aplanatic lens on the system error is made, and a calibration method is proposed to eliminate the system error. In addition, the differential Zernike polynomials fitting method is introduced for wavefront retrieval. Before our experiment, we have designed several grating pitches and their corresponding order selection mask parameters. In the final comparative experiment with ZYGO interferometer, the wavefront diagnosis system exhibits both high precision and repeatability.