Specular Surfaces Research Articles

Comparison of Camera Calibration and Measurement Accuracy Techniques for Phase Measuring Deflectometry

Phase measuring deflectometry (PMD) is a competitive method for specular surface measurement that offers the advantages of a high dynamic range, non-contact process, and full field measurement; furthermore, it can also achieve high accuracy. Camera calibration is a crucial step for PMD. As a result, a method based on the calibration of the entrance pupil center is introduced in this paper. Then, our proposed approach is compared with the most popular photogrammetric method based on Zhang’s technique (PM) and Huang’s modal phase measuring deflectometry (MPMD). The calibration procedures of these three methods are described, and the measurement errors introduced by the perturbations of degrees of freedom in the PMD system are analyzed using a ray tracing technique. In the experiment, a planar window glass and an optical planar element are separately measured, and the measurement results of the use of the three methods are compared. The experimental results for the optical planar element (removing the first 6 terms of the Zernike polynomial) show that our method’s measurement accuracy reached 13.71 nm RMS and 80.50 nm PV, which is comparable to accuracy values for the interferometer.

Reflectivity of light on axisymmetric radiation receiver of cylindrical parabolic solar energy concentrator

This series of articles is dedicated to the development of general theoretical approach to definition of characteristic of cylindrical-parabolic solar energy concentrator with axisymmetric radiation receiver, making it possible to optimize its parameters and to improve its optical efficiency. In this article, characteristics of electromagnetic radiations reflected on the interface with semiconductor receiver–converters of radiations into other kinds of energy or with their protection coatings, have been studied related to the lows of geometrical optics for relationships for incidence, reflection and refraction angles. Reflectivity and reflected aggregate fluxes of direct (nonreflected from the specular concentrator) radiation from the surface of axisymmetric receiver depending on refraction index n of rays by radiation receiver have been defined. The values have been defined and the dependences have been studied for reflectivities from the surface of receiver of radiation flow incident on the specular surface, for various concentrator design structures, including the following configurations: the axis of radiation receiver coincides with the focus of specular surface, the focus of specular surface coincides with the lower generatrix of receiver and for various positions of the focus of specular surface inside the receiver, on its symmetry plane.

Leaf Surface Reflectance Does Not Affect Biophysical Traits Modelling from VIS-NIR Spectra in Plants with Sparsely Distributed Trichomes

In this study, we examine leaf reflectance as the main optical property used in remote sensing of vegetation. The total leaf reflectance consists of two main components: a diffuse component, originating from the leaf interior, and a component reflected directly from the leaf surface. The latter contains specular (mirror-like) reflectance (SR) and surface particle scattering, driven by the surface roughness. Our study aimed to (1) reveal the effects of key leaf structural traits on SR in 400–2500 nm, and (2) compare the performance of PLSR models of leaf biophysical properties based on the total reflectance and SR removal reflectance. Four Arabidopsis thaliana structural surface mutants and six Hieracium species differing in trichome properties were studied. PCA did not reveal any systematic effect of trichome density, length, and morphology on SR. Therefore, the results do not support the hypothesis that leaves with denser and longer trichomes have lower SR and higher total reflectance than the smooth leaves. SR removal did not remarkably improve PLSR models of biophysical traits (up to 2% of RMSE). Thus, in herbaceous dorsiventral leaves with relatively sparse trichomes of various morphology and without apparent waxy surface, we cannot confirm that SR removal significantly improves biophysical trait prediction.

Characterization of specular freeform surfaces from reflected ray directions using experimental ray tracing

Abstract. The applications of freeform surfaces in optical components and systems are increasing more and more. Therefore, appropriate measurement techniques are needed to measure these freeform surfaces for verification. This task is still a challenge for most measurement techniques. In this paper, we propose a measurement technique for optical and other specular freeform surfaces based on experimental ray tracing. This technique is able to measure form and mid-spatial-frequency deviations simultaneously. The focus will be set on the sensing technique and the measurement uncertainties in the setup. As the measurement technique is described, an estimation of the influence of different uncertainties based on simulations is given. The result from an experimental measurement is evaluated in relation to the influence of the uncertainties. A comparison measurement for evaluation is given.

Active speckle deflectometry based on 3D digital image correlation.

A method based on 3D digital image correlation (DIC) to measure the shape of specular surface is proposed. The proposed active speckle deflectometry (ASD) utilizes a stereo-camera system to monitor the liquid crystal display (LCD), which is deliberately moved during the measurement. Another testing camera (TC) is used to capture the single-shot speckle pattern displayed on the LCD screen after reflection by the test surface. With this proposal, the movement of the LCD screen can be arbitrary as long as the TC can capture the reflection of speckle pattern. The distance as well as the direction of the movement is not required to be known. The coordinates of the point source are determined by applying the 3D DIC technique with the monitoring stereo-cameras (MSC) before and after the movement of the LCD screen, then the slope and surface shape are obtained. The measurement accuracy of this method is evaluated by measuring a flat glass with a diameter of about 80 mm, compared with the measurement results of interferometer, the shape measurement difference is 0.278um in root mean square (RMS). The shape of two wafers is also measured, and the measurement results are compared to that of the traditional phase measuring deflectometry (PMD). ASD has the advantages of fast measurement, low cost, arbitrary LCD movement, tolerance for the out-of-plane shape of the LCD screen. It provides a new method for specular surface measurement.

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.

One-shot deflectometry for high-speed inline inspection of specular quasi-plane surfaces

Deflectometry is a non-contact optical technique which uses a simple system setup to measure and inspect specular surfaces. Conventional deflectometry methods usually need very complicated system calibration and time-consuming phase-shifting methods, which are not practical for high-speed measurement. But if the measurement target is a quasi-plane surface where the local slope is directly modulated by phase, the complicated system calibration can be neglected. In this study, to reduce error, we investigated and analyzed error contribution when measuring quasi-plane surfaces. A simple and high speed one-shot phase retrieval method is proposed for quickly measuring surface phase. We used a single high carrier-frequency composite pattern for pattern projection, and derivation of the captured fringe pattern for phase retrieval. A phase-related slope is obtained for slope calculation and the 3D shape is reconstructed by integrating the surface slopes. We verified our proposed method by measuring representative quasi-plane surfaces and comparing results with conventional methods.

Photography-based real-time long-wave infrared scattering estimation technique.

The scattered light distribution of surfaces in the long-wave infrared (λ∼8-12µm) is measured using a small set of thermal camera images. This method can extract scatter patterns considerably faster than standard laboratory bidirectional reflectance distribution function measurements and is appropriate for passive homogeneous surfaces. Specifically, six images are used in this study, each taken with respect to a thermal light source at an angle ranging from 10° to 60° to the normal of the surface. This data is deconvolved with the shape of the light source to estimate the scattering pattern. Both highly specular (black Masonite) and diffuse (painted drywall) surfaces are tested. Errors between the estimated scattering distribution and a directly measured one using a goniometer stage and quantum-cascade laser (QCL) are less than or equal to 3% except for extremely specular surfaces where viable QCL measurements cannot be made due to the increased relative contribution of speckle noise.

Infrared phase measuring deflectometry by using defocused binary fringe.

Three-dimensional surface information acquisition of specular objects plays an important role in the fields of automobile industry, aerospace, cultural relic protection, intelligent robotics, equipment manufacturing, and so on. Most of the existing specular surface measurement methods are based on focused sinusoidal fringe patterns, so there are certain requirements for the range of the depth of field (DOF) of the camera on the focus position. However, for many specular surfaces with a large gradient, the tested objects may not always be in the DOF of the camera, so sinusoidal fringe patterns are defocused to be vulnerable to the noise. In this Letter, a new infrared phase measuring deflectometry (PMD) based on defocused binary fringe is proposed that combines a binary fringe defocusing technique and direct PMD. The measurement principle and the corresponding system calibration method are described. The feasibility and measurement accuracy of fringe defocus in specular measurement are studied in principle. The experimental results on several specular objects show that the proposed method can effectively measure specular surfaces out of the DOF of the camera.

Fugitive methane detection using open-path stand-off chirped laser dispersion spectroscopy.

We report an open-path chirped laser dispersion spectrometer capable of detecting the atmospheric methane concentration above the background using both specular and diffusive reflective surfaces via two distinct operation modes in a stand-off detection configuration. The system is integrated with simultaneous ranging functionality, which enables average concentration measurements for varying optical pathlengths. The system was first tested for accuracy and characterized to achieve sensitivity of 2.9ppm-m/Hz1/2 and pathlength precision of 0.2m/Hz1/2 with a controlled release of methane outside the laboratory. The instrument was subsequently field-deployed in the proximity of a natural gas compressor station for fugitive methane detection. The instrument successfully detected methane plumes and narrowed down the location of the plume through multi-path measurement. The field measurements were verified by a co-located reference mobile methane sensor.

3D reconstruction of the specular surface using an iterative stereoscopic deflectometry method.

Phase measuring deflectometry (PMD) is an effective technique for three-dimensional measurement of specular surfaces. However, the ambiguity of monoscopic PMD and the time-consuming searching process of stereoscopic PMD are challenges for specular surface reconstruction. To solve it, we propose an iterative reconstruction algorithm for the stereoscopic phase measuring deflectometry system free of the time-consuming searching process for each pixel. An arbitrary seed point on the specular surface is accurately obtained via a coarse-to-fine optimization means without any other expensive and complicate auxiliaries. Then, a plane with the height of seed point is set as the initial surface form for the iteration, in which the pinhole model is used to find the linear relation to update the surface form. The converging height is the output as the final result. Simulations and experiments verify the feasibility and efficiency of our proposed method based on the stereoscopic phase measuring deflectometry system. The accuracy and robustness are comprehensively evaluated as well.

A novel defect detection method with eliminating dust for specular surfaces based on structured-light modulation analysis technique

In modern industrial manufacturing, the demand for the surface inspection of specular objects has become more and more urgent. In this paper, a novel defect detection method with eliminating dust is systematically proposed to inspect surface defects of specular objects. The proposed approach can be applied to the defect detection of silicon wafers, optical lens, cell phone cover glass, etc. Structured-Light Modulation Analysis Technique (SMAT) is adopted to detect defects and dust simultaneously. Meanwhile, according to the discrepancy of polarization states between dust and other places, polarized illumination and a linear polarizer are exploited to photograph dust image exclusively. The dust image is used to remove dust from the modulation image; thus, the purified defect distribution can be obtained. Relevant experiments are also performed to prove the effectiveness of this proposed approach.

Non-Immersion Ultrasonic Cleaning: An Efficient Green Process for Large Surfaces with Low Water Consumption

Ultrasonic cleaning is a developed and widespread technology used in the cleaning industry. The key to its success over other cleaning methods lies in its capacity to penetrate seemingly inaccessible, hard-to-reach corners, cleaning them successfully. However, its major drawback is the need to immerse the product into a tank, making it impossible to work with large or anchored elements. With the aim of revealing the scope of the technology, this paper will attempt to describe a more innovative approach to cleaning large area surfaces (walls, floors, façades, etc.) which involves applying ultrasonic cavitation onto a thin film of water, which is then deposited onto a dirty surface. Ultrasonic cleaning is an example of the proliferation of green technology, requiring 15 times less water and 115 times less power than conventional high-pressurized waterjet cleaning mechanisms. This paper will account for the physical phenomena that govern this new cleaning mechanism and the competition it poses towards more conventional pressurized waterjet technology. Being easy to use as a measure of success, specular surface cleaning has been selected to measure the degree of cleanliness (reflectance) as a function of the process’s parameters. A design of experiments has been developed in line with the main process parameters: amplitude, gap, and sweeping speed. Regression models have also been used to interpret the results for different degrees of soiling. The work concludes with the finding that the proposed new cleaning technology and process can reach up to 98% total cleanliness, without the use of any chemical product and with very low water and power consumption.

Phase measuring deflectometry for obtaining 3D shape of specular surface: a review of the state-of-the-art

Phase measuring deflectometry (PMD) is a superior technique to obtain three-dimensional (3D) shape information of specular surfaces because of its advantages of large dynamic range, noncontact operation, full-field measurement, fast acquisition, high precision, and automatic data processing. We review the recent advances on PMD. The basic principle of PMD is introduced following several PMD methods based on fringe reflection. First, a direct PMD (DPMD) method is reviewed for measuring 3D shape of specular objects having discontinuous surfaces. The DPMD method builds the direct relationship between phase and depth data, without gradient integration procedure. Second, an infrared PMD (IR-PMD) method is reviewed to measure specular objects. Because IR light is used as a light source, the IR-PMD method is insensitive to the effect of ambient light on the measured results and has high measurement accuracy. Third, a proposed method is reviewed to measure the 3D shape of partial reflective objects having discontinuous surfaces by combining fringe projection profilometry and DPMD. Then, the effects of error sources that mainly include phase error and geometric calibration error on the measurement results are analyzed, and the performance of the 3D shape measurement system is also evaluated. Finally, the future research directions of PMD are discussed.

Open GL–Open CL Solar Radiation Pressure Modeling with Time-Varying Spacecraft Geometries

A method for the fast computation of spacecraft force and torque due to solar radiation pressure (SRP) is presented. A faceted model is employed that tracks which elements of a time-varying geometry are exposed to the sunlight, but sunlight reflections are not modeled. The method uses the highly parallel execution capabilities of commodity graphics processing unit (GPU) and the Open Graphics Library (OpenGL) and Open Compute Language (OpenCL) to render a spacecraft mesh on the GPU. A custom-developed OpenGL render pipeline computes the per-model facet SRP forces and torques that are summed on the GPU before the resultant spacecraft force and torque vectors are copied back to the CPU bound process. The process is validated on spherical and cubic test shapes. The evaluation accommodates spacecraft self-shadowing and is capable of accounting for arbitrary spacecraft articulation. Material properties are encoded with the model to provide realistic specular, diffuse, and absorption surface light interactions. Numerical simulations illustrate the impact of geometric fidelity and articulated surfaces. The faceted OpenGL-OpenCL method is up to an order of magnitude faster on integrated GPU hardware than high-end graphics card as the process is not demanding on the GPU and benefits from the fast memory transfer of on-chip processors.

Height measurement of solder bumps using two-wavelength parallel four-step phase shifting digital holography.

Digital holography (DH) with two wavelengths (TW) that are close to each other was applied to height measurement of solder bumps having spherical specular surfaces with diameters of ∼20µm and heights of ∼20µm. We employed the parallel phase shifting method for instantaneous image capturing, and we improved the spatial resolution of our TW-DH system having two beams with different wavelengths that traveled in opposite directions in the interferometer. It gave 74-times higher repetition and 2.4-times higher spatial resolution than those in our previous DH system based on the Fourier transform method.

On-Machine Calibration Method for In Situ Stereo Deflectometry System

Deflectometry is a promising method for the in situ measurement of specular surfaces. However, the accuracy of this method largely depends on the geometrical calibration, which suffers from a low convergence rate and local minima during the optimization. Off-machine calibration results in a change in geometry parameters due to the repeated disassembly of the measurement system and change of ambient temperature, which largely affects the measurement accuracy. This article presents a flexible on-machine calibration method with fewer intermediate parameters to accelerate the optimization for the in situ stereo deflectometry system. An inclined mirror with a fixed angle was mounted on the spindle of an ultraprecision machine tool and used as a reference plane reflecting the fringe displayed on the display screen. The reflected fringe captured by the cameras was encoded by an inclined mirror moving along the $C$ -axis and the $Z$ -axis of the machine tool. The preknowledge of the movement distance of the ultraprecision machine tool along the $Z$ -axis and rotation angle of the spindle with a nanoscale accuracy was used to largely reduce the number of intermediate variables involved in the optimization, thus improving the convergence speed of the global optimization method and calibration reliability. The experiment results show that this calibration method has higher calibration accuracy, measurement accuracy, and reliability compared to the conventional off-machine calibration method.

Theoretical analysis on imaging technique of local unevennesses occurred on specular surface objects

Theoretical analysis on imaging technique of local unevennesses occurred on specular surface objects

Shining Light on Retroreflectivity: Perceived Brightness of Surfaces with Varying Reflective Properties

Typical road users appear not to understand retroreflectivity despite nightly exposure to retroreflective materials like road signs. A critical benefit of retroreflective materials is a robustness to changes in entrance angle, the angle at which light strikes the material. This study aims to measure observers’ perceived brightness judgments of surfaces representing three types of reflection (diffuse, specular, and retroreflective) when viewed under manipulations of entrance angle. Perceived brightness will be assessed before and during a demonstration including illumination from a source positioned near the observer’s eyes. Prior to the demonstration, observers are hypothesized to predict specular and retroreflective surfaces will have a consistent brightness despite changes in entrance angle. Seeing the retroreflectivity demo is expected to result in increased perceived brightness of only the retroreflective surfaces in the more extreme entrance angle conditions. Watching visual demonstrations of reflection, however, is expected to produce an enhanced appreciation that retroreflective (but not specular or diffuse) surfaces remain bright despite large changes in entrance angle. This evidence may eventually increase demand for retroreflective markings by vulnerable road users.

Impact of surface reflection on microbial inactivation in a UV LED treatment duct

Modeling of a UV-C LED air treatment duct showed that use of highly reflective duct surfaces significantly increased microbial inactivation and minimized the impact of LED positions on inactivation levels. This result was independent of the number of LEDs used. Four wall reflection resulted in higher inactivation rates than solely side-side or top–bottom surface reflection, but beyond three reflections of intensity the number of LEDs controlled the maximum inactivation. The model consisted of a radiation routine which calculated fluence rate from LED intensities and specular surface reflection, integrated with a fluid flow routine which determined air residence time and an inactivation routine which combined microbe inactivation rate constants with fluence levels. Model predictions compared well with data from a 2-cm square test treatment system that achieved 99.9% inactivation of Staphylococcus aureus at low flow rates. These results can guide treatment system designs and expand modeling capabilities for such systems.