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As an important branch of computational optics, the light field can simultaneously record the spatial intensity and propagation direction of light, which provides a new way to realize three-dimensional (3D) measurement. Although light field measuring system (LFMS) can quickly detect nontransparent surfaces, specular light affects the overall measurement result. In this paper, an LFMS based on a diffuse light source and polarization technology is proposed to eliminate specular light and obtain a more accurate 3D reconstruction result in one exposure. During the measurement, a series of images corresponding to continuously varying polarization angle are captured, in which the value(V)-channel value distribution is used as the criterion of specular light removal efficiency, and hence, the image at the optimal polarization angle is obtained, in which the influence of specular light is removed. The depth map and specular light removal efficiency are experimentally compared with those of the traditional LFMS. The experimental results show that the depth map error can be greatly reduced, which further verifies the feasibility of the proposed method and system. The present study can be extended to the traditional optical system or other machine vision measurement methods involving specular light.

The internal structure of hair consists of three layers: the cuticle, the cortex, and the medulla, and there are multiple membrane structures inside the cuticle. Light incident on these subwavelength structures is subject to scattering and interference phenomena, and the reflected highlights vary depending on the viewing angle. This phenomenon of light coloration due to the microstructure is called structural coloring or photonic coloration. It shows a unique specular highlight and determines the magnitude of the specular highlight, which is not observable in the linear optics of CG. In the case of hairs, this structural coloring or photonic coloration occurs in addition to the simple surface reflections and the backscattering lights, which penetrate the hair and are absorbed by the melanin pigment. In the present report, we mainly discuss the effects of the structural coloring first caused by the multi-layered structure in the cuticle region on the hair surface, simulating the electromagnetic field using the NS-FDTD (Non-Standard Finite Difference in Time Domain) method. In addition, we also discuss the backscattering phenomena inside the hair fibers.

In this paper, a novel, to the best of our knowledge, iterative approach for highlight removal is proposed using lenselet-based plenoptic cameras without multiple exposures. An unsupervised k-means clustering approach that relates unsaturated pixels to chromatic dispersion based on the intrinsic decomposition and dichromatic reflection model is proposed to recover unsaturated highlights. Meanwhile, an adaptive direction method along with a Gaussian probability distribution model is designed to recover the saturated highlights. Finally, a method that combines the specular residual ratio with information entropy is built to quantitatively evaluate the quality of highlight removal. Generally, our method not only fully removes specular highlights, but also has low spatial complexity of image acquisition, more stability, and outstanding restoration for complex scenes.

This study examined perceptual differentiation of specular from diffuse shading for the recovery of surface colour and gloss. In Experiment 1, we parametrically varied the mesoscale relief height of globally planar surfaces, specular sharpness and the orientation of the surface relative to the light source. We obtained psychophysical matches for perceived colour saturation and value (HSV), but also considered whether the main effects could be influenced by colour space used when transforming data to perceptually-uniform CIE LCH space. Results revealed strong interactions between perceived colour attributes and the lighting conditions, the structure of specular reflections, and surface relief. Declines in saturation were observed with increasing specular roughness (using an HSV colour representation), but no similar decline was observed in chroma (using a CIE LCH colour representation). Experiment 2 found strong negative correlations between perceived gloss and specular roughness. Perceived gloss also depended on mesoscopic relief height and orientation of the surface relative to the light source. Declines in perceived gloss moderately accounted for the variability in colour saturation and value matches obtained in Experiment 1. We found information about perceived specular coverage could further improve the model’s accountability of perceived colour saturation and lightness (Experiment 3). These findings together suggest that perceived colour saturation and colour value depends on the visual system’s ability to distinguish the underlying diffuse shading from specular highlights in images.

Simulation of light reflection from specular surfaces is a core problem of computer graphics. Existing solutions either make the approximation of providing only a large-area average solution in terms of a fixed BRDF (ignoring spatial detail), or are specialized for specific microgeometry (e.g. 1D scratches), or are based only on geometric optics (which is an approximation to more accurate wave optics). We design the first rendering algorithm based on a wave optics model that is also able to compute spatially-varying specular highlights with high-resolution detail on general surface microgeometry. We compute a wave optics reflection integral over the coherence area; our solution is based on approximating the phase-delay grating representation of a micron-resolution surface heightfield using Gabor kernels. We found that the appearance difference between the geometric and wave solution is more dramatic when spatial detail is taken into account. The visualizations of the corresponding BRDF lobes differ significantly. Moreover, the wave optics solution varies as a function of wavelength, predicting noticeable color effects in the highlights. Our results show both single-wavelength and spectral solution to reflection from common everyday objects, such as brushed, scratched and bumpy metals.

Endoscopy provides a convenient way to access the inner structures of various organs. The endoscopic images provide an immediate observation and help diagnosis and therapy. Shape reconstruction from endoscopic images further provides real scale factor for image-guided navigation. However, specular highlights, bright patches of light appearing on the imaged surface, mask the real image texture and result in erroneous reconstruction. Therefore, the detection of specular highlights is essential for accurate reconstruction. In this study, we divide the images into homogeneous regions by color quantization and spatial segmentation. Then, a thresholding technique based on histogram of the pixel intensity values is used. Finally, we check the gray level consistency for each region to avoid over segmentation. The experimental results show that the proposed method can achieve successfully detection.

This paper presents a novel structured light approach for the 3D reconstruction of specular surface. The binary shifting strip is adopted as structured light pattern instead of conventional sinusoidal pattern. Based on the framework of conventional High Dynamic Range (HDR) imaging technique, an efficient means is first introduced to estimate the camera response function. And then, dynamic range of the generated radiance map is compressed in the gradient domain by introducing an attenuation function. Subject to the change of lighting conditions caused by projecting different structured light patterns, the structure light image with middle exposure level is selected as the reference image and used for the slight adjustment of the primary fused image. Finally, the regenerated structured light images with well exposing condition are used for 3D reconstruction of the specular surface. To evaluate performance of the method, some stainless stamping parts with strong reflectivity are used for the experiments. And the results showed that, different specular targets with various shapes can be precisely reconstructed by the proposed method.

A new surface profilometry technique is proposed for profiling a wafer surface with both diffuse and specular reflective properties. Most moir&eacute, projection scanning techniques using triangulation principle work effectively on diffuse reflective surfaces, on which the reflected light beams are assumed to be well captured by optical sensors. In reality, this assumption is no longer valid when measuring a semiconductor wafer surface having both diffuse and specular reflectivities. To resolve the above problem, the proposed technique uses a dual optical sensing configuration by engaging two optical sensors at two different viewing angles, with one acquiring diffuse reflective light and the other detecting at the same time specular surface light for achieving simultaneous full-field surface profilometry. The deformed fringes measured by both sensors could be further transformed into a 3-D profile and merged seamlessly for full-field surface reconstruction. Several calibration targets and industrial parts were measured to evaluate the feasibility and accuracy of the developed technique. Experimental results showed that the technique can effectively detect diffuse and specular light with repeatability of one standard deviation below 0.3 &micro, m on a specular surface and 2.0 &micro, m on a diffuse wafer surface when the vertical measuring range reaches 1.0 mm. The present findings indicate that the proposed technique is effective for 3-D microscale surface profilometry in in-situ semiconductor automated optical inspection (AOI).

We entertain the use of light dependent resistors as a viable option as measuring sensors in optics laboratory experiments or classroom demonstrations. The main advantages of theses devices are essentially very low cost, easy handling and commercial availability which can make them interesting for instructors with limited resources. Simple calibration procedures were developed indicating a precision of $\sim 5\% $ for illuminance measurements. Optical experiments were carried out as proof of feasibility for measurements of reflected and transmitted light and its quality results are presented. In particular, the sensor measurements allowed to verify the angular distribution of a Lambertian reflective material, to observe transmitted and reflected specular light on a glass slab as function of the incoming angle of a light beam, and to estimate glass refractive index with values averaging $1.51\pm0.06$ in satisfactory agreement with the expected 1.52 value., 13 pages, 8 figures

We examined whether perception of color saturation and lightness depends on the three-dimensional (3D) shape and surface gloss of surfaces rendered to have different hues. In Experiment 1, we parametrically varied specular roughness of predominantly planar surfaces with different mesoscopic relief heights. The orientation of surfaces was varied relative to the light source and observer. Observers matched perceived lightness and chroma (effectively saturation) using spherical objects rendered using CIE LCH color space. We observed strong interactions between perceived saturation and lightness with changes in surface orientation and surface properties (specular roughness and 3D relief height). Declines in saturation and increases in lightness were observed with increasing specular roughness. Changes in relief height had greater effects on perceived saturation and lightness for blue hues compared with reddish and greenish hues. Experiment 2 found inverse correlations between perceived gloss and specular roughness across conditions. Experiment 3 estimated perceived specular coverage and found that a weighted combination of perceived gloss and specular coverage could account for perceived color saturation and lightness, with different coefficients accounting for the perceptual experience for each of the three hue conditions. These findings suggest that perceived color saturation and lightness depend on the separation of specular highlights from diffuse shading informative of chromatic surface reflectance.

Specular highlights on the surface of an object can serve as cues for estimating illumination color, because highlights on many objects directly reflect illumination colors. In addition, we experience that illumination colors typically have chromaticities near the daylight locus in daily life, and this experience may possibly affect our estimation of the illumination color. In this study, we investigated the effects of specular highlights and object colors on the estimation of the illumination colors by human observers using images of a single object with specular highlights as stimuli. In the experiments, the colors of all pixels on an object with specular highlights changed from the reference D65 chromaticity to a test chromaticity in a trial. Observers judged whether this chromaticity change appeared to be induced by changing the object or by changing the illumination. In the control experiment, observers performed the same task for a stimulus of a mat object without specular highlights. Different values of hue and saturation were used as the test chromaticities. In the results, the chromaticity changes appeared more likely to be caused by changing the illumination when the test color had lower saturation. In addition, the ratios of “illumination changed” responses were slightly higher for objects with specular highlights than for objects without specular highlight. However, the ratios of “illumination changed” responses averaged across observers were not largely different across color directions. These results suggest that specular highlights slightly contribute to the estimation of illumination color, and that the daylight locus does not largely affect the estimation of the illumination color.

A method for light sources estimation is proposed in this paper. The method utilizes feature points in cast shadows to estimate near light source positions from estimated source directions using specular highlights. There are several methods that can be used to estimate light sources from scene images, using either cast shadows or specular highlights. However, most of them are limited to directional light sources. The proposed method can estimate the positions and intensities of multiple near point light sources. Specular highlights on an object of known geometry are first used for light source direction estimation. Then, a discontinuity point in the object shape and the corresponding cast shadow on a ground plane are used for light source position estimation. Feature points obtained from an image of the cast shadow, however, can be inaccurate due to various factors. Information on diffused light reflected from Lambertian ground-plane surface is subsequently used to improve estimation accuracy. Experimental results were used to evaluate the performance of the proposed method. Key words: Light source estimation, light source recovery, augmented reality.

The measurement of the particle size distribution plays an important role in mineral processing. Due to the high costs and time-consumption of the screening process, modern machine vision methods based on the acquisition and analysis of recorded photographic images. But the image analysis methods used so far, do not provide information on the three-dimensional shape of the grain. In the coal industry, the application scope of these methods is substantially limited by the low reflectivity of the black coal particle surface. These circumstances hinder proper segmentation of coal stream surface image. The limited information contained in two-dimensional image of the raw mineral stream surface, makes it difficult to identify proper size of grains partially overlapped by other particles and skewed particles. Particle height estimation based on the shadow length measurement becomes very difficult in industrial environment because of the fast movement of the conveyor belt and because of spatial arrangement of these particles, usually touching and overlapping. Method of laser triangulation connected with the movement of the conveyor belt makes it possible to create three-dimensional depth maps. Application of passive triangulation methods (e.g. stereovision) can be impeded because of the low contrast of the black coal on the black conveyor belt. This forces the use of active triangulation methods, directly identifying position of the analyzed image pixel. High contrast of the image can be obtained by a direct pointwise laser lighting. For the simultaneous identification of the entire section of the raw material stream it is useful to apply a linear laser (a planar sheet of the laser light). There have been presented basic formulas for conversion of pixel position on the camera CCD matrix to the real-word coordinates. A laboratory stand has been described. This stand includes a linear laser, two high-definition (2Mpix) cameras and stepper motor driver. The triangulation head moves on the rails along the belt conveyor section. There have been compared acquired depth maps and photographic images. Depth maps much better describe spatial arrangement of coal particles, and have a much lower noise level resulting from the specular light reflections from the shiny fragments of the particle surface. This makes possible an identification of the coal particles partially overlapped by other particles and obliquely arranged particles. It enables a partial elimination or compensation of image disturbances affecting the final result of the estimated particle size distribution. Because of the possibility of the reflected laser beam overriding by other particles it is advantageous to use a system of two cameras. Results of the experimental research confirmed the usefulness of the described method in spite of low reflectance factor of coal surface. The fast detection of changes in particle size distribution makes possible an on-line optimization of complex technological systems - especially those involving coal cleaning in jigs - thus leading to better stabilization of quality parameters of the enrichment output products. An additional application of the described method can be achieved by measuring the total volume of the stream of the transported materials. Together with the measurement signal from the belt conveyor weight it makes possible to estimate the bulk density of the raw mineral stream. The low complexity of the signal processing in the laser triangulation method is associated with the acquisition of high contrast images and analysis based on simple trigonometric dependencies.

본 논문은 경면체의 3차원 형상을 측정하기 위한 광학시스템에 관한 것으로, 특히 LED 배열로 이루어진 점광원과 하프미러, 이미지센서로 구성된 비전시스템을 구성하여 광로분석을 수행하고 효과적인 근사화 방법을 제시한다. 실험을 통하여 취득된 영상 내의 점광원의 상대적 위치변화로부터 경면체의 3차원형상의 효과적 추정이 가능함을 보인다. 【An optical method to measure the three-dimensional (3D) shape of a surface with specular reflection is proposed. The proposed method is based on the analysis of the geometric path of the light from a point source, and the relative displacements of points in the reflection image. The 3D shape of a concave mirror is shown to be determined approximately via experiments, where the vision system consists of LED array illumination, a half-mirror, and an imaging sensor.】

The intensity value recorded by terrestrial laser scanning (TLS) systems is significantly influenced by incidence angles. Most existing models focus on the diffuse reflection of rough surfaces and ignore the specular reflection, despite that both reflections simultaneously exist in all natural surfaces. At large incidence angles, specular reflection can be neglected. However, laser detectors can receive a portion of specular reflection at small incidence angles. Specular reflection can lead to additional increase in the original intensity data and even highlight phenomenon on scanned targets, especially those with a relatively smooth or highly reflective surface. In this paper, a new empirical method is proposed to correct the intensities of highlight regions caused by the specular reflection. The intensity from the specular reflection is obtained by subtracting the intensity caused by diffuse reflection and instrumental effects from the original intensity. The proposed method is tested and validated on different targets scanned by Faro Focus3D 120. Results imply that the proposed method can effectively eliminate the highlight phenomenon in TLS for 3-D point cloud representation by intensity and intensity image interpretation.

Computer simulated stimuli can provide a flexible method for creating artificial scenes in the study of visual perception of material surface properties. Previous work based on this approach reported that the properties of surface roughness and glossiness are mutually interdependent and therefore, perception of one affects the perception of the other. In this case roughness was limited to a surface property termed bumpiness. This paper reports a study into how perceived gloss varies with two model parameters related to surface roughness in computer simulations: the mesoscale roughness parameter in a surface geometry model and the microscale roughness parameter in a surface reflectance model. We used a real-world environment map to provide complex illumination and a physically-based path tracer for rendering the stimuli. Eight observers took part in a 2AFC experiment, and the results were tested against conjoint measurement models. We found that although both of the above roughness parameters significantly affect perceived gloss, the additive model does not adequately describe their mutually interactive and nonlinear influence, which is at variance with previous findings. We investigated five image properties used to quantify specular highlights, and found that perceived gloss is well predicted using a linear model. Our findings provide computational support to the 'statistical appearance models' proposed recently for material perception.

In dynamic scenes, relative motion between the object, the observer, and/or the environment projects as dynamic visual information onto the retina (optic flow) that facilitates 3D shape perception. When the object is diffusely reflective, e.g. a matte painted surface, this optic flow is directly linked to object shape, a property found at the foundations of most traditional shape-from-motion (SfM) schemes. When the object is specular, the corresponding specular flow is related to shape curvature, a regime change that challenges the visual system to determine concurrently both the shape and the distortions of the (sometimes unknown) environment reflected from its surface. While human observers are able to judge the global 3D shape of most specular objects, shape-from-specular-flow (SFSF) is not veridical. In fact, recent studies have also shown systematic biases in the perceived motion of such objects. Here we focus on the perception of local shape from specular flow and compare it to that of matte-textured rotating objects. Observers judged local surface shape by adjusting a rotation and scale invariant shape index probe. Compared to shape judgments of static objects we find that object motion decreases intra-observer variability in local shape estimation. Moreover, object motion introduces systematic changes in perceived shape between matte-textured and specular conditions. Taken together, this study provides a new insight toward the contribution of motion and surface material to local shape perception. (C) 2015 The Authors. Published by Elsevier Ltd.

The intensity value recorded by terrestrial laser scanning (TLS) systems is significantly influenced by the incidence angle. The incidence angle effect is an object property, which is mainly related to target scattering properties, surface structures, and even some instrumental effects. Most existing models focus on diffuse reflections of rough surfaces and ignore specular reflections, despite that both reflections simultaneously exist in all natural surfaces. Due to the coincidence of the emitter and receiver in TLS, specular reflections can be ignored at large incidence angles. On the contrary, at small incidence angles, TLS detectors can receive a portion of specular reflections. The received specular reflections can trigger highlight phenomenon (hot-spot effects) in the intensity data of the scanned targets, particularly those with a relatively smooth or highly-reflective surface. In this study, a new method that takes diffuse and specular reflections, as well as the instrumental effects into consideration, is proposed to eliminate the specular reflection effects in TLS intensity data. Diffuse reflections and instrumental effects are modeled by a polynomial based on Lambertian reference targets, whereas specular reflections are modeled by the Phong model. The proposed method is tested and validated on different targets scanned by the Faro Focus3D 120 terrestrial scanner. Results imply that the coefficient of variation of the intensity data from a homogeneous surface is reduced by approximately 38% when specular reflections are considered. Compared with existing methods, the proposed method exhibits good feasibility and high accuracy in eliminating the specular reflection effects for intensity image interpretation and 3D point cloud representation by intensity.

Due to the strong reflection property of materials with smooth surfaces like ceramic and metal, it will cause saturation and the highlight phenomenon in the image when taking pictures of those materials. In order to solve this problem, a new algorithm which is based on reflection component separation (RCS) and priority region filling theory is designed. Firstly, the specular pixels in the image are found by comparing the pixel parameters. Then, the reflection components are separated and processed. However, for ceramic, metal and other objects with strong specular highlight, RCS theory will change color information of highlight pixels due to larger specular reflection component. In this situation, priority region filling theory was used to restore the color information. Finally, we implement 3D experiments on objects with strong reflecting surfaces like ceramic plate, ceramic bottle, marble pot and yellow plate. Experimental results show that, with the proposed method, the highlight caused by the strong reflecting surface can be well suppressed. The highlight pixel number of ceramic plate, ceramic bottle, marble pot and yellow plate, is decreased by 43.8 times, 41.4 times, 33.0 times, and 10.1 times. Three-dimensional reconstruction results show that highlight areas were significantly reduced.

We propose a method to estimate the 3-D shape of surfaces with specular reflection, using a model of the difference in appearance between images reflected from a flat surface and a curved surface. First, we analyze the geometry of spatial reflection from a specular surface and how reflected light varies due to a curved surface. This is used to estimate 3-D shape. The proposed method is shown to be effective in experiments using illumination from spatially distributed light sources and a camera capturing the reflected light from curved, specular surfaces.

SummaryRetinal images are produced by interactions between a surface’s 3D shape, material properties, and surrounding light field. In order to recover the 3D geometry of a surface, the visual system must somehow separate aspects of image structure generated by a surface’s shape from structure generated by its material properties or the light field in which it is embedded. Attributing image structure to the wrong physical source would cause the visual system to interpret changes in one physical property (such as reflectance) as changes in another (such as shape). Many previous studies have shown that the visual system does not conflate image structure generated by specular reflectance with 3D shape [1–6], but they did not assess the physical conditions where it would be computationally most difficult to disentangle these different sources of image structure. Here, we show that varying the specular roughness and curvature of surfaces embedded in natural light fields can strongly modulate perceived shape. Despite the complexity of these interactions, we show how an image’s gradient structure mediates its interpretation as a specular reflection or a change in 3D shape. Our findings provide a coherent explanation of when and why specular reflections impact perceived shape and reveal how the static surface properties, simplified light fields, and experimental methods used in previous studies may explain their inconsistent results.

Real time paper surface-web measurement is one of the challenging research fields. The traditional laboratory method has many limitations and is unable to measure the entire tambour during the manufacturing process. It has been necessary to develop an online technique that could measure the surface topography in real time. An optical technique was developed, based on laser triangulation, and is applied to develop a new prototype device, which characterizes high speed paper-web surfaces over a wide scale of spatial wavelengths spectrum and computes the surface roughness in real time. The used multi channel pulsed laser diode, source of illumination onto the paper-web, is of benefit due to its low coherence length and is capable to deliver a powerful burst of light beam over a 1 µs duration, which delivers energy of 100 µJ per pulse. The short exposure time avoids blurriness in the acquired images which could possible due to the high speed and vibrations on the paper-web.The laser beam is shaped into a narrow line-of-light using cylindrical lenses and is projected onto a paper-web surface, which covers a physical length of about 210 mm. The created line-of-light cross section full width at half maximum, FWHM Gaussian distribution, is 2-3 pixels on the image. The line-of-light is projected onto the paper-web perpendicular to the plane of the surface. The low angled, low specular, reduced coherence length, scattered reflected laser line is captured by the 3 CCD sensors, which are synchronized with the laser source. The low specular light ensures to avoid saturation of the imaging sensors if the surface is very smooth, and obliquely captures the z-directional fine feature of the surface.The scattered phenomenon of the reflected light is responsible for the surface irregularity measurements. The basic image processing algorithm is applied in order to remove noise and cropped the images widthwise so that only pixels above a preset threshold gray level can be processed, which enables efficient real time measurement. The image is transformed into a 1D array using the center of gravity, COG. The accuracy and precision of the COG depends on the line-of-light FWHM, which, in turn, is responsible for the accuracy, noise and the resolution of the developed technique. The image subpixel resolution achieved is 0.01 times a pixel and uuncertainty in the raw data is 0.43 µm while it is 0.05 µm in the rms roughness.The signal processing steps combining the B-Spline filter and the filter in the spatial frequency domain were employed in order to separate roughness, waviness, and form and position error in the raw profile. The prototype is designed to measure online surface roughness and to characterize surface in a spatial wavelength spectrum from 0.09 to 30 mm, which is extendable to any required spatial range in order to cover a wide scale surface feature such as micro roughness, macro roughness and waviness. It is proven that exploitation of a simple laser triangulation technique could lead to an improvement in the overall quality and efficiency in the paper and paperboard industries and it can also be of potential interest for the other surface characterization problems.

This paper introduces a novel method for recovering light directions and camera parameters using a single sphere. Traditional methods for estimating light directions using spheres either assume both the radius and center of the sphere being known precisely, or they depend on multiple calibrated views to recover these parameters. In this paper, it will be shown that light directions can be uniquely determined from specular highlights observed in a single view of a sphere without knowing or recovering the exact radius and center of the sphere. Besides, given multiple views of the sphere, it will be shown that the focal length and the relative positions and orientations of the cameras can be determined using the recovered sphere and light directions. Closed form solutions for estimation of light directions and camera poses are presented, and an optimization procedure for estimation of the focal length is introduced. Experimental results on synthetic and real data demonstrates both the accuracy and robustness of the proposed method.

It is well known that ambient light reduces the contrast ratio. However binocular human perception is often better than expected by monocular measurements. We evaluated this by subjects and adapted the measurement set-up to binocular vision. Typical mobile displays were examined and we verified the binocular visual perception of small specular light sources.

This paper presents a new Phase Measuring Deflectometry (PMD) method to measure specular object having\ud discontinuous surfaces. A mathematical model is established to directly relate absolute phase and depth, instead of phase\ud and gradient. Based on the model, a hardware measuring system has been set up, which consists of a beam splitter to\ud change the optical path, and two LCD screens to display the same sinusoidal fringe patterns. By using model-based and\ud machine vision method, system calibration is accomplished to provide the required parameters and conditions. The\ud verification tests are given to evaluate the effectiveness of the developed system. The 3D shape of an artificial step\ud having multiple specular surfaces and a concave mirror has been measured. Initial experimental results show that the\ud proposed measurement method can obtain 3D shape of specular objects with discontinuous surface effectively.

A mathematical model of space target illumination characteristics is established based on the basic theory of radiation by considering geometry, background, and material characteristics of the space target. Using the model, the spatial distribution of scattering light intensity from the space target is calculated with the modeling and blanking technique of target when being illuminated by the sun. The relations of specular reflection with the position, geometry, materials and other attributes of the space target are analyzed. Furthermore, the effect of specular reflection on space target detecting is discussed. A method of characteristic simulation of space target is presented. The simulation result indicates that polyhedral structure, mirror surface, or solar sail is easy to cause specular reflection. It shows the effect of specular reflection is helpful for space target detection and identification.

This article proposes to analyze the relation between gloss and roughness of surfaces of polypropylene samples abraded at different magnitudes. Experiments consist of mechanical polishing at different grades, roughness profiles measurement, and specular light reflectometry with three incidence angles of resulting surfaces. About 100 roughness parameters were considered in this investigation and an original statistical methodology combining the conventional one-way ANOVA to the recent and powerful Computer Based Bootstrap Method (CBBM) is presented to select which of them is the most relevant, first, to characterize the polypropylene surfaces abraded at different magnitude and, second, to model the physical interaction of these abraded surfaces with a white light beam. The statistical treatment of the experimental results of this investigation shows that the fractal dimension is the most relevant roughness parameter in the two cases. As far as the optical properties are concerned, it is shown that the fractal dimension of an abraded surface is closely related to the local curvature radii of peaks which are known to play a major role in physical interactions with a white light beam. POLYM. ENG. SCI., 56:103–117, 2016. © 2015 Society of Plastics Engineers

The dynamic inspection and reverse engineering of specular freeform surfaces is a challenge so far largely unsolved. Products produced in high quantities in industrial environments are still inspected manually which is labour intensive, expensive, monotonous and subjective. We propose a novel deflectometric hardware setup and methodology to overcome this shortfall. The reflection of a line laser from a moving specular surface is captured on a translucent screen. The resulting complex line provides detailed data about the surface gradients at the intersection of the laser light plane and the surface but cannot create correspondence between points of reflections and surface points. The latter is inherent in a traditional active light scanning setup whose data alone will, for specular surfaces, be subject to high levels of noise. We propose a novel method that by simulation of the reflection fuses the two data sets to allow digitalising moving specular surfaces with the high slope sensitivity of deflectometric methods.

Our visual system relies on the image structure generated by the interaction of light with objects to infer their material properties. One widely studied surface property is gloss, which can provide information that an object is smooth, shiny or wet. Studies have historically focused on the role of specular highlights in modulating perceived gloss. Here we show in human observers that glossy surfaces can generate both bright specular highlights and dark specular 'lowlights', and that the presence of either is sufficient to generate compelling percepts of gloss. We show that perceived gloss declines when the image structure generated by specular lowlights is blurred or misaligned with surrounding surface shading and that perceived gloss can arise from the presence of lowlights in surface regions isolated from highlights. These results suggest that the image structure generated by specular highlights and lowlights is used to construct our experience of surface gloss.

Real time paper surface-web measurement is one of the challenging research fields. The traditional laboratory method has many limitations and is unable to measure the entire tambour during the manufacturing process. It has been necessary to develop an online technique that could measure the surface topography in real time. An optical technique was developed, based on laser triangulation, and is applied to develop a new prototype device, which characterizes high speed paper-web surfaces over a wide scale of spatial wavelengths spectrum and computes the surface roughness in real time. The used multi channel pulsed laser diode, source of illumination onto the paper-web, is of benefit due to its low coherence length and is capable to deliver a powerful burst of light beam over a 1 µs duration, which delivers energy of 100 µJ per pulse. The short exposure time avoids blurriness in the acquired images which could possible due to the high speed and vibrations on the paper-web.The laser beam is shaped into a narrow line-of-light using cylindrical lenses and is projected onto a paper-web surface, which covers a physical length of about 210 mm. The created line-of-light cross section full width at half maximum, FWHM Gaussian distribution, is 2-3 pixels on the image. The line-of-light is projected onto the paper-web perpendicular to the plane of the surface. The low angled, low specular, reduced coherence length, scattered reflected laser line is captured by the 3 CCD sensors, which are synchronized with the laser source. The low specular light ensures to avoid saturation of the imaging sensors if the surface is very smooth, and obliquely captures the z-directional fine feature of the surface.The scattered phenomenon of the reflected light is responsible for the surface irregularity measurements. The basic image processing algorithm is applied in order to remove noise and cropped the images widthwise so that only pixels above a preset threshold gray level can be processed, which enables efficient real time measurement. The image is transformed into a 1D array using the center of gravity, COG. The accuracy and precision of the COG depends on the line-of-light FWHM, which, in turn, is responsible for the accuracy, noise and the resolution of the developed technique. The image subpixel resolution achieved is 0.01 times a pixel and uuncertainty in the raw data is 0.43 µm while it is 0.05 µm in the rms roughness.The signal processing steps combining the B-Spline filter and the filter in the spatial frequency domain were employed in order to separate roughness, waviness, and form and position error in the raw profile. The prototype is designed to measure online surface roughness and to characterize surface in a spatial wavelength spectrum from 0.09 to 30 mm, which is extendable to any required spatial range in order to cover a wide scale surface feature such as micro roughness, macro roughness and waviness. It is proven that exploitation of a simple laser triangulation technique could lead to an improvement in the overall quality and efficiency in the paper and paperboard industries and it can also be of potential interest for the other surface characterization problems.

Real time paper surface-web measurement is one of the challenging research fields. The traditional laboratory method has many limitations and is unable to measure the entire tambour during the manufacturing process. It has been necessary to develop an online technique that could measure the surface topography in real time. An optical technique was developed, based on laser triangulation, and is applied to develop a new prototype device, which characterizes high speed paper-web surfaces over a wide scale of spatial wavelengths spectrum and computes the surface roughness in real time. The used multi channel pulsed laser diode, source of illumination onto the paper-web, is of benefit due to its low coherence length and is capable to deliver a powerful burst of light beam over a 1 µs duration, which delivers energy of 100 µJ per pulse. The short exposure time avoids blurriness in the acquired images which could possible due to the high speed and vibrations on the paper-web.The laser beam is shaped into a narrow line-of-light using cylindrical lenses and is projected onto a paper-web surface, which covers a physical length of about 210 mm. The created line-of-light cross section full width at half maximum, FWHM Gaussian distribution, is 2-3 pixels on the image. The line-of-light is projected onto the paper-web perpendicular to the plane of the surface. The low angled, low specular, reduced coherence length, scattered reflected laser line is captured by the 3 CCD sensors, which are synchronized with the laser source. The low specular light ensures to avoid saturation of the imaging sensors if the surface is very smooth, and obliquely captures the z-directional fine feature of the surface.The scattered phenomenon of the reflected light is responsible for the surface irregularity measurements. The basic image processing algorithm is applied in order to remove noise and cropped the images widthwise so that only pixels above a preset threshold gray level can be processed, which enables efficient real time measurement. The image is transformed into a 1D array using the center of gravity, COG. The accuracy and precision of the COG depends on the line-of-light FWHM, which, in turn, is responsible for the accuracy, noise and the resolution of the developed technique. The image subpixel resolution achieved is 0.01 times a pixel and uuncertainty in the raw data is 0.43 µm while it is 0.05 µm in the rms roughness.The signal processing steps combining the B-Spline filter and the filter in the spatial frequency domain were employed in order to separate roughness, waviness, and form and position error in the raw profile. The prototype is designed to measure online surface roughness and to characterize surface in a spatial wavelength spectrum from 0.09 to 30 mm, which is extendable to any required spatial range in order to cover a wide scale surface feature such as micro roughness, macro roughness and waviness. It is proven that exploitation of a simple laser triangulation technique could lead to an improvement in the overall quality and efficiency in the paper and paperboard industries and it can also be of potential interest for the other surface characterization problems.

Spectral image acquisition as well as color image is affected by several illumination factors such as shading, gloss, and specular highlight. Spectral invariant representations for these factors were proposed for the standard dichromatic reflection model of inhomogeneous dielectric materials. However, these representations are inadequate for other characteristic materials like metal. This paper proposes a more general spectral invariant representation for obtaining reliable spectral reflectance images. Our invariant representation is derived from the standard dichromatic reflection model for dielectric materials and the extended dichromatic reflection model for metals. We proof that the invariant formulas for spectral images of natural objects preserve spectral information and are invariant to highlights, shading, surface geometry, and illumination intensity. It is proved that the conventional spectral invariant technique can be applied to metals in addition to dielectric objects. Experimental results show that the proposed spectral invariant representation is effective for image segmentation.

In order to reconstruct a surface which has specular reflection from the grayscale information contained in a single two-dimensional (2D) image, a specular shape from shading (SFS) using the fast marching method is presented. Under assumptions that the camera performs an orthographic projection whose direction is equal to the direction of the light source and that the Phong model is used to depict the reflectance property of the surface, the image irradiance equation for specular surface is derived and it is a first-order nonlinear partial differential equation (PDE). By transforming it into a standard Eiknoal PDE, the fast marching method is applied to compute a solution of the resultant Eiknoal equation. Experiments are tested on both a synthetic vase image and a sphere image and the results demonstrate the effectiveness of the presented specular SFS method.

This paper presents a novel technique for measuring the three-dimensional (3-D) shapes of specular surfaces. In this technique, an LCD monitor plane is used as the diffusive light source. It can be vertically moved to two or more different positions. At each position, sinusoid fringe patterns are displayed on the LCD plane and reflected by the measured specular surface. The distorted fringe patterns are captured with a camera, so that the phase distributions at these positions are measured. From the phases, the locus of the incident ray for each pixel is determined in the least squares sense, and further the three-dimensional shape of the specular surface is reconstructed. In so doing, the restrictions and limitations of the existing techniques in computational complexities, phase ambiguities, and error accumulations are eliminated. The validity of this technique has been demonstrated by experimental results.

This paper presents a novel method for estimating specular roughness and tangent vectors, per surface point, from polarized second order spherical gradient illumination patterns. We demonstrate that for isotropic BRDFs, only three second order spherical gradients are sufficient to robustly estimate spatially varying specular roughness. For anisotropic BRDFs, an additional two measurements yield specular roughness and tangent vectors per surface point. We verify our approach with different illumination configurations which project both discrete and continuous fields of gradient illumination. Our technique provides a direct estimate of the per-pixel specular roughness and thus does not require off-line numerical optimization that is typical for the measure-and-fit approach to classical BRDF modeling.

In this paper, we present a rigorous theoretical formulation of the fundamental problem—indirect illumination from area sources via curved ideal specular surfaces. Intensity and area factors are introduced to clarify this problem and to rectify the radiance from these specular surfaces. They take surface geometry, such as Gaussian curvature, into account. Based on this formulation, an algorithm for integrating ideal specular transfers into global illumination is also presented. This algorithm can deal with curved specular reflectors and transmitters. An implementation is described based on wavefront tracing and progressive radiosity. Sample images generated by this method are presented.

A high-accuracy full field 3-D surface profilometer using digital structured fringe projection is presented in the article. In the proposed method, a depth-focus response curve can be established by performing a confocal scanning along the optical axis of the measurement system when a digital fringe is controlled and projected by a digital micromirror device (DMD). To avoid specular light diffusive problems, the developed method projects spatially encoded digital fringe patterns with modulated light intensity onto a shinny lens surface, in order to achieve full field and high accuracy measurement. Some of spherical microlenses have been measured to attest the feasibility of the developed approach. The depth measurement resolution can reach up to 0.1μm and the averaged measurement error was verified to be a submicro scale.

Scattered light from a surface contains a huge amount of information including surface slope, roughness, pattern, texture, etc. In this paper, the relationship between the intensity distribution of scattered light from a surface and the 3D surface topography will be investigated. From the geometrical point of view the intensity distribution of scattered light from a surface may be modelled with three components, the specular spike, specular lobe and diffuse lobe. In the situation when light is scattered from a relatively smooth surface whose roughness is much larger than the wavelength of the incident light, the scattered light intensity distribution will consist of only a diffuse lobe and a specular lobe, which are well described by a combination of the Lambertian and Torrance–Sparrow geometrical models. Based on the light scattering phenomenon, in this paper a scanning method capable of measuring the 3D surface topography with 500 mm scanning width, high scanning speed, and micron resolution is presented. The 3D topography measurement is implemented by using a linear photodiode array to measure scattered light intensity distribution at different angles against the surface normal. Then the specular lobe of the scattered light will be extracted at each scanning point to calculate the surface normal distribution. Sequentially, the 3D surface topography is obtained by using gradient integration techniques.

The random variation in gloss often observed in images produced in electrophotographic printers has been examined by an analytical technique that combines the capabilities of a microdensitometer with a goniophotometer. The technique is called microgoniophotometry and measures both the spatial and the angular distribution of the specular component of reflected light. The analysis provides information about the spatial variation of specularly reflected light at all angles through which the specular light is reflected, not just at the equal/opposite angle at which gloss is traditionally measured. The results of this analysis have lead to an optical model of the random spatial variation in gloss. The results indicate that dry toner is typically not completely fused and can be described as a surface composed of two distinct regions. These two regions differ in the extent of fusing that has occurred, as manifested by their differences in specular reflectance characteristics. The difference in reflectance is manifested primarily in their different angular distributions of specular light and also in their spatial frequency.

During the past several years, considerable work has been presented on the methods for measuring and modeling the observed reflectance properties of materials. However, most of these works have been done under controlled lighting configurations, and require a constant bidirectional reflectance distribution function (BRDF), which is a measure of the amount of light scattered by a medium from one direction into another, or a homogeneous material over the object regions. This paper discusses a method to estimate the reflectance properties of a textured object under uncontrolled illumination. Assuming that the object has a constant specular reflectance property over its surface, the BRDF can be approximated as a constant specular component and a spatially varying diffuse component. The proposed method will first approximate the illumination distribution of a scene from a set of high dynamic range (HDR) radiance maps of a light probe. Next, a patch on the surface of an object is selected; several HDR intensity maps of the patch taken under different viewing conditions are used to estimate the average specular reflectance parameters of this patch. With the separation of the specular component and the diffuse component of each sample, and using the separated diffuse component, the diffuse reflectance parameters for each point over the surface are next estimated. The test results show that the method is effective for textured objects with constant specular reflectance properties. The method can also be directly applied to objects with piecewise-smooth material, which means that although the surface of an object does not have a constant specular reflectance property; when sub-divided into many smaller meshes, the specular parameter of each mesh can be regarded as uniform.

Computer vision plays an important role in various applications. However, the accuracy of computer vision is often degraded by specular reflection. To solve this problem, a system utilizing a high-speed camera and a strobe for removing specular reflection has been shown to be effective. This method utilizes a principle of estimation using images with luminances varied by the flickering of a strobe. However, the method generates noise due to the difference in position of specular reflection among images in video imaging. In this paper, the authors propose a method combining optical flow with the above-mentioned method. The proposed method enables the removal of specular reflection for video imaging without generating noise. Experimental results and comparison with other methods of specular reflection removal show the effectiveness of the proposed method.

Color imaging depends strongly on the illumination conditions. Semi-specular surfaces impose ambiguity in colorimetric analysis and the specular reflection component has to be treated differently depending on the material. To exclude the specular component, special configuration of photometric devices is required. Such conditions cannot easily be met for images of irregular surfaces. I propose a simple method of structured light illumination for separating the specular and diffuse components of semi-specular surfaces using color camera and a calibrated flat-screen monitor.

Specular reflection generally decreases the saturation of surface colors, which will be possibly confused with other colors that have the same hue but lower saturation. Traditional methods for specular reflection separation suffer this problem of hue-saturation ambiguity, producing over-saturated specular-free images quite often. We proposed a two-step approach to solve this problem. In the first step, we produce an over-saturated specular-free image by global chromaticity propagation from specular-free pixels to highlighted ones. Then we recover the saturation based on priors of the piecewise constancy of diffuse chromaticity as well as the spatial sparsity and smoothness of specular reflection. We achieve this through increasing the achromatic component of diffuse chromaticity, while the magnitudes of increments are determined by linear programming under the constraints derived from the priors. Experiments on both laboratory and natural images show that our method can separate the specular reflection while preserving the saturation of the underlying surface colors.

In this paper, a method is proposed to measure the shapes of specular surfaces with one-shot-projection of structured laser patterns. By intercepting the reflection of the reflected laser pattern twice with two diffusive planes, the closed form solution is achieved for each reflected ray. The points on the specular surface are reconstructed by computing the intersections of the incident rays and the reflected rays. The proposed method can measure both static and dynamic specular shapes due to its one-shot-projection, which is beyond the capability of most of state of art methods that need multiple projections. To our knowledge, the proposed method is the only method so far that could yield the closed form solutions for the dynamic and specular surfaces.

Stray light in an endoscope largely contributes to whether a signal can be detected or not. This FRED analysis used a novel endoscope designed for the fallopian tubes to show how common endoscope elements cause stray light contamination, and to offer suggestions on how to mitigate it. Standard and advanced optical raytracing was performed. Raytrace reports determined which ray paths caused the highest power and irradiance distributions after reflecting one or more times from an element in the system. The analysis revealed that the cover plate introduced significantly more stray light into the system than other endoscope components. The imaging lenses and variable stop reflectivity had a negligible impact on the signal. To obtain acceptable signal-to- noise ratio, the source numerical aperture (NA) was lowered to 0.35 and 0.25 to keep the stray light within the same order of magnitude and an order of magnitude lower, respectively than the desired signal. There was a single specular reflection off of the cover plate distal surface. This illumination reflected back into the imaging fiber without first scattering off the tissue, which resulted in high stray power at the back of the imaging lenses. The specular light appeared brighter at higher source NAs and saturated the desired signal at the edge of the imaging fiber. An NA between 0.25 and 0.35 provides maximum illumination to image the tissue, with minimal stray light degrading the desired signal.

Gloss meters are commonly used to measure characteristics of the specular light reflected from materials. Such meters are based on illumination and detection at equal, opposite angles. The particular angle, θ, and other parameters of instrument geometry, are well known to play major roles in the results produced by a given meter, so several standards have been developed for gloss measurements that specify the geometry and optical characteristics of gloss meters (TAPPI T480; ISO-2813 (1994); ASTM-D523-89 (1999)). Nevertheless, the reason why gloss meter readings change with θ, and the reasons why meters of the same 8 produce different readings seem not to be well understood. The focus of the study described in this paper has been on exploring these two effects. A quantitatively model of a generic gloss meter was constructed from Fresnel's law of surface reflection combined with empirical models of bidirectional reflectance distribution function (BRDF). Comparison of the model with experimental data strongly indicates that the width of the BRDF, and therefore the roughness of the surface, plays the major role in governing the reading from a gloss meter. Differences in index of refraction, by comparison, appear to play only a minor role. In addition, differences in gloss readings produced by instruments of the same angle, θ, were found to be the result of differences in the instrument angles of acceptance. The results of these studies suggest that it may be possible to make better use of conventional gloss meter measurements by making measurements at multiple angles, θ, rather than just a single angle.

A study has been carried out on the effect of the optical density of inks on the specular reflectance of a printed surface. Samples of cyan, magenta, and yellow ink were printed with a liquid electrophotographic, an offset lithographic, and a thermal transfer printer. Specular reflectance was measured using a goniophotometric instrument to produce the bidirectional reflectance distribution function. The results clearly demonstrated that the amount of specular light reflected from a printed surface depends strongly on the optical density of the ink. A linear correlation was observed between the total amount of specular light reflected from the surface and the square of the transmittance of the ink layer. A highly transparent ink (e.g., yellow ink measured with red light) reflected approximately twice as much specular light than a highly absorbing ink (e.g., yellow with blue or cyan with red). It is suggested that specular reflections from surfaces below the ink layer can contribute significantly to overall specular reflectance of a printed image.