Specular Region Research Articles

Computational ghost imaging with adaptive intensity illumination for scenes featuring specular surfaces

Imaging a target scene with specular surfaces is a daunting challenge for both direct imaging and indirect computational imaging techniques. The intense specular reflection component during the measurement severely degrades the quality of the reconstructed image, resulting in a substantial loss of scene information. To address this issue, we propose a computational ghost imaging (CGI) method with adaptive intensity illumination. Capitalizing on the encoded imaging feature of CGI, this method enables effective imaging of target scenes with specular surfaces through two series of measurements, eliminating the necessity for additional optical components. Based on the position and intensity information of pixels in the specular regions from the first series of measurements, our method modulates the illumination patterns to weaken the intensity of the specular region in the second series of measurements. Simulation and experimental results demonstrate that the utilization of these modulated illumination patterns for target scene measurement effectively mitigates interference from the specular surface during imaging. Consequently, the reconstructed image is capable of presenting more detailed information about the target scene other than the specular regions. Our work introduces a novel approach for imaging target scenes with specular surfaces and broadens the scope of applications for CGI in reality.

High dynamic range 3D measurements based on space-time speckle correlation and color camera.

Structured light (SL) based three-dimensional (3D) measurement struggles to estimate high dynamic range (HDR) scenes, where both high and low reflectivity parts exist simultaneously. This paper proposes a method through the joint design and optimization of hardware and algorithms, in which only four frames are required to realize the 3D reconstruction of HDR scenes. The height information of each sub-area in the scene under test can be encoded effectively by temporally projecting two sets of complementary speckle patterns onto target surface. To decode the corresponding patterns captured by the cameras, we design a stereo matching strategy consisting of space-time binary feature (ST-BIF) descriptor preliminary screening and zero-mean normalized cross-correlation (ST-ZNCC) final retrieval. The ST-BIF descriptor based on neighborhood comparison is designed to describe the space-time relative intensity change of projected speckles. Besides the HDR adaptability, the ST-BIF descriptor can effectively improve the matching speed. In addition, the measurable dynamic range can be further improved by fusing all channel disparities as evaluated results, benefitting from the different response of R, G and B channels in color camera to monochromatic light. Experiments are conducted to demonstrate the feasibility of the proposed method. The results indicate that our method achieves the root mean square error 0.2516mm (vs. 1.0668 by commonly used ZNCC) and an average coverage rate up to 94.87% (vs. 93.35% by commonly used ZNCC). Furthermore, the experimental results show that the proposed method can achieve 3D reconstruction of HDR scenes including specular reflection region.

Specular Detection on Glossy Surface Using Geometric Characteristics of Specularity in Top-View Images.

In an autonomous driving assistance system (ADAS), top views effectively represent objects around the vehicle on a 2D plane. Top-view images are therefore widely used to detect lines in ADAS applications such as lane-keeping assistance and parking assistance. Because line detection is a crucial step for these applications, the false positive detection of lines can lead to failure of the system. Specular reflections from a glossy surface are often the cause of false positives, and since certain specular patterns resemble actual lines in the top-view image, their presence induces false positive lines. Incorrect positions of the lines or parking stalls can thus be obtained. To alleviate this problem, we propose two methods to estimate specular pixels in the top-view image. The methods use a geometric property of the specular region: the shape of the specular region is stretched long in the direction of the camera as the distance between the camera and the light source becomes distant, resulting in a straight line. This property can be used to distinguish the specular region in images. One estimates the pixel-wise probability of the specularity using gradient vectors obtained from an edge detector and the other estimates specularity using the line equation of each line segment obtained by line detection. To evaluate the performance of the proposed method, we added our methods as a pre-processing step to existing parking stall detection methods and investigated changes in their performance. The proposed methods improved line detection performance by accurately estimating specular components in the top-view images.

Multiple Diffuse Coding Metasurface of Independent Polarization for RCS Reduction

In this article, a multiple diffuse coding metasurface (MDCM) of independent polarization is designed to control the propagation direction of diffuse reflections under different polarizations and to improve the monostatic and bistatic RCS (radar cross section) reduction effect. First, a method for controlling the distribution range and propagation direction of the diffuse field is studied, and the diffuse field distribution of the random phase metasurface is optimized by a genetic algorithm to improve the uniformity of the diffuse scattering distribution. Then, the random phase distribution is superimposed on the periodic gradient phase distributions of the linear and hedge types in the orthogonal direction so that the main propagation direction of the diffuse metasurface deviates from the specular reflection region under different polarizations, showing single and two diffuse beams. Finally, the anisotropic unit cell with a rectangle inside and an improved Jerusalem cross on the outside is employed as the basic coding element of the MDCM due to its independent polarization phase response. The numerical and experimental results show that the MDCM features multiple diffuse scattering, independent polarization and angle insensitivity and can efficiently improve the monostatic and bistatic RCS reduction effect simultaneously. Because the scattered energies are redirected away from the specular reflection direction, the specular scattering reduction effect is better than the isotropic diffuse metasurface. The proposed method increases the difficulty of detection by single or netted radar and has the potential for the applications of stealth techniques.

Perceptual brightness-based inverse tone mapping for high dynamic range imaging

This paper proposes a novel high dynamic range imaging method based on inverse tone mapping for generating a high dynamic range image from a single low dynamic range image. With the increasing expectations for image quality, displays providing high dynamic range images have been released. However, although low dynamic range images are currently commonly used, they do not use the high representation capability of high dynamic range displays. Moreover, high dynamic range contents are scarce. Therefore, high dynamic range imaging techniques are gaining attention. The proposed method involves adaptive specular region detection to determine the bright and dark areas for high dynamic range imaging, output mapping range decision to consider perceptual brightness, final mapping-level decision to prevent degradation of perceptual image quality, and inverse pixel mapping for luminance mapping. The simulation results of the proposed method yielded an HDR-VDP-2 quality score of 59.51, which was higher than those of the conventional methods. Furthermore, the execution time of the proposed algorithm was 0.66 s, which had the lowest complexity compared to the conventional methods. For subjective performance evaluation, we analyzed the perceived image quality on the display with the DRIM, which confirmed the superiority of the proposed method over the conventional methods.

Selection of Spectral Resolution and Scanning Speed for Detecting Green Jujubes Chilling Injury Based on Hyperspectral Reflectance Imaging

Hyperspectral imaging is a non-destructive method for the detection of chilling injury in fruit. However, the limitation of this technique is the lacking of an appropriate working parameters and a feasible discriminating model for chilling on-line sorting. This research was aimed to select the optimal spectral resolution, scanning speed, and classification model for green jujube chilling injury detection based on hyperspectral reflectance imaging. Criminisi algorithm was firstly carried out to reconstruct the specular reflection region in spectral images before deriving mean spectra, and thus the optimal wavelengths were selected by random frog. Results showed that the Criminisi algorithm presented a desirable ability of spectral image inpainting. The linear discriminant analysis (LDA) achieved overall accuracies of 98.3% and 93.3% for two-class and three-class classification, respectively, at the speed of 20 mm/s with the spectral resolution of 5.03 nm based on selected spectral features. The results demonstrated that 20 mm/s with the spectral resolution of 5.03 nm was more feasible for the detection of green jujube chilling injury in hyperspectral imaging system due to a higher scanning efficiency, but a less data size.

A Study of Wind Direction Effects on Sea Surface Specular Scattering for GNSS-R Applications

A modeling study investigating the influence of wind direction on spaceborne global navigation satellite system reflectometry (GNSS-R) near-specular observations of the sea surface is reported. The study first focuses on a purely specular geometry under plane wave incidence, for which it is shown using the theorem of reciprocity and reflection symmetry that up-down wind variations are identically zero. It is also shown that “single-scattering” approximations of rough surface scattering predict no variations with wind direction of any kind for a purely specular geometry, while higher order approximations (such as the second-order small-slope approximation) can predict up/cross wind differences. Examples of these variations are reported and found to be small. Because the delay doppler maps (DDMs) measured in GNSS-R include some nonspecular contributions even for “specular” portions of the DDM, the second part of the study performs an examination of near-specular DDM variations with wind direction using the widely used geometrical optics approximation of surface scattering for a surface described with the non-Gaussian Cox–Munk slope probability density function. Variations with wind direction of the normalized radar cross section (NRCS) mapped onto the surface are examined, and again, it is shown that these variations are small for surface portions contributing to the near-specular portion of the DDM. In addition, it is shown that the dependencies of the bistatic NRCS on wind direction are such that differing portions of the surface “glistening zone” have differing phase shifts in their dependence on wind direction, causing the wind dependencies of the final near-specular DDM to be negligible. The final results of the study suggest that any wind direction dependence in spaceborne GNSS-R should be sought only in portions of the DDM away from the specular region. These results provide information to guide analyses of the wind direction information available in current GNSS-R missions such as TDS-1 and Cyclone Global Navigation Satellite System.

Dynamic Searching and Classification for Highlight Removal on Endoscopic Image

Endoscopic imaging is a common clinical modality to inspect surficial abnormality grew on the internal organs inside human body. Covered by tissue fluid, surface of these anatomies tend to be glossy, showing specular reflections from the illumination source. In this paper, we present a novel method for specular region separation and restoration from only a single image. Distinguishing from segmentation methods using simple threshold, our solution treats the separation of highlight pixels as a binarization problem based upon a supervised learning classification algorithm. Also, we propose a multiscale dynamic image expansion and fusion based method to restore the highlighted region. It takes full advantages of propagating the regions with similar structure features to specular regions. Experimental results on the removal of the endoscopic image with specular reflections demonstrate improved efficiency by the proposed method compared to commonly used techniques.

Radar Response of Off-Specular Bistatic Scattering to Soil Moisture and Surface Roughness at L-Band

This letter investigates the bistatic radar response of soil moisture and surface roughness of bare soil surfaces at L-band using the advanced integral equation model (AIEM). It focuses on the use of bistatic geometries away from the specular region. To better explore the potential of bistatic scattering for soil moisture sensing, both polarized and angular scattering coefficients, and their combinations, are evaluated using a defined sensitivity index. The results show that sensitivity is enhanced in a bistatic mode compared with the monostatic case. Using a combination of dual polarized and angular data suppresses an undesired impact of the surface correlation function. Moreover, the forward region is preferred to soil moisture sensing regardless of the surface correlation function. Among the combinations investigated, the dual angular observation reduces the influence of surface roughness, preserves a good sensitivity to soil moisture response, and thus seems to be a good candidate for soil moisture sensing in bistatic configuration.

Reflective Inverse Diffusion

Phase front modulation was previously used to refocus light after transmission through scattering media. This process has been adapted here to work in reflection. A liquid crystal spatial light modulator is used to conjugate the phase scattering properties of diffuse reflectors to produce a converging phase front just after reflection. The resultant focused spot had intensity enhancement values between 13 and 122 depending on the type of reflector. The intensity enhancement of more specular materials was greater in the specular region, while diffuse reflector materials achieved a greater enhancement in non-specular regions, facilitating non-mechanical steering of the focused spot. Scalar wave optics modeling corroborates the experimental results.

Depolarization in polarizing supermirrors

We present data on depolarizing effects in polarizing mirrors. At typical magnetizing field strengths used in polarizing devices, depolarizations rise up to the percent level in the specular region and are shown to be successfully suppressed to 10−4 when increasing the magnetizing field. We show evidence linking a part of this depolarization to lateral correlation of the magnetization fluctuations in the ferromagnetic layers. Effects of the supermirror factor (m), wavelength and incidence angle are studied. The findings are applied to a crossed supermirror geometry and we report a neutron beam polarization of 99.97(1)% for a beam of wavelength λ=5.3Å,Δλλ=0.1 (FWHM).

Effects of a measurement floor on Mueller matrix measurements in a dual rotating retarder polarimeter bidirectional scatter distribution function system.

Since a measurement of the bidirectional scatter distribution function (BSDF) of a material is proportional to the intensity of the scattered light, a BSDF measurement system with the addition of a dual rotating retarder polarimeter can be used to calculate the Mueller matrix of a scatterer. One advantage of a BSDF system using a laser source is its large dynamic range, which allows the measurement of scattered light both near to and away from the specular region. As BSDF measurements move away from the specular region and into a more diffuse-scatter region, the measured signal decreases and may approach the system's measurement floor. Therefore, BSDF and Mueller-matrix measurements are dependent not only on the scatter from the sample but also on the noise floor of the system. By analyzing numerically created bidirectional reflectance distribution function data, we show that since the noise floor of a system is typically constant, the Mueller-matrix measurement at the noise floor appears to be that of a perfect depolarizer. Therefore, as the BSDF measurement space moves away from the high-signal region and the noise floor is approached, the Mueller matrix assigned to the sample artificially approaches that of a perfect depolarizer. The rate and location in scatter-angle space of this shift is dependent on the BSDF of the material and on the signal-to-noise ratio in the system. Therefore, caution must be taken when drawing conclusions about measured Mueller matrices for scattered light, particularly in measurement regions where the measured signal approaches the system floor.

Determination of particle-induced structural disorder depth profile in crystals using the grazing-angle incidence hard x-ray backscattering diffraction technique**This paper is dedicated to Professor Dr Petros H Bezirganyan on the occasion of his 95th birthday on 15th December 2011.

In this theoretical paper, we propose an x-ray imaging method for determination of particle-induced structural disorder depth profile in the crystalline materials based on the extremely sensitive, high-resolution, and non-destructive grazing-angle incidence hard x-ray backscattering diffraction technique. A peculiar value of the Bragg angle is discovered within the specular beam suppression angular region for which the curve of x-ray reflectivity is very close to the profile of the corresponding structural disorder. The coincidence presents a unique opportunity for the direct registration of the structural disorder depth profile in particle-irradiated crystals.

Bistatic scattering analysis for two-dimensional rough sea surfaces using an angular composite model

An angular composite model (ACM) is applied to analyse the bistatic scattering from two-dimensional sea surfaces. When the local scattered angle lies in the specular region, Kirchhoff approximation (KA) is used to evaluate the specular reflection, which predominates the total scattering in this region; the small perturbation method (SPM) is employed to deal with the diffuse reflection dominated by small roughness, as the local scattered angle is situated out of the specular region. This avoids dwelling on the tough question about how to choose the proper cut-off wave number like in the well-known two-scale model (TSM). Numerical illustrations are carried out for bistatic scattering coefficients versus wind speed, incident angle, incident frequency and polarization state, respectively. The comparison of numerical results of the ACM and other experimental and theoretical models in several configurations shows that the ACM is robust to give accurate numerical evaluations for bistatic scattering of rough sea surfaces.

Mie scattering in the time domain Part II The role of diffraction

The p=0 term of the Mie-Debye scattering amplitude contains the effects of external reflection and diffraction. We computed the reflected intensity in the time domain as a function of the scattering angle and delay time for a short electromagnetic pulse incident on a spherical particle and compared it to the predicted behavior in the forward-focusing region, the specular reflection region, and the glory region. We examined the physical consequences of three different approaches to the exact diffraction amplitude, and determined the signature of diffraction in the time domain. The external reflection surface wave amplitude gradually replaces the diffraction amplitude in the angular transition region between forward-focusing and the region of specular reflection. The details of this replacement were studied in the time domain.

An angular cutoff composite model for investigation on electromagnetic scattering from two-dimensional rough sea surfaces

Based on the local configuration angle division to select the corresponding method for electromagnetic scattering calculation from rough sea surface, this paper presents an angular cutoff composite model: when the local scattered angle is in the specular region that is given by an approximately 20 degrees cone around the specular direction, the Kirchhoff approximation is applied to evaluate the specular reflection, which dominates the total scattering in this region; the small perturbation method is employed to handle the diffuse reflection which is predominant as the local scattered angle is situated out of the specular region. Numerical results are compared with those of experimental and theoretical models in several configurations as a function of incident angle, wind speed, wind direction. The comparison of numerical results of other experimental and theoretical models in several configurations shows that the new composite model is robust to give accurate numerical evaluations for the sea surface scattering.

Gloss and goniocolorimetry of printed materials

AbstractAngular distribution of reflectance spectra and of colorimetric parameters for printed papers (varnished and not) and foils, clear and metallized, were measured by goniospectrophotometry, with a photomultiplier for visible region, or a spectrocolorimeter as a detector, keeping the light source in a fixed position, −45° anormally. All reflectance spectra were related to near‐Lambertian BaSO4 powder white standard, relative to which the color coordinates were calculated. The photometric cosine law holds for an ideal diffuser; consequently, all color parameters are significantly perturbed by gloss in a large region, several 10's of degrees around the specular angle. The reflectance peak in the specular angle region of glossy material can be well approximated by the Lorentz function, especially in the vicinity of the specular angle. Lightness, chroma, and hue angle in the gloss angle region suffer as a result of high changes and extremes depending on the gloss of the material tested. This is what complicates color measurements and full determination or definition of the complex appearance of printed materials. The change of chroma with varnishing of glossless materials was clearly elucidated. It was found that “bottom color gloss” considerably influences the appearance of printed metallized foils or other prints on high glossy substrates. © 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 335–342, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10177

A comparison of perturbation theory and the small-slope approximation for acoustic scattering from a rough interface for a Biot medium

In this paper, the lowest order small-slope approximation (SSA) scattering cross section for Biot theory is derived. Numerical results are obtained for both backscattering and bistatic scattering using a modified power law spectrum, and these results are compared with those of lowest order perturbation theory (PT). Frequencies ranging from 100 Hz to 3 kHz are used for surfaces with RMS heights h of 0.1 and 1 m and a correlation length l of 10 m. The angle of incidence for the bistatic results is limited to 45/spl deg/. It is found that for the smaller surface height roughness (h = 0.1 m), the SSA and PT give the same results for frequencies up to almost 1 kHz for both backscattering and bistatic scattering. For h = 1 m, the SSA and PT backscatter results are in good agreement at all frequencies for incident grazing angles up to approximately 45/spl deg/. For the bistatic results, the SSA and PT results agree only at low grazing angles of scatter. In the specular region, the results differ significantly.

On the classical behavior, coherent and incoherent contributions to scattering of atoms from a surface

In this work we calculate the coherent and incoherent contributions to the diffraction probability of atoms scattered by a surface. We are interested in exploring the relative importance of each of these contributions, and compare them with results obtained from molecular dynamics calculations. To achieve this goal, we employed a method developed by Heller that consists of writing the incident plane wave as a sum of Gaussian wave packets, propagating them independently by using the time dependent Schrödinger equation, and constructing the scattered wave function by adding coherently the scattered packets. For the system studied, the molecular dynamics results show the largest intensity in the specular region and also display a classical rainbow structure. On the other hand, the quantum results exhibit diffraction features, with the coherent contribution accounting for most of the total intensity probability.

Off-specular reflection from flat interfaces

The reflection of x ray or neutron from a flat interface with density or compositional fluctuations was studied. An integral equation relating the reflection intensities to the interfacial fluctuations was derived in the Fraunhofer limit. At the specular condition this integral equation, although derived for the off-specular intensities, has an expression identical to a well-established one for the specular component. This integral equation was applied to a flat surface onto which gold spheres of uniform size were randomly adhered. The out-of-plane components for both the real wave and the time reversed wave were approximated by the solution from the specular component. Based on the calculated off-specular intensities near the specular region, an approximation scheme was proposed to estimate the in-plane correlation length.