Novel LED-based radiation source and its application in diffuse reflectometry and polarization measurements

An LED sphere radiator (LED-SR) was constructed to improve the accuracy in spectral radiance factor measurements performed with the robot-based gonioreflectometer at PTB. Its properties with respect to the spectral range and coverage, the temporal stability, and the homogeneity of the radiation field are presented. Two types of matte ceramic reflection standards were used for spectral radiance factor validation measurements comparing the standardly used halogen sphere radiator (Halogen-SR) and the LED-SR. Due to its designed spectral range at the border between the visible and the UV-A spectral range, the LED-SR is well suited for many applications in diffuse reflectometry. Its use for absolute radiance factor measurements and investigations of the fluorescence properties of diffuse reflecting samples is shown. Reliable polarization-resolved measurements at wavelengths below 430 nm could be carried out with PTB’s gonioreflectometer for the first time due to the beneficial signal-to-noise ratio of the LED-SR.

The surface of (4) Vesta in visible light as seen by Dawn/VIR

Aims. We analyzed the surface of Vesta at visible wavelengths, using the data of the Visible and InfraRed mapping spectrometer (VIR) on board the Dawn spacecraft. We mapped the variations of various spectral parameters on the entire surface of the asteroid, and also derived a map of the lithology. Methods. We took advantage of the recent corrected VIR visible data to map the radiance factor at 550 nm, three color composites, two spectral slopes, and a band area parameter relative to the 930 nm crystal field signature in pyroxene. Using the howardite-eucrite-diogenite meteorites data as a reference, we derived the lithology of Vesta using the variations of the 930 and 506 nm (spin-forbidden) band centers observed in the VIR dataset. Results. Our spectral parameters highlight a significant spectral diversity at the surface of Vesta. This diversity is mainly evidenced by impact craters and illustrates the heterogeneous subsurface and upper crust of Vesta. Impact craters also participate directly in this spectral diversity by bringing dark exogenous material to an almost entire hemisphere. Our derived lithology agrees with previous results obtained using a combination of infrared and visible data. We therefore demonstrate that it is possible to obtain crucial mineralogical information from visible wavelengths alone. In addition to the 506 nm band, we identified the 550 nm spin-forbidden one. As reported by a laboratory study for synthetic pyroxenes, we also do not observe any shift of the band center of this feature across the surface of Vesta, and thus across different mineralogies, preventing use of the 550 nm spin-forbidden band for the lithology derivation. Finally, the largest previously identified olivine rich-spot shows a peculiar behavior in two color composites but not in the other spectral parameters.

Methods Calculating the Slab Radiance Factor

One of the most critical problems of realistic visualization of the real-world objects is physically adequate modeling of their reflection of light. Reflection of light by objects occurs both from the surface and the bulk of matter (scattering). Accounting for the light reflection from the surface of objects was solved almost a century ago based on its representation as a Fresnel randomly rough surface. Scattering by a bulk of matter is the subject of radiation transfer theory, which has only recently received its known completion in the form of discrete transfer theory. Strict analytical methods for solving the radiation transport equation (RTE) are often not highly effective for calculating the radiance factor. For a long time, in the absence of effective numerical methods for solving problems and the unavailability of high-speed computers for practical calculations, approximate methods for solving RTE were developed. However, their accuracy and applicability limits were poorly defined. The discrete transfer theory allowed us to evaluate the existing approximate methods for solving the UPI, their accuracy, and the efficiency of application for calculating the radiance factor. It is shown that the most effective method is the method of synthetic iterations. The method is based on the selection of the solution anisotropic part based on a small-angle approximation of the RTE solution. The solution regular part can be calculated by any approximation. Then a simple iteration from the complete solution is performed to refine the angular distribution of the radiance factor.

Light Reflection from Real Surfaces: Probabilistic Model of the Layer Radiance Factor

The article is devoted to the modelling of light reflection from real surfaces. Most of the existing models are often created to solve a certain task and therefore can not be used in other realms. To create a model useful for different purposes, we propose a reflective surface representation as a scattering layer bounded by the diffuse bottom and a randomly rough Fresnel upper boundary. Such an approach allows one to account for light polarization in the scattering layer and slope correlation of the randomly rough boundary which paves the way for the observation of some physical effects that take place in nature (for instance, statistical lens emergence). The first results of the light reflection modelling were taken at such initial parameters as to compare to those obtained in other research. Former occurred to be qualitatively of the same form as the latter. The model needs to undergo further validation in numerous experiments to prove its serviceability for different types of reflective surfaces.

Image-Based Measurement of Structural Color using Spectral Camera

We perceive structural colors by optical phenomena such as light interference and diffraction caused by a fine structure of the object surface. One of the characteristics of structural colors is that a wavelength distribution of light changes depending on an incident angle of a light source and a viewing angle. Generally, for color evaluation and reproduction, it is required to acquire reflection characteristics of objects. Therefore, BRDF (Bidirectional Reflectance Distribution Function) is often used as a function that represents reflection characteristics depending on incident and viewing angles. In this study, we measured BRDF of structural colors based on a method to acquire image-based material reflection characteristics using a spectral camera. The measurement was performed by aligning an optical axis of a spectral camera with a structural color sample and changing an irradiation angle of a light source. Reflection characteristics were represented by using a radiance factor, which was a ratio between a spectral radiance of white material and that of structural color. From measurement results, we confirmed an angle-dependent radiance factor. Finally, based on a measured spectral radiance of a structural color sample, we spectrally reproduced the structural color using a spectral projector based on model fitting of spectral data.

The surface of (1) Ceres in visible light as seen by Dawn/VIR

Aims. We study the surface of Ceres at visible wavelengths, as observed by the Visible and InfraRed mapping spectrometer (VIR) onboard the Dawn spacecraft, and analyze the variations of various spectral parameters across the whole surface. We also focus on several noteworthy areas of the surface of this dwarf planet. Methods. We made use of the newly corrected VIR visible data to build global maps of a calibrated radiance factor at 550 nm, with two color composites and three spectral slopes between 400 and 950 nm. We have made these maps available for the community via the Aladin Desktop software. Results. Ceres’ surface shows diverse spectral behaviors in the visible range. The color composite and the spectral slope between 480 and 800 nm highlight fresh impact craters and young geologic formations of endogenous origin, which appear bluer than the rest of the surface. The steep slope before 465 nm displays very distinct variations and may be a proxy for the absorptions caused by the O2− → Fe3+ or the 2Fe3+ → Fe2+ + Fe4+ charge transfers, if the latter are found to be responsible for the drop in this spectral range. We notice several similarities between the spectral slopes and the abundance of phyllosilicates detected in the infrared by the VIR, whereas no correlation can be clearly established with carbonate species. The region of the Dantu impact crater presents a peculiar spectral behavior – especially through the color and the spectral slope before 465 nm – suggesting a change in composition or in the surface physical properties that is not observed elsewhere on Ceres.

On Variants of the Main Atmospheric Correction Formula

The article studies the accuracy of the main formula of atmospheric correction allowing us to determine albedo of a underlying (earth) surface based on radiance factor of solar light reflected by the system of atmosphere and underlying surface. The problem of atmospheric correction is considered in three-dimensional geometry with spatial non-uniformity of the underlying surface taken into account. It is demonstrated that the accuracy of albedo recovery depends on the used variant of the main formula.

Mathematical Model of a Surface Radiance Factor

The article is devoted to the creation of a surface radiance factor mathematical model. The basis of the model is the solution of the boundary value problem of the radiative transfer equation (RTE). The surface is considered as a structure consisting of several turbid layers, each of which is characterized by its optical parameters. The top of the structure is randomly rough, uncorrelated, Fresnel. The lower boundary reflects perfectly diffusely. The complexity of solving the RTE boundary value problem for real layers is due to the fact that the suspended particles in each layer are always much longer than the wavelength. This leads to a strong anisotropy of the radiance angular distribution according to Mie theory. The solution comes down to a system of equations by the discrete ordinates method that consists of several hundred of differential equations. Subtraction of the anisotropic part from the solution based on an approximate analytical solution of the RTE allows avoiding this problem. The approximation is based on a slight decrease in the anisotropic part of the angular spectrum. The matrix-operator method determines the general solution for a complex multilayer structure. The calculation speed can be increased without compromising the accuracy of the solution with the help of the synthetic iterations method. The method consists of two stages: the first one repeats the described one with a small number of ordinates; on the second one the iteration of it is performed. The model is realised in the Matlab software.