Directional Thermal Radiation Research Articles

A layered metastructure based on brewster angle for Janus ultra-wideband tunable directional thermal radiation

In this proposal, we introduce a Janus layered metastructure (JLM) composed of three distinct structures. The first and third structures (S1 and S3) are designed based on Brewster and Bragg reflection laws, which enable the selection of a specific incident angle of electromagnetic waves (EMWs) by adjusting the refractive indices of the materials. By matching the thickness of the dielectric layer to the optical wavelength, S1, and S3 operate across the 30–300 μm band, achieving angle selection (AS) of EMWs in the THz band. The second structure (S2) consists of a single layer of graphene and SiO2 arranged in quasi-periodic sequences, which absorbs EMWs in the THz band and achieves broadband absorption without angle sensitivity. By combining the characteristics of these three structures, we achieve a broadband directional thermal radiation covering the 30–300 μm band. Additionally, the adjustability of the AS of S1 and S3 and the absorption effect of S2 enable Janus ultra-wideband tunable directional thermal radiation at large angles depending on the specific needs.

Multilayer-structured fibrous membrane with directional moisture transportability and thermal radiation for high-performance air filtration

Abstract The demand of high-performance filter media for the face masks is urgent nowadays due to the severe air pollution. Herein, a highly breathable and thermal comfort membrane that combines the asymmetrically superwettable skin layer with the nanofibrous membrane has been fabricated via successive electrospinning and electrospraying technologies. Thanks to high porosity, interconnected pore structure, and across-thickness wettability gradient, the composite membrane with a low basis weight of 3.0 g m−2 exhibits a good air permeability of 278 mm s−1, a comparable water vapor permeability difference of 3.61 kg m−2 d−1, a high filtration efficiency of 99.3%, a low pressure drop of 64 Pa, and a favorable quality factor of 0.1089 Pa−1, which are better than those of the commercial polypropylene. Moreover, the multilayer-structured membrane displays a modest infrared transmittance of 92.1% that can keep the human face cool and comfort. This composite fibrous medium is expected to protect humans from PM2.5 and keep them comfortable even in a hygrothermal environment.

On the Derivation of Geometric Optical Kernels for Directional Thermal Radiation

AbstractThe derivation of widely used geometric optical (GO) kernels in bidirectional reflectance distribution function models, that is, LiSparseReciprocal kernel (KGOLSR) and LiDenseReciprocal kernel, was based on two important assumptions: (1) The shaded components are perfect black and (2) the contributions of two sunlit components are identical. Different from the bidirectional reflectance, thermal radiation directionality effect mainly results from component temperature differences, suggesting the above assumptions are not applicable in most situations. Therefore, this study derived GO kernels for thermal radiation based on temperature differences rather than illumination differences. Specifically, four GO kernels, that is, KGO4 with considering sunlit/shaded vegetation and sunlit/shaded soil, KGO3 with considering sunlit/shaded soil and vegetation, KGO2 with considering vegetation and soil, and KGOg only considering the hottest sunlit soil, have been developed. By using a comprehensive simulated data set, their performances have been thoroughly evaluated and the comparison with KGOLSR has also been analyzed in depth. Results showed that (1) KGO4 had the highest accuracy and KGO3 was the second; in the case of only two available angles, KGOg performed best. (2) Variables such as component temperature, component emissivity, solar zenith angle, and the percentage of tree crown cover mainly affected the comparison result between KGOLSR and KGO2; KGOLSR would have a better performance for a scene with a stronger vegetation effect. Moreover, the best values of two structure characteristics, that is, the crown shape parameter b/r and relative height h/b, for these five kernels have been determined, which can provide instruction for practical application.

Evaluation of Three Parametric Models for Estimating Directional Thermal Radiation from Simulation, Airborne, and Satellite Data

An appropriate model to correct thermal radiation anisotropy is important for the wide applications of land surface temperature (LST). This paper evaluated the performance of three published directional thermal radiation models—the Roujean–Lagouarde (RL) model, the Bidirectional Reflectance Distribution Function (BRDF) model, and the Vinnikov model—at canopy and pixel scale using simulation, airborne, and satellite data. The results at canopy scale showed that (1) the three models could describe directional anisotropy well and the Vinnikov model performed the best, especially for erectophile canopy or low leaf area index (LAI); (2) the three models reached the highest fitting accuracy when the LAI varied from 1 to 2; and (3) the capabilities of the three models were all restricted by the hotspot effect, plant height, plant spacing, and three-dimensional structure. The analysis at pixel scale indicated a consistent result that the three models presented a stable effect both on verification and validation, but the Vinnikov model had the best ability in the erectophile canopy (savannas and grassland) and low LAI (barren or sparsely vegetated) areas. Therefore, the Vinnikov model was calibrated for different land cover types to instruct the angular correction of LST. Validation with the Surface Radiation Budget Network (SURFRAD)-measured LST demonstrated that the root mean square (RMSE) of the Moderate Resolution Imaging Spectroradiometer (MODIS) LST product could be decreased by 0.89 K after angular correction. In addition, the corrected LST showed better spatial uniformity and higher angular correlation.

Quantum Heat Engine Using Electromagnetically Induced Transparency.

We report an experiment demonstrating the generation of directional thermal radiation with a spectral brightness that is about 9 times greater than that of the ambient pumping reservoir. The experiment is based on the recent proposal for a nontraditional quantum heat engine and uses cold Rb atoms, electromagnetically induced transparency, and photon correlation spectroscopy [Phys. Rev. A 94, 053859 (2016)PLRAAN2469-992610.1103/PhysRevA.94.053859].

Estimation of Upward Longwave Radiation From Vegetated Surfaces Considering Thermal Directionality

Surface upward longwave radiation (SULR) is an important component of the surface energy balance and is closely related to land surface temperature and emissivity. The estimation of SULR plays an important role in the study of surface energy circulation and climate change. State-of-the-art methods to estimate SULR, including the physical method and the hybrid method, are conducted without considering directional thermal radiation (DTR), which may induce a large error in the estimation, particularly over sparsely vegetated surfaces. In this paper, we modified the physical temperature–emissivity algorithm by combining a directional emissivity model (FRA97) and a kernel-driven DTR model to estimate the SULR of vegetated surfaces while considering the thermal directionality of the land surface. The most suitable kernel-driven model and an angle combination of the DTR were selected from six kernel-driven models and five angular combinations. The sensitivity of the proposed algorithm to the input parameters was also analyzed. The proposed algorithm was then validated with the Wide-angle infrared Dual-mode line/area Array Scanner (WiDAS) data set and longwave radiation data of automatic meteorological stations from the Heihe Watershed Allied Telemetry Experimental Research experiment. The results showed that the five-angle combination with large-angle intervals performs the best. When the leaf area index (LAI) is less than 1.2, the RossThick-LiSparseR model performs the best; when LAI is larger than 1.2, the RossThick-LiDenseR model is the most accurate. The SULR is not sensitive to surface downward longwave radiation and LAI, is slightly sensitive to leaf and soil emissivity at certain LAIs, and is highly sensitive to DTR, which may greatly affect the accuracy of the estimated SULR. The root-mean-square error (RMSE) and the mean bias error (MBE) of the SULR estimated using the WiDAS data and the proposed algorithm are 5.618 and −1.642 W/m2, respectively, thereby improving the estimation accuracy by as much as 7.479 and 10.511 W/m2 at most in terms of RMSE and MBE, respectively, compared with the results calculated without considering the DTR.

Directional thermal emission control by coupling between guided mode resonances and tunable plasmons in multilayered graphene

Directional thermal radiation is attracting interest because of its applicability to thermal management systems and mid-infrared light sources. Plasmons in a single graphene layer are tunable by the chemical potential, while the lateral wavenumber of the plasmon dispersion is too large for the directional coupling to the far field. In this paper, we achieve directional thermal radiation by utilizing tunable plasmons in multilayered graphene. The lateral wavenumber of the plasmon is shown to be reduced as the number of graphene layers increases, and the reduction is analytically explained. The thermally excited graphene plasmon couples to the guided mode resonance in a silicon grating through evanescent waves so as to realize angular-selective far-field emission. We develop a modal analysis in order to investigate the coupling condition. In addition, the directional thermal emission including asymmetric one can be tuned by varying the chemical potential of graphene layers. The calculated emissivity obtained by changing both the chemical potential and the height of the grating suggests the control of the angular heat flux profile.

Determination of optimum viewing angles for the angular normalization of land surface temperature over vegetated surface.

Multi-angular observation of land surface thermal radiation is considered to be a promising method of performing the angular normalization of land surface temperature (LST) retrieved from remote sensing data. This paper focuses on an investigation of the minimum requirements of viewing angles to perform such normalizations on LST. The normally kernel-driven bi-directional reflectance distribution function (BRDF) is first extended to the thermal infrared (TIR) domain as TIR-BRDF model, and its uncertainty is shown to be less than 0.3 K when used to fit the hemispheric directional thermal radiation. A local optimum three-angle combination is found and verified using the TIR-BRDF model based on two patterns: the single-point pattern and the linear-array pattern. The TIR-BRDF is applied to an airborne multi-angular dataset to retrieve LST at nadir (Te-nadir) from different viewing directions, and the results show that this model can obtain reliable Te-nadir from 3 to 4 directional observations with large angle intervals, thus corresponding to large temperature angular variations. The Te-nadir is generally larger than temperature of the slant direction, with a difference of approximately 0.5~2.0 K for vegetated pixels and up to several Kelvins for non-vegetated pixels. The findings of this paper will facilitate the future development of multi-angular thermal infrared sensors.

A bi-directional gap model for simulating the directional thermal radiance of row crops

Row crops are a kind of typical vegetation canopy between discrete canopy and continuous canopy. Kimes et al. studied the directional thermal radiation of row crops using the geometrical optical model, which simplified row structure as “box” and neglected the gap among foliage and did not consider the emissivity effects. In this work we take account of the gaps along illumination and viewing directions and propose a bi-direction gap model on the basis of the idea of gap probability of discrete vegetation canopy introduced by “Li-Strahler” and inter-correlation of continuous vegetation developed by Kuusk. It can be used to explain “hot spot” effects in thermal infrared region. The gap model has been validated by field experiment on winter wheat planted in shape of rows and results show that the gap model is better than Kimes’ model in describing the directionality of thermal infrared emission for row crops. Keywords: DOI:

Revealing of major factors in the directional thermal radiation of ground objects

This paper is devoted to eliminating the noise from the measuring of directional thermal radiation for ground objects. Specifically, we think that the noise is mainly due to the variance of components in the field of view of the sensor with the view angle changing and to the heat balance change on the ground during the period of measurement. The authors present two new observation methods named as “constant area method by thermal camera” and “concurrent method by dual sensors” respectively. The experiments show that the data obtained by these methods abide by some regularly directional distribution, which is totally different from the data from the former methods. The analysis of the major factors in the directionality of thermal radiation is also made in the paper.

Component temperatures inversion for remote sensing pixel based on directional thermal radiation model

When the remote sensing pixel is composed of multiple components and a non-isothermal surface, its directional signature of thermal-infrared radiation is mainly determined by the 3D structure of the pixel. In this paper, we present our simple directional thermal radiation model to describe the relation between the pixel thermal emission and the pixel’s component parameters, and invert the model to get the component temperatures. For the inversion algorithm, we focus on how to use the information of given observations in a more effective way. The information content in inversion procedure is studied. Our forward model and inversion method are validated using indoor directional measurement data.

Spectral and directional thermal radiation characteristics of selective sufaces for solar collectors

Abstract Surface systems suitable for collectors of solar heat energy are examined. Spectral radiation characteristics of metallic substrates coated with semiconductor and antireflection layers are considered. Directional spectral reflectances of several surface coatings are presented and evaluated. Commercial chemical dip treatments of copper and steel are shown to give selective characteristics desirable for low temperature collectors such as solar water heaters.