Validation of the North American ASTER Land Surface Emissivity Database (NAALSED) version 2.0 using pseudo-invariant sand dune sites

Knowledge of the Land Surface Emissivity (LSE) in the Thermal Infrared (TIR: 8-12 µm) part of the electromagnetic spectrum is essential to derive accurate Land Surface Temperatures (LSTs) from spaceborne TIR measurements. This study focuses on validation of the emissivity product in the North American ASTER Land Surface Emissivity Database (NAALSED) v2.0 — a mean seasonal, gridded emissivity product produced at 100 m spatial resolution using all Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) scenes from 2000 to 2008 over North America (http://emissivity.jpl.nasa.gov). The NAALSED emissivity product was validated over bare surfaces with laboratory measurements of sand samples collected at nine pseudo-invariant sand dune sites located in the western/southwestern USA. The nine sand dune sites cover a broad range of surface emissivities in the TIR. Results show that the absolute mean emissivity difference between NAALSED and the laboratory results for the nine validation sites and all five ASTER TIR bands was 0.016 (1.6%). This emissivity difference is equivalent to approximately a 1 K error in the land surface temperature for a material at 300 K in the TIR.     

A Graphical Method for Calculating Reflection Errors in Radiation Thermometry

This article presents a graphical method to calculate reflection corrections for radiation thermometry. The method is based on the observation that the measured radiance of a target is a linear combination of radiances dependent on the emissivity of the target. The method is most easily implemented as a nomogram, enabling thermometer users to estimate reflection corrections immediately when measurements are taken. The nomogram also provides a visual means of explaining the impact of reflection errors on measurements and for explaining the impact of measurement uncertainties on temperature measurements corrected for reflections.     

The effects of emissivity on the performance of steel in furnace tests

The role that surface emissivity plays in the standard furnace test (BS476) is considered for steel sections. Steel samples coated with either a low-emissivity paint or a high-emissivity paint were subjected to furnace tests and cone calorimeter tests in order to quantify the degree to which emissivity affects performance. The cone calorimeter experiments were designed primarily to determine the emissivity of the coatings and to compare with the manufacturer's estimates. However, a welcome additional benefit of this analysis was an estimate of the average convection heat transfer coefficient h for horizontal test specimens in the cone calorimeter. Our measurements suggest that h has been significantly underestimated (in some cases by at least 50%) in the literature to date. Most studies appear to assume a value for h that is close to the value for free convection for a hot plate with hot surface uppermost (something in the region 10–15 W m⁻² K⁻¹). Our results suggest that a figure closer to 28 W m⁻² K⁻¹ is more appropriate. The furnace tests showed that emissivity has a low-order effect on performance and so we are able to conclude that convective heat transfer is dominant in these situations.     

New radiation thermometer for near room temperature

The authors have developed a new radiation thermometer for the measurement of near room temperature with high signal-to-noise ratio (S/N). The thermometer, utilizing an HgCdTe semiconductor sensor, is compact and insensitive to temperature changes of the surroundings. Its major breakthrough owes much to the fact that the sensor is cooled and controlled by Peltier thermoelectric devices and that the sensor serves as reference radiator. A well designed optical system with a chopper and a concave mirror shuts off stray radiation, thus improving the S/N.

On-line experiments in a steel sheet manufacturing process using a prototype thermometer proved the validity of this new radiation thermometer.     

The North American ASTER Land Surface Emissivity Database (NAALSED) Version 2.0

Thermal Infrared (TIR) data are supplied by instruments on several satellite platforms including the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER), which was launched on the Terra satellite in 1999. ASTER has five bands in the TIR and a spatial resolution of 90 m. A mean seasonal, gridded, Land Surface Temperature and Emissivity (LST&E) database has been produced at 100 m spatial resolution using all the ASTER scenes acquired for the months of Jan-Mar (winter) and Jul-Sep (summer) over North America. Version 2.0 of the North American ASTER Land Surface Database (NAALSED) (http://emissivity.jpl.nasa.gov) has now been released and includes two key refinements designed to improve the accuracy of emissivities over water bodies and account for the effects of fractional vegetation cover. The water adjustment replaces ASTER emissivity values over inland water bodies with a measured library emissivity spectrum of distilled water, and then re-calculates the surface temperatures using a split-window algorithm. The accuracy of ASTER emissivities over vegetated surfaces is improved by applying a fractional vegetation cover adjustment (TES_Pv) to the ASTER Temperature Emissivity Separation (TES) calibration curve. Comparisons of NAALSED emissivity spectra with in-situ data measured over a grassland in Northern Texas resulted in a combined absolute difference for all five ASTER bands of 1.0% for the summer emissivity data, and 0.1% for the winter data—a 33-50% improvement over the original TES results.     

Intercomparison of versions 4, 4.1 and 5 of the MODIS Land Surface Temperature and Emissivity products and validation with laboratory measurements of sand samples from the Namib desert, Namibia

Eight new refinements were implemented in the MODIS Land Surface Temperature and Emissivity (LST&E) product suite when transitioning from version 4 (V4) to version 5 (V5). The refinements were designed to improve the spatial coverage, stability, and accuracy of the product suite. Version 4.1 (V4.1) is an interim collection which uses V5 input products (MOD02, MOD03, MOD07, MOD10, and MOD35), but the LST&E retrieval algorithm is unchanged from V4 in which the split-window and day/night temperature retrieval algorithms are only partially incorporated, and not fully incorporated as in V5. A test dataset for the V4.1 product was produced by MODAPS for a 3-month period from July through September 2004, and after an initial evaluation period, it was decided to generate the V4.1 product from mission period 2007001 onwards as a continuation of previous years of V4 data. This paper compares MODIS retrieved surface emissivities between V4, V4.1 and V5 using the level-3 MODIS daily LST&E product, MOD11B1.Comparisons of MOD11B1 retrieved surface emissivity during the Jul-Sep 2004 test period with lab measurements of sand samples collected at the Namib desert, Namibia result in a combined mean absolute emissivity difference for bands 29 (8.55 µm), 31 (11 µm) and 32 (12 µm) of 1.06%, 0.65% and 1.93% for V4, V4.1 and V5 respectively. Maximum band 29 emissivity differences with the lab results were 4.10%, 2.96% and 8.64% for V4, V4.1 and V5 respectively. These results indicate that over arid and semi-arid areas, users should consider using MODIS V4 or V4.1 data instead of V5. Furthermore, users should be careful not to develop time series from a mixture of product versions that could introduce artifacts at version boundaries.     

地表温度热红外遥感反演的研究现状及其发展趋势

区域性或全球性的地表温度, 只有通过遥感手段才能获得, 在诸多应用中是一个非常重要的参数。地表温度反演是热红外遥感研究的热点和难点之一, 大气校正、温度与比辐射率的分离是必须考虑的两个重要方面。近年来有关的研究非常多, 主要反演方法可分为5 类: 单通道方法、分裂窗(双波段) 方法、多波段温度- 比辐射率分离方法、多角度温度反演方法和多角度与多通道相结合的方法。这些方法都各有利弊, 如何提高反演的精度和模型的适用性是地表温度热红外遥感的未来发展趋势, 理论和实验相结合的多种信息源的综合应用成为必然的要求。     

Effect of water emissivity on solar still efficiency

The proper installation of a solar still can reduce the heat loss from the basin to ambient and significantly increase the still efficiency. In this case, the use of a dye resulted in minimal improvement in still efficiency. The reduction in the water emissivity will reduce the radiation heat transfer from water to glass. This resulted in a substantial improvement in still efficiency and offers great potential for solar still usage. The two methods above are mutually exclusive and could be used concurrently for best results. The numerical predictions reported agree favorably with the most recently published experimental work in a similar configuration.     

Low-emitting surfaces prepared by applying transparent aluminum-doped zinc oxide coatings via a sol-gel process

Polycrystalline transparent conductive oxide thin films based on aluminum-doped zinc oxide (AZO) have been prepared on soda-lime glass by using an inorganic sol-gel process and the dip-coating technique. The multilayered films were crystallized on the substrate and subsequently annealed in a reducing atmosphere to enhance the number of free charge carriers. Significant characteristics of the functional coatings, such as crystallinity, surface smoothness, layer thickness, electrical conductivity, as well as the optical properties in a large spectral range between 0.25 and 35 μm were analyzed. The obtained samples are highly transparent with a visual transmittance above 0.85 and show specific resistivities of up to 1.6·10^− 3 Ω·cm. Applying an optimized AZO-coating with at most 500 nm is sufficient to reduce the surface emittance from 0.89 to less than 0.45 in the infrared.     

Emissivity characteristics of polished aluminum alloy surfaces and assessment of multispectral radiation thermometry (MRT) emissivity models

Emissivity characteristics were measured for several polished aluminum alloy samples over the spectral range of 2.05-4.72 μm and temperatures of 600-800 K. Overall, aluminum alloys buck the general trend of increasing emissivity with increasing temperature for metallic surfaces in the infrared range. Only AL 7150 follows the expected trend, while the emissivity of the other alloys decreases between 600 and 700 K and increases between 700 and 800 K, and the emissivity of commercially pure aluminum (AL 1100) decreases monotonically with increasing temperature. The experimental results are used to assess the accuracy of popular multispectral radiation thermometry (MRT) emissivity models for temperature measurement. It is shown that drastic changes in the shape of emissivity distribution preclude the use of a single function to accurately represent every band of the measured spectrum. Better predictions are achieved using the simplest form of MRT emissivity models and minimum number of wavelengths required by the model. Two relatively simple models are identified for best overall predictions for different alloys and temperatures. Despite the relative success of these two models, this study clearly demonstrates that improvements are required in both instrumentation and emissivity models to achieve acceptable accuracy in the implementation of radiation thermometry in the aluminum industry.