Experimental measurements for studying angular and spectral variation of thermal infrared emissivity

One condition for precise multiangle algorithms for estimating sea and land surface temperature with the data from the Advanced Along Track Scanning Radiometer is accurate knowledge of the angular variation of surface emissivity in the thermal IR spectrum region. Today there are very few measurements of this variation. The present study is conducted to provide angular emissivity measurements for five representative samples (water, clay, sand, loam, gravel). The measurements are made in one thermal IR broadband (8-13 microm) and three narrower bands (8.2-9.2, 10.3-11.3, and 11.5-12.5 microm) at angles of 0 degrees-60 degrees (at 5 degrees increments) to the surface normal. The results show a general decrease in emissivity with increasing viewing angles, with the 8.2-9.2-microm channel the most sensitive to this dependence and sand the sample showing the greatest variation.     

Simultaneous measurements of skin sea surface temperature and sea surface emissivity from a single thermal imagery

A novel method, to our knowledge, to measure simultaneously the thermal emissivity and skin temperature of a sea surface has been developed. The proposed method uses an infrared image that includes a sea surface and a reference object located near the surface. By combining this image with sky radiation temperature, we retrieve both skin sea surface temperature and sea surface emissivity from the single infrared image. Because the method requires no knowledge of thermal radiative properties of actual sea surfaces, it can be used even for a contaminated sea surface whose emissivity is hard to determine theoretically, e.g., oil slicks or slicks produced by biological wastes. Experimental results demonstrate that the estimated emissivity agrees with the theoretical prediction and, also, the recovered temperature distribution of skin sea surface has no appreciable high-temperature area that is due to reflection of the reference object. The method allows the acquisition of match-up data of radiometric sea surface temperatures that precisely correspond to the satellite observable data.     

Unpolarized infrared emissivity with shadow from anisotropic rough sea surfaces with non-Gaussian statistics

The emissivity of two-dimensional anisotropic rough sea surfaces with non-Gaussian statistics is investigated. The emissivity derivation is of importance for retrieval of the sea-surface temperature or equivalent temperature of a rough sea surface by infrared thermal imaging. The well-known Cox-Munk slope probability-density function, considered non-Gaussian, is used for the emissivity derivation, in which the skewness and the kurtosis (related to the third- and fourth-order statistics, respectively) are included. The shadowing effect, which is significant for grazing angles, is also taken into account. The geometric optics approximation is assumed to be valid, which means that the rough surface is modeled as a collection of facets reflecting locally the light in the specular direction. In addition, multiple reflections are ignored. Numerical results of the emissivity are presented for Gaussian and non-Gaussian statistics, for moderate wind speeds, for near-infrared wavelengths, for emission angles ranging from 0 degrees (nadir) to 90 degrees (horizon), and according to the wind direction. In addition, the emissivity is compared with both measurements and a Monte Carlo ray-tracing method.     

Effect of atmospheric radiance errors in radiometric sea-surface skin temperature measurements

Errors in measurements of sea-surface skin temperature (SSST) caused by inappropriate measurements of sky radiance are discussed; both model simulations and in situ data obtained in the Atlantic Ocean are used. These errors are typically caused by incorrect radiometer view geometry (pointing), temporal mismatches between the sea surface and atmospheric views, and the effect of wind on the sea surface. For clear-sky, overcast, or high-humidity atmospheric conditions, SSST is relatively insensitive (<0.1 K) to sky-pointing errors of ?10 degrees and to temporal mismatches between the sea and sky views. In mixed-cloud conditions, SSST errors greater than ?0.25 K are possible as a result either of poor radiometer pointing or of a temporal mismatch between the sea and sky views. Sea-surface emissivity also changes with sea view pointing angle. Sea view pointing errors should remain below 5 degrees for SSST errors of <0.1 K. We conclude that the clear-sky requirement of satellite infrared SSST observations means that sky-pointing errors are small when one is obtaining in situ SSST validation data at zenith angles of <40 degrees . At zenith angles greater than this, large errors are possible in high-wind-speed conditions. We recommend that high-resolution inclinometer measurements always be used, together with regular alternating sea and sky views, and that the temporal mismatch between sea and sky views be as small as possible. These results have important implications for the development of operational autonomous instruments for determining SSST for the long-term validation of satellite SSST.     

Measurements of the infrared emissivity of a wind-roughened sea surface

Spectral statistical-analysis techniques were developed and applied to high-spectral-resolution infrared measurements of the sea surface. The effective incidence angle of a ship-borne instrument in typical at-sea conditions was found to introduce errors of up to 0.7 K in sea-surface temperature retrievals at a 55 degrees view angle. The sea-surface emissivity was determined over the 8-12-microm window at view angles of 40 degrees and 55 degrees and at wind speeds up to 13 ms(-1). The emissivity was found to increase in magnitude with increasing wind speed, rather than decrease, as predicted by widely used parameterizations. Use of these parameterizations can cause significant bias in remote sensing of sea-surface temperature in noncalm conditions.     

Effect of the observation length on the two-dimensional shadowing function of the sea surface: application on infrared 3-13-microm emissivity

An analytical approach of the two-dimensional emissivity of a rough sea surface in the infrared band is presented. The emissivity characterizes the intrinsic radiation of the sea surface. Because the temperature measured by the infrared camera depends on the emissivity, the emissivity is a relevant parameter for retrieving the sea-surface temperature from remotely sensed radiometric measurements, such as from satellites. This theory is developed from the first-order geometrical-optics approximation and is based on recent research. The typical approach assumes that the slope in the upwind direction is greater than the slope in the crosswind direction, involving the use of a one-dimensional shadowing function with the observed surface assumed to be infinite. We introduce the two-dimensional shadowing function and the surface observation length parameters that are included in the modeling of the two-dimensional emissivity.     

Polarized infrared emissivity of one-dimensional Gaussian sea surfaces with surface reflections

Surface reflection is an important phenomenon that must be taken into account when studying sea surface infrared emissivity, especially at large observation angles. This paper models analytically the polarized infrared emissivity of one-dimensional sea surfaces with shadowing effect and one surface reflection, by assuming a Gaussian surface slope distribution. A Monte Carlo ray-tracing method is employed as a reference. It is shown that the present model agrees well with the reference method. The emissivity calculated by the present model is then compared with measurements. The comparisons show that agreements are greatly improved by taking one surface reflection into account. The Monte Carlo ray-tracing results of sea surface infrared emissivity with two and three reflections are also determined. Their contributions are shown to be negligible.     

大口径高发射率面型黑体辐射源的研制

黑体辐射源作为定标标准器,在红外测量设备的辐射定标中具有重要作用。为应对大口径红外测量设备的辐射定标工作需求,设计了1台辐射面积为400mm×400mm的面型黑体辐射源。采用多路控温和连接固定冷源的方式对黑体进行温度控制;通过热仿真确定合适的传热模型,同时结合高发射率涂层工艺与辐射面的结构设计使黑体具备高发射率,辐射面有效发射率可达到0.992;在真空环境下,利用标准铂电阻温度计测量得到黑体辐射面源的温度均匀性偏差最大为0.101K,稳定性平均值为0.018K/10min,该黑体辐射光源能够满足现阶段大口径红外测量设备的使用需求。     

Blackbody calibration sources of high accuracy for a spaceborne infrared instrument: the Along Track Scanning Radiometer

We describe the two 140-mm-aperture simulated blackbody sources used for the on-board calibration of the Along Track Scanning Radiometer, a spaceborne thermal infrared instrument for the accurate remote sensing of sea surface temperature, in operation since 1991. The design of these spaceborne sources, which operate at ≈ -10 °C and ≈ +30 °C, allows them to meet their unprecedented accuracy goal, namely a 3σ uncertainty in their brightness temperature of <0.1 K for the whole mission. This performance is empirically demonstrated in the laboratory and in flight by long-term temperature readout tests, temperature uniformity measurements, and direct emissivity measurements.     

不同发射率下红外热图像的非稳态温度场测量研究

通过分析红外测温公式,研究物体发射率对红外热图像测温精度的影响,并找出影响物体发射率的因素。根据经典计算方法,发现了物体发射率在红外热图像测温应用中的局限。从红外热图像的角度对发射率的计算进行了建模,基于修正物体发射率得到提高红外热图像对非稳态温度场测量精度的模型。对铸铁、不锈钢进行了3组测温实验,红外热图像直接测量的最大误差分别为2.1℃、1.7℃、2.3℃,而所提出的方法测量的最大误差分别为1.0℃、0.7℃、1.6℃。通过对比分析3组测量结果数据验证了测量方法的可行性。     

Emissivity of rough sea surface for 8-13 num: modeling and verification

The emissivity model for rough sea surface [Remote Sensing Environ. 24, 313-329 (1988)] is inspected in light of the measured surface emissivity. In the presence of moderate wind (5 m/s or less), the emissivity model is found to be adequate for small to moderate view angles. For large view angles, the discrepancy between the computed and the measured emissivity is large, but one can reduce this considerably by incorporating the reflected sea surface emission into the emissivity model. In addition, examination of the spectral variation of the observed and computed emissivity suggests the need for refined measurements of the complex refractive index. An improved model is constructed to calculate the rough sea surface emissivity that can be used to provide accurate estimates of sea surface skin temperatures from remotely sensed radiometric measurements. An important feature of the improved model is that the computed sea surface emissivity is only weakly dependent on wind speed for most view angles used in practice.     

Surface temperature correction for active infrared reflectance measurements of natural materials

Land surface temperature algorithms for the moderate resolution imaging spectroradiometer satellite instrument will require the spectral bidirectional reflectance distribution function (BRDF) of natural surfaces in the thermal infrared. We designed the spectral infrared bidirectional reflectance and emissivity instrument to provide such measurements by the use of a Fourier transform infrared spectrometer. A problem we encountered is the unavoidable surface heating caused by the source irradiance. For our system, the effects of the heating can cause a 30% error in the measured BRDF The error caused by heating is corrected by temporally curve fitting the radiance signal. This curve-fitting technique isolates the radiance caused by reflected irradiance. With this correction, other factors dominate the BRDF error. It is now ~5% and can be improved further. The method is illustrated with measurements of soil BRDF.     

High-Precision and High-Resolution Measurements of Thermal Diffusivity and Infrared Emissivity of Water–Methanol Mixtures Using a Pyroelectric Thermal Wave Resonator Cavity: Frequency-Scan Approach

The thermal diffusivity and effective infrared emissivity of water–methanol mixtureswere measured at atmospheric pressure and ambient temperature using a pyroelectric thermal-wave resonator cavity. The applied frequency-scan method allows keeping the cavity length fixed, which eliminates instrumental errors and substantially improves the precision and accuracy of the measurements. A theoretical model describing conduction and radiation heat transfer in the cavity was developed. The model predictions and the frequency-scan experimental data were compared, showing excellent agreement. The measurements were performed for methanol volume fractions of 0, 0.5, 1, 2, 5, 10, 20, 40, 75, and 100%. The fitted thermal diffusivity and effective emissivity vs. concentration results of the mixtures were compared to literature theoretical and experimental data. The maximum resolution of 0.5% by volume of methanol in water by means of the thermal-wave cavity method is the highest reported to date using thermophysical techniques. Semi-empirical expressions for the mixture thermal diffusivity and infrared emissivity as functions of methanol concentration have been introduced. The expression for infrared emissivity is consistent with the physical principle of detailed balance (Kirchhoff’s law). The expression for thermal diffusivity was found to describe the data satisfactorily over the entire methanol volume-fraction range.     

Validation of fascod3 and modtran3: comparison of model calculations with ground-based and airborne interferometer observations under clear-sky conditions

The validation of fascod3 and modtran3 against ground-based and airborne high-resolution Michelson interferometer measurements under clear-sky conditions is presented. Important considerations including water vapor continuum, frequency-dependent sea surface emissivity in the IR window region, and spectral resolution of modtran3 in the comparison of model calculations with high-resolution interferometer measurements are discussed. Our results indicate that it is not adequate to assume sea surface emissivity of 1.0 [?(ν) = 1.0] or a constant in the simulation of upwelling radiance observed by the airborne Michelson interferometer. The use of spectral emissivity (frequency-dependent emissivity) leads to much better agreement between model calculations and interferometer measurements in the IR window region from 750.0 to 1050.0 cm(-1). This could have important implications for the retrieval of sea surface temperature, thin cirrus properties, and aerosol parameters because of the sea surface emissivity of 1.0 assumption commonly used by many researchers. Comparisons of modtran3 calculations with interferometer measurements show that the agreement might not be adequate at the nominal resolution of 2.0 cm(-1), and further spectral degradation might be necessary to improve the agreement between measurements and modtran3 calculations. modtran should be used with caution for relatively high spectral resolution remote-sensing applications.     

Emissitivity spectra obtained from field and laboratory measurements using the temperature and emissivity separation algorithm

Surface emissivities play an important role in thermal remote sensing, since knowledge of them is required to estimate land surface temperature with enough accuracy. They are also important in other environmental or geological studies. We show the results obtained for the emissivity spectra of different natural surfaces (water, green, and senescent vegetation) by applying the temperature and emissivity separation (TES) algorithm to ground-based measurements collected at the field with a multiband thermal radiometer. The results have been tested with data included in spectral libraries, and rms errors lower than 0.01 have been found, except for senescent vegetation. Two methods are also proposed to apply the TES algorithm to measurements achieved in the laboratory: (i) by heating the sample and (ii) using a box with reflective walls.     

Transient infrared temperature measurements of liquid-fuel surfaces: results of studies of flames spread over liquids

An infrared thermograph technique with an 8-12-mum spectral range was used to measure transient two-dimensional profiles of liquid (1-propanol) surface temperatures. An IR camera was placed over the liquid, allowing us to observe the fuel surface through propanol vapor. To use this technique, one must know the emissivity of the liquid surface and the IR absorption of both the liquid propanol and the propanol vapor. The emissivity of the liquid propanol was determined with a fine thermocouple temperature measurement, IR absorption with the propanol vapor was calibrated with a blackbody source, and IR absorption with a liquid propanol was theoretically estimated. The accuracy of our infrared thermograph technique proved to be better than 97% in detecting the liquid-surface temperature with a temperature sensitivity of 0.1 degrees C and a time response of 30 ms.     

Emissivity and reflection model for calculating unpolarized isotropic water surface-leaving radiance in the infrared. I: Theoretical development and calculations

Although published sea surface infrared (IR) emissivity models have gained widespread acceptance for remote sensing applications, discrepancies have been identified against field observations obtained from IR Fourier transform spectrometers at view angles approximately > 40 degrees. We therefore propose, in this two-part paper, an alternative approach for calculating surface-leaving IR radiance that treats both emissivity and atmospheric reflection in a systematic yet practical manner. This first part presents the theoretical basis, development, and computations of the proposed model.     

Multiband fiber optic radiometry for measuring the temperature and emissivity of gray bodies of low or high emissivity

Infrared fiber optic radiometry was used for noncontact thermometry of gray bodies whose temperature was close to room temperature (40-70 degrees C). We selected three gray bodies, one with high emissivity (epsilon = 0.97), one with medium emissivity (epsilon = 0.71), and one with low emissivity (epsilon = 0.025). We carried out optimization calculations and measurements for a multiband fiber optic radiometer that consisted of a silver halide (AgClBr) infrared-transmitting fiber, a dual-band cooled infrared detector, and a set of 18 narrowband infrared filters that covered the 2-14-microm spectral range. We determined the optimal spectral range, the optimal number of filters to be used, and the optimal chopping scheme. Using these optimal conditions, we performed measurements of the three gray bodies and obtained an accuracy of better than 1 degrees C for body temperature and for room temperature. An accuracy of 0.03 was obtained for body emissivity.     

基于高发射率标靶的物体表面温度快速精确测量研究

针对某些工业现场物体表面温度测量要求快速且精确,红外测温仪直接测温虽然快速,但准确性较低,提出了一种基于高发射率标靶的物体表面温度快速精确测量方法。采用高发射率涂料涂层制作标靶,通过在测量现场使用该标靶,可以避免物体表面发射率对物体表面红外温度测量准确性的影响,同时,对比直接测量和标靶测量结果,消除环境温度、大气温度、测量距离、大气衰减等因素的影响,从而实现物体表面温度测量的快速和准确性。实验结果与理论分析表明,该方法能有效提高物体表面红外温度测量精度,平均测量误差小于1%。     

Radiative properties characterization of ZrB2–SiC-based ultrahigh temperature ceramic at high temperature

Experimental investigations of radiative property on pre-oxidized ZrB₂–SiC–15 vol.%–C ultrahigh temperature ceramic (ZSC UHTC) at high temperature range of 1100–1800 °C were performed. By Fourier transform infrared radiant (FT-IR) spectrometer, spectral emissivity was measured in the wavelength region between 3 and 18 μm. Total normal emissivity was calculated using spectral emissivity data via theoretical formula. It has been found that high emissivity for all the testing specimens was presented, and the total normal emissivity is between 0.65 and 0.92 with temperature range from 1100 to 1800 °C. Moreover, the total normal emissivity of pre-oxidized ZSC ceramic decreased non-monotonously as the temperature increased. The total normal emissivity decreased as the testing temperature increased from 1100 to 1800 °C, whereas the total normal emissivity at the testing temperature of 1600 °C was higher than that of 1400 and 1800 °C. Macroscopical surface morphology and microstructure were carried out before and after the testing.