Variation Of Spectral Emissivity Research Articles

Thermal emission measurements of ordinary chondrite mineral analogs in a simulated asteroid environment: 2. Representative mineral mixtures

We investigate a suite of synthetic mineral mixtures designed to act as bulk mineralogical analogs to H, L, and LL ordinary chondrite meteorites in order to probe how the thermal emission characteristics of such materials change between ambient and simulated asteroid environmental conditions. Due to the parent body link with certain S-type asteroids, studying these analog mixtures in an environment that is relevant to actual asteroid surfaces advances our understanding of the thermal emission properties of one of the most common regolith types among the main-belt and near-Earth asteroid populations. The observed changes in spectral emissivity features due to cold, vacuum conditions are not as large as previously observed for single mineral (silicate) samples. We interpret this difference to be the result of metallic and opaque components weakening near-surface thermal gradients in the mixtures. As such, we predict that near-surface thermal gradients on ordinary chondrite parent bodies (e.g., S-type asteroids) are likely much weaker than would be inferred from cold, vacuum measurements of individual mineral components. We tested whether the increased spectral contrast observed in fine-grained (<25 μm) samples measured in a cold, vacuum environment increases the efficacy of least squares linear unmixing methods. It is found that the accuracy of such models does not improve relative to measurements made under ambient conditions, thus linear unmixing models are not expected to yield accurate estimates of the modal mineralogy of airless planetary surfaces if they are dominated by fine-grained regolith. Mixtures with coarse particle sizes (125–250 μm) that were modeled using the coarse particle size endmembers yielded results that were largely independent of the environmental conditions, but with larger errors in spectral fits for samples measured in a simulated asteroid environment. At simulated asteroid environmental conditions, the bulk silicate composition and metal content play a more important role in determining the thermal state and brightness temperature of the sample than at ambient conditions. Modest changes in metal content (10–25 wt%) lead to large differences in the brightness temperature of a sample. Under simulated asteroid conditions, an ~10 K increase in maximum brightness temperature that tracks with increased iron content is observed at fine particle sizes (<25 μm) between each of the analog ordinary chondrite groups. Based on these results, it may be difficult to distinguish H, L, and LL compositions of suspected ordinary chondrite parent bodies using only Earth- or space-based thermal emission spectra. This is inferred from the spectral similarity of the analog mixtures and the absence of significant variation in spectral emissivity associated with reported differences in bulk metal content for ordinary chondrites.

Emissivity model of steel 430 during the growth of oxide layer at 800–1100 K and 1.5 μm

This work studied the variation in spectral emissivity with growth of oxide layer at the different temperatures. For this reason, we measured the normal spectral emissivity during the growth of oxide layer on the sample surface at a wavelength of 1.5 μm over a temperature range 800–1100 K. In the experiment, the temperature was measured by the two thermocouples, which were symmetrically welded onto the front surface of specimens. The average of their readings was regarded as the true temperature. The detector should be perpendicular to the specimen surface as accurately as possible. The variation in spectral emissivity with growth of oxide layer was evaluated at a certain temperature. Altogether 11 emissivity models were evaluated. The conclusion was gained that the more the number of parameters used in the models was, the better the fitting accuracy became. On the whole, all the PEE models, the four-parameter LEE model and the five-parameter PFE, PLE and LEE models could be employed to well fit this kind of variation. The variation in spectral emissivity with temperature was determined at a certain thickness of oxide film. Almost all the models studied in this paper could be used to accurately evaluate this variation. The approximate models of spectral emissivity as a function of temperature and oxide-layer thickness were proposed. The strong oscillations of spectral emissivity were observed, which were affirmed to arise from the interference effect between the two radiations stemming from the oxide layer and from the substrate. The uncertainties in the temperature of steel 430 generated only by the surface oxidization were approximately 4.1–10.7 K in this experiment.

Spectral emissivity modeling of steel 201 during the growth of oxidation film over the temperature range from 800 to 1100 K in air

This study explores the spectral emissivity modeling of steel 201 during the growth of oxidation film over the temperature range from 800 to 1100K at 1.5μm. The radiance coming from the specimen is received by an InGaAs photodiode detector. The specimen temperature is obtained by averaging the two platinum–rhodium thermocouples, which are tightly welded in the front surface of specimen near the measuring area viewed by the detector. The variation of spectral emissivity with the temperature is studied at a given heating time. The variation of spectral emissivity with the heating time is evaluated at a definite temperature. The strong oscillations of spectral emissivity are observed and discussed in detail, which originate from the interference effect between the radiation stemming from the oxidization film on the specimen surface and the radiation coming from the specimen surface. The measurement uncertainties of spectral emissivity contributed only by the surface oxidization are about 3.2–14.1%. At a given heating time, the variation of spectral emissivity with the temperature abides well by a simple analytic functional form. And at a definite temperature, the variation of spectral emissivity with the heating time can also be well reproduced by fitting except for the periodical oscillations.

Effect of surface oxidization on the spectral emissivity of steel 304 at the elevated temperature in air

Effect of surface oxidization on the spectral emissivity of steel 304 is studied over the temperature range from 830 to 1100K at 1.5μm. The experimental setup is composed of three modules: the optical receiving system, the specimen heating and temperature-controlling system, and the signal-controlling and data-computing system. The temperature of steel 304 specimens is measured by averaging the two R-type platinum–rhodium thermocouples. The radiant energy stemming from the surface specimens is received by an InGaAs photodiode detector. The growth law of oxidization film on the surface of steel 304 specimens is observed carefully. The variation of spectral emissivity with the heating-duration time is studied at a given elevated temperature. The dependence of spectral emissivity on the temperature is evaluated at a definite heating-duration time. The interference enhancement of spectral emissivity appears mainly during the initial heating-duration period, in particular at a relatively low elevated temperature. The interference effect between the radiation stemming from the oxidization film on the specimen surface and the radiation coming from the surface of specimens is discussed, which has been affirmed to be responsible for the interference enhancement of spectral emissivity. The analytic model between the spectral emissivity of steel 304 specimens and the temperature of specimen surface is evaluated at several definite heating-duration times. The conclusion is derived that the polynomial functional form is very suitable for fitting the experimental results obtained at a given heating-duration time at 830–1100K.

Experimental study of the relationships between the spectral emissivity of brass and the temperature in the oxidizing environment

Effect of surface oxidization on the spectral emissivity of brass is studied over the temperature range from 800 to 1070K at the wavelength of 1.5μm. The temperature of brass surface is measured by averaging the two R-type platinum–rhodium thermocouples. The radiant energy emitted by the brass surface is received by an InGaAs photodiode detector. Two kinds of relationships between the spectral emissivity and the temperature are investigated in the oxidizing environment at the elevated temperature. One is the variation of spectral emissivity with the heating-duration time at the given temperature. The other is the variation of spectral emissivity with the temperature at the given heating-duration time. The interference effect of radiation coming from the brass surface and coming from the oxidization film is discussed when the oxidation film on the surface is grown. The resonant structures of spectral emissivity are observed during the whole heating period, in particular at the early stage of heating duration. The analytic formula of spectral emissivity versus the temperature is derived at the heating-duration time of 30, 60, 90, 120, 150, 180, 210, 240, 270 and 300min, respectively. The conclusion is obtained that coefficients of analytic expressions between the spectral emissivity and the temperature are different from each other for the experimental results obtained at the different heating-duration time, though the polynomial functional form is suitable to fit all the measurements obtained in the present work.

Temperature and emissivity separation from multispectral thermal infrared observations

Knowledge of the surface emissivity is important for determining the radiation balance at the land surface. For heavily vegetated surfaces, there is little problem since the emissivity is relatively uniform and close to one. For arid lands with sparse vegetation, the problem is more difficult because the emissivity of the exposed soils and rocks is highly variable. With multispectral thermal infrared (TIR) observations, it is possible to estimate the spectral emissivity variation for these surfaces. We present data from the TIMS (Thermal Infrared Multispectral Scanner) instrument, which has six channels in the 8- to 12-μm region. TIMS is a prototype of the TIR portion of the ASTER (Advanced Spaceborne Thermal Emission and Reflection radiometer) instrument on NASA's Terra (EOS-AM1) platform launched in December 1999. The Temperature Emissivity Separation (TES) algorithm, developed for use with ASTER data, is used to extract the temperature and six emissivities from the six channels of TIMS data. The algorithm makes use of the empirical relation between the range of observed emissivities and their minimum value. This approach was applied to the TIMS data acquired over the USDA/ARS Jornada Experimental Range in New Mexico. The Jornada site is typical of a desert grassland where the main vegetation components are grass (black grama) and shrubs (primarily mesquite) in the degraded grassland. The data presented here are from flights at a range of altitudes from 800 to 5000 m, yielding a pixel resolution from 3 to 12 m. The resulting spectral emissivities are in qualitative agreement with laboratory measurements of the emissivity for the quartz rich soils of the site. The derived surface temperatures agree with ground measurements within the standard deviations of both sets of observations. The results for the 10.8- and 11.7-μm channels show limited variation of the emissivity values over the mesquite and grass sites indicating that split window approaches may be possible for conditions like these.

Temperature measurements by optical pyrometry during the epitaxial growth of semiconductors

The true and apparent temperature of samples during the deposition of III-V layers by molecular beam epitaxy changes as a result of the variation in spectral emissivity ϵ with layer thickness. Taking into account the infrared optical properties of these materials, we modelized the variations of the true sample temperature and the apparent temperature (as determined by pyrometric measurement) during the growth. We limited our study to deposits involving at least one absorbing material (at the pyrometer wavelength), for example GaSb, InAs or InSb. We showed that our simple model can agree reasonably with experiments in the 400−500 °C temperature range.

Retrieval of land and sea surface temperature using NOAA-11 AVHRR· data in north-eastern Brazil

Abstract The split-window method is investigated and a simple airborne atmospheric correction model for thermal data is proposed. Analysis shows that the atmospheric transmittance has a quadratic behaviour with the water vapour content in the first few kilometres of a wet atmosphere. The effect of the emissivity is evaluated for the retrieval of surface temperature using data from NOAA-11 AVHRR channels 4 and 5 for two extreme atmospheres. The results indicate that a spectral emissivity variation (Δe) in channels 4 and 5 of ±0.01 is not as important for a wet atmosphere as for a dry atmosphere. The split-window algorithm developed in this work has its parameters dependent on the atmospheric state and the values of these parameters are determined by using radiosonde profiles. Data from eighty-five radiosondes have been used to determine and check the local seasonal equatorial split-window parameters. The results of surface temperature retrieval show that the local seasonal equatorial and daily split-window...

AVHRR split window temperature differences and total precipitable water over land surfaces

Abstract Measurements from the thermal infrared split window channels of the AVHRR sensor were investigated for their relationship to the total atmospheric water vapour amount over land surfaces. The difference in brightness temperature between the AVHRR channel 4 and 5 (10·3–11·3μm and 11·4–12·3μm respectively) was found to be a linear function of total precipitable water, for several stations in differing climatic regimes. For each individual location the total precipitable water was estimated with a standard error ranging from 0·22 to 0·48 cm for the complete range of conditions from wet to dry season or summer to winter. For mid-latitude continental locations there is very little influence of atmospheric aerosols on the relationship while for the African Sahel region the effect of large airborne particulates with a silicate component introduces a significant effect at large values of aerosol optical depth due to absorption. The influence of spectral emissivity variation in the split window region was also observed for arid regions where there is a significant quartz component to the soil. It is concluded that for regional retrieval of precipitable water, this technique provides sufficient accuracy for application to correction of near-infrared satellite data such as AVHRR channel 2 (0·71 –0·98 μm), however the site specific relation between T 4-T 5 and PW needs to be established with independent PW measurements.

Temperature-independent spectral indices in thermal infrared bands

In order to perform spectral analysis in the thermal infrared bands, temperature-independent thermal infrared spectral indices (TISI) are derived from observable thermal infrared radiances. Being temperature-independent and simply related to spectral emissivities of the observed surface, these indices are as easy to use to perform spectral analysis as reflectances in the visible and near infrared spectral domains. Examples of such indices are given for TIMS and AVHRR. Several properties of these indices are discussed, particularly their sensitivity to spectral emissivity variation, their efficient combination, and the derivation of relative spectral emissivity. These indices are quite complementary to NDVI and can lead to stronger results if used together rather than separately. For instance, in the examples worked out, these indices are more sensitive to bare soils characteristics than NDVI. Furthermore, these indices can be tailored to weight certain bands more heavily than others, giving to these indices a wide range of application. They may, for instance, be used for geologic compositional mapping. Practical examples from TIMS and AVHRR data are shown and briefly discussed with a possible applications to the determination from AVHRR data of the surface temperature and surface emissivity.

A simplified technique to compute spectral atmospheric radiation

The spectral distribution of longwave atmospheric radiation arriving at the earth's surface is composed of contributions from atmospheric layers which absorb and re-emit longwave radiation. A procedure to estimate this radiation requires, as input, the temperature and pressure variations with altitude and the distribution of absorbing gases in the atmosphere. Such detailed information about the atmosphere at a given location and time is rarely known. However, local variation of the amount of water vapor is generally known, and ozone undergoes known seasonal variations. These two gases along with carbon dioxide are the main emitters of longwave radiation in the atmosphere. A rigorous technique to compute spectral atmospheric radiation is used together with atmospheric data from a Standard Atmosphere to examine the variation of spectral emissivity with the amount of absorbing gases in the atmosphere. A simple spectral correlation is thereby developed which is then applied to the other Standard Atmospheres utilizing their water vapor amount as the only input. Results obtained from this correlation are compared with those obtained from detailed computations for each Standard Atmosphere. Good agreement is observed at all the wavelengths considered except in a 1 μm region wherein the differences are somewhat larger. It is demonstrated that for any geographic latitude and season the spectral longwave atmospheric radiation can be computed from a simple correlation using precipitable water vapor amount as the only input parameter.