Spectral Emissivity & Emittance

San Diego CA, USA –Surface Optics’ ET10 measures emissivity values in two most commonly used spectral regions, 3 to 5 and 8 to 12 microns.

Its main application is to produce emissivity values for the infrared cameras.

Advanced IR cameras require the input of an emissivity value for accurate temperature calculations. The emissivity values obtained from tables can be far from real leading to large temperature uncertainties.

The ET10 can be used in the lab or in the field and on small or large objects. With the ET10 one can measure emissivity of any surface in just a few seconds.

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The Heat Island Group at Lawrence Berkeley National Laboratory, Berkeley, CA have measured solar reflectance of roofing samples with an UV-VIS-NIR Spectrometer with an integrating sphere and they measured the spectral emittance of the samples with a FTIR Spectral Emissometer. The following writeup and graphs are from their webpageat eetd.lbl.gov/HeatIsland/CoolRoofs/Samples.html

“Below are examples of complete reflectance and emittance data for several metal roofing samples made of cool roofing materials. These measurements show examples of complete laboratory information needed to determine radiative heat exchange by a roof which, in turn, can be used to estimate peak roof temperatures.

“The spectral solar reflectance is the total reflectance (diffuse and specular) as a function of wavelength, across the solar spectrum (wavelengths of 0.3 to 2.5 µm). It is used to compute the overall solar reflectance, using a standard solar spectrum as a weighting function. It also contains the information in the visual range (0.4 to 0.7 µm) which is sufficient to compute the color coordinates for color matching with other materials.

“The spectral thermal emittance (the graphs on the right) contains the information for computing the overall thermal emittance, using a blackbody curve as the weighting function. The spectral range is about 5 to 40 µm. If the spectral thermal emittance is approximately a horizontal line (a “gray” body), then the overall emittance is adequate for computing longwave radiative radiative exchange between the roof and the atmosphere. If the spectral thermal emittance deviates markedly from a horizonal line, then the details of the spectral emittance and the atmospheric emittance are necessary for a complete computation.

Note that the hunter green sample (middle graph) looks green to the eye because of the reflectance “bump” at 0.5 µm. The average solar reflectance, at 0.086, is almost as low as black (zero).”"The burgundy sample (bottom graph) looks red due to the increase in reflectance near 0.7 µm. The visible reflectance is only about 0.1, but the relatively high reflectance in the near infrared (0.7 to 2.5 µm) yields an overall solar reflectance of 0.226.”The emittance for all these samples is roughly 0.9, with an abrupt fall-off near 6 µm. Link to: Roof Heat Transfer > Emittance”

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Galvalume (top graph), due to the inclusion of aluminum metal in the zinc anti-corrosion coating, is more reflective to sunlight than traditional galvanized steel which has a solar reflectance around 0.5.A further coating, with a clean acrylic material (low graph), can be used to raise the infrared emittance without significantly changing the solar reflectance.

Surface Optics Corporation (SOC) operates a world-class measurement facility equipped for the most demanding spectral measurement tasks for spectral directional and bidirectional reflectance measurements for modeling, simulation, special effects and more.

Spectral measurements can be made in wavelength regions from the ultraviolet to long wave infrared and include one or all of the following types of reflectance measurements:

Directional or hemispheric reflectance: the fraction of the light incident on a sample at a given angle that is reflected back into the hemisphere.

Bidirectional Reflectance Distribution Function (BRDF): the distribution of light, described as a function of two angles, reflected back into the hemisphere from light incident at a given angle on a sample.

Monostatic Bidirectional Reflectance(enhanced backscatter measurement): a small portion of the BRDF measured at the direct backscattered angle using a laser interferometric reflectometer.

SOC also develops and expands on its off-the-shelf library of optical properties data for a variety of materials. This library can be purchased in whole or in part at considerable savings over the cost of individual measurements.

For more information on our database and its contents contact SOC.

You can also download the Optical Properties Database brochure.

A list of FAQs regarding the database, and information on using the databases in 3D sensor simulation.

1. Spectral Reflectance Data for (52) rocks, (29) soils, (28) vegetation types, (41) construction materials, (38) paints, and (12) fabrics from 0.3 to 25 microns.
2. Hemispherical, Directional, Diffuse and Specular
3. Surface temperatures versus time-of-day, climate and orientation
4. Complete solution for visual and infrared radiance simulation.

3D Models for Sensor Simulation

SOC is constantly developing computationally efficient polygonal models for accurate sensor simulation.

Unlike visual simulation models, sensor models require an intimate understanding of the physical nature and physics responsible for the signature of an object.

SOC’s extensive background in both Infrared and Radar sensor simulation and analysis is incorporated into all of our 3D models.

The Raytek North America website includes a table for the emissivity of a large range of non-metallic materials that includes common building materials, ceramics, glasses and natural materials including ice & water in as many as four wavelength regions.

Wavebands covered include 1.0 micrometer (micron), 5.0 microns, 7.9 microns and the 8-14 micron band. No specific data and the limits of the various wavebands and there are many instances where the wavelength region is labelled as “nr” meaning “Not Recommended”.

A Basic Guide to Thermography A PDF Download from the Land Instruments website - (Last modified 15/11/04 -433kb - 14 pages). It contains a table of effective spectral emissivities over the 8 to 14 micrometer waveband for several common industrial and building materials as part of its introduction to Infrared Thermography.