Problems Measuring Emittance

There are, unfortunately, many problems in measuring emittance. Measurement of emittance is subject to errors due to atmospheric absorption. For non-gray samples, spectral detail is needed. There are no modern reference standards, although the National Institute of Standards and Technology (NIST) is working on it. Most available published data on emittance were obtained in the 1960s. Also, the best equipment available originate from the 1960s. Emittance is important to understanding roof heat transfer. Link to: Roof Heat Transfer. Roofs with low emittances may lead to higher energy use. Link to: Emittance Impacts Energy Use.

Sample emittance measurements for some examples of cool roofing materials are available. Link to: Cool Roofing Samples

Emittance measurements on smooth materials, such as glass, can be readily performed with standard equipment, but measurements of diffusely scattering materials used for roofing are technically challenging. A more conventional approach would use a gold-surfaced integrating sphere, but can only cover the infrared out to 20 µm.

Spectral Emittance Technique

The black body curve shows the distribution of thermal radiation of interest here, and the "spectrometer measurement" shows a typical measurement for an emissive sample. The "noise" between 5 and 8 µm and between 20 and 40 µm is actually due to the absorption of water and carbon dioxide in the air between the emitting sample and the spectrometer window. The top curve is the spectral emittance of our reference black coating, which in turn was measured relative to a blackbody cavity.

Similar Materials Can Have Very Different Emittances

Spectral emittances of two aluminum coatings can be very different.

In cooperation with current work at Oak Ridge National Laboratory (ORNL), we characterized these two aluminum roof coatings. These emittance curves show that these materials are not gray bodies, for which the curve would be flat and horizontal.

To learn more about the instruments we use to characterize roofing samples in our laboratory, link to: Coof Roofs Instrumentation