Emissivity

Emissivity

The emissivity of a material is a measure of its ability to radiate thermal energy in comparison to a perfect black body. Emissivity is computed, using the Stefan Boltzmann law, as the ratio of thermal radiation from the surface to thermal radiation from a perfect blackbody at the same absolute temperature and wavelengths, under the same viewing conditions. It is a dimensionless number, ranging from 0 (for a perfect reflector or white body) to 1 (for a black body). The emissivity value depends on the material and nature of the surface, as well as the wavelength.

The three main methods for measuring infrared emissivity are calorimetric, radiometric, and reflection methods. The calorimetric method is based on heating the sample and analyzing the dissipated Stefan–Boltzmann power. Radiometry measures the surface temperature using an infra-red thermometer. In reflective emissivity measurements, the intensity of the radiation reflected from a surface is measured by infrared (FTIR) and Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy or by Attenuated Total Reflection (ATR).

The most common application of emissivity is in non-contact temperature measurements using Infrared thermometers and pyrometers and for thermal imaging. It is essential for thermal engineering of furnaces, heat transfer equipment, and building and equipment insulation calculations. Emissivity has been used to determine planetary temperatures and for space vehicle material and insulation design. Earth surface emissivity is important for thermal remote sensing used in climatology, meteorology, and oceanography.

Common Uses of Emissivity

• Non-contact temperature measurement
• Thermal engineering of radiative equipment such as furnaces
• Insulation systems engineering
• Design and quality control of coated glass, low emission flat glass and architectural glass.
• Design of low emissivity coatings for glass including polymer composite coatings and liquid coatings with nanocomposites
• Rapid thermal processing (RTP) is a technique of importance in the semiconductor industry that requires the measurement of emissivities
• Radiometric Emissivity Imaging (REI) is a technique to detect open cracks on static and moving surfaces

Advantages of Emissivity

• Allows non-contact temperature estimation
• Enables precise thermal modeling and design
• Wavelength and temperature correlation
• Spot measurements are possible

 Limitations of Emissivity

• Interference from background radiation and transmitted radiation
• Dependence on the nature and finish of the surface

 Industrial Applications of Emissivity

• Thermal engineering
• Remote sensing
• Aerospace
• Solar flat plate collectors
• Building Glass systems design
• HVAC systems design
• Insulation systems design
• Semiconductors
• Non-destructive testing