|Emissivity & other infrared-optical properties FAQs|
C9. What is the emissivity of a layer of gas?
C10. Where can I find information on the emissivity of a given surface?C11. How can I measure the emissivity of a surface using an IR-thermometer? C12. What is the difference between emissivity and emittance? C13. What is a radiant barrier? C14. What is a low-e coating? C15. What is low-e glass? C16. What is a selective absorber? C17. Is a knowledge of emissivity important for contactless temperature measurements?
The emissivity of a surface is the ratio of the energy radiated from it to that from a blackbody at the same temperature, the same wavelength and under the same viewing conditions.
At high temperatures or in evacuated environments thermal radiation is the main mode of heat transfer. Total emissivity governs the amount of thermal radiation lost or gained by an object and can therefore either cool or heat it, respectively.
Emissivity is used in the Planck radiation equation to calculate the temperature of a surface when its thermal radiation is measured using a pyrometer. Emissivity is used in equations to calculate heat transfer by thermal radiation. In many cases the emissivity value can be enhanced (e.g. through polishing, roughening, shaping of a material surface) to improve heating or cooling by thermal radiation.
Spectral emissivity measurements are difficult and require great care, as demonstrated by round-robin exercises. This is largely because there are so many factors that can influence a measurement, among them: temperature measurement (sample surface, blackbody), temperature control (uniform, stable temperatures), low signal-to-noise and possible material sample variation (oxidation or sample-to-sample variation).
It is possible in principle, using Maxwell's equations and material electro-optical properties, to calculate the emissivity of a very restricted range of materials, namely optically smooth and homogeneous metals or dielectrics. There are also geometric models for determining the emissivity of opaque materials with known local surface emissivity and assumed roughness profile. It is also possible to calculate the effective emissivity of a cavity or enclosure based on local surface emissivity and cavity shape. But these are all special cases and in general it is not possible to predict emissivity values for real materials.
For calculation of radiative heat transfer the parameter that is mainly used is the total hemispherical emissivity. Most of the software used for those calculations does not consider the directional and spectral variations of emissivity. For non-contact thermometry (pyrometry and infrared thermography), the directional spectral emissivity has to be considered. That parameter characterises the emission of the surface in a particular direction and in a particular spectral band.
C7.What is the emissivity of painted metal surfaces and how does it depend on layer thickness?
The emissivity of a painted surface usually approaches the emissivity of the paint for a sufficient layer thickness, usually 2 or 3 thin coats. The emissivity of painted surfaces usually increases with temperature due to broadening of the absorption bands of the chemical components in the paint. A metal surface painted with a thin layer (1 coat) might have a change in emissivity from 0.95 at 500 °C to 0.4 at 50 °C.
C8. Which surfaces behave like a grey body?
In practice none! (For a grey body the emissivity is independent of wavelength.) The emissivity of all real surfaces changes with wavelength, although some surfaces change very little and so are close to a grey body. The ideal grey body concept is useful for simplified equations in heat transfer calculations.
C9. What is the emissivity of a layer of gas?
The emissivity of a layer of gas depends on the chemical composition, path length, pressure and temperature. The spectral emissivity of a given slab of gas can be calculated with details and features at temperatures around ambient with good precision, e.g. using databases like HITRAN and HITEMP. Equations and tables can be found in the literature to be used in heat transfer calculations. A hot gas cell can be used for measuring the emissivity of a given gas mixture.
C10. Where can I find information on the emissivity of a given surface?
Look at the evitherm website for a start. However, it might be difficult to find accurate values of the emissivity for a given specific surface in practise. Surface machining, treatment and handling all affect the emissivity value. Several laboratories offer emissivity measurement services - see services offered by test & calibration labs and NMIs.
C11. How can I measure the emissivity of a surface using an IR-thermometer?
You need a calibrated IR-thermometer and you must know the surface temperature precisely, e.g. using a calibrated thermo couple. Adjust the emissivity setting until the IR-thermometer reading equals the temperature of the surface.
C12. What is the difference between emissivity and emittance?
Emittance describes the ability of a body to emit radiation. It is defined as the ratio of the thermal radiation emitted by a surface to a blackbody, which is at the same temperature. Emissivity is an ideal concept since it is restricted to the case of the emittance of an optically smooth, opaque and homogeneous material.
C13. What is a radiant barrier?
Heat flow in buildings occurs in three ways: conduction, convection and radiation. Radiant barriers address heat flows by radiation. Thermal radiation is the range of electromagnetic waves emitted by a material due to its temperature alone. Warm surfaces, such as roof decks warmed by the sun, will radiate or emit their heat to cooler interior building surfaces. A radiant barrier is a low emissivity surface placed in the path of thermal radiation and it will block the radiant heat transfer.
C14. What is a low-e coating?
A low-e coating is a coating with a very low emissivity in the infrared region. In windows for example a low-e-coating is used, which has a low emissivity in the infrared region and which is transparent in the visible region.
C15. What is low-e glass?
Thin metallic layers are applied to the glass substrate during the float process, (pyrolytic or hard coat), or after the float process, (sputter or soft coat), giving the glass excellent solar control properties.
C16. What is a selective absorber?
A solar absorber, which has a high absorbance in the visible and near infrared region up to 2500 nm and a low emissivity in the infrared region, is called a selective absorber.
C17. Is a knowledge of emissivity important for contactless temperature measurements?
For contactless temperature measurements a knowledge of the surface emissivity is very important, because the emitted thermal radiation depends not only on temperature but also on the emissivity of the surface. Additionally, radiation from the surroundings that is reflected by the surface also influences the measurement and can be significant when the material surface and surroundings are at closely similar temperatures.
C18. What is infrared thermography?
Infrared thermography is the technique of creating fully analysable images from the thermal radiation given off by a subject, by means of an infrared camera. This image is called a thermogram and is often presented as a colour-coded image, where blue indicates 'cold' and red is hot.