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This is a subject that is widely misunderstood in the security industry. It is a measure of the efficiency of heat radiation and absorption, it affects the rate of transfer, not if there will be transfer. To establish transfer of energy it is necessary that a temperature difference exist and transfer will take place from the hotter surface to the cooler one. Temperature difference will establish an energy transfer, emissivity will control the rate of energy transfer. It is commonly thought that a target with a different emissivity than the background can be detected even if there is no temperature difference between them, but this is not so. Any exchange of energy between the target and the detector must pre-suppose a temperature differential. This is the reason that PIR sensors "go to sleep" when the background temperature is similar to the intruders temperature.

In a very loose analogy one might think of emissivity as the size of a pipe connecting the bottoms of two tanks of water. Tank A has initially has a higher level than tank B so gravity will equalize the levels via the pipe. The difference in level is analogous to temperature. If the pipe is small, low emissivity, the time taken to equalize will be longer than if the pipe is large, high emissivity. In the end, independent of the pipe size, the levels will equalize; i.e. become the same temperature. That is unless more water is added to tank A. This is analogous to tank A being an active energy source in which case levels, temperatures, may never equalize.

Only three things can happen to the energy radiated from from one surface to another. The transferred energy is absorbed, reflected, transmitted through the surface or a combination thereof. The emissivity factor gives the ratio of absorbed vs. reflected/transmitted energy. An emissivity factor of 0.9 absorbs 90% of the received energy and reflects/transmits/re-radiates 10%. Some materials transmit energy at some wavelengths while absorbing energy at others so emissivity is a spectral parameter.

When the heat gained equals the heat lost by reflection/re-radiation and transmission, thermal equilibrium is reached. If the second surface temperature is raised above ambient it may also become a radiator to surounding cooler surfaces.

How important is emissivity in sensor design and operation? Well it is sometimes hyped up to be very mysterious but it is mostly common sense. Most common building materials, clothing and human skin have high emissivity factors of 0.9 and higher. Among common materials only polished metals have low emissivity. However, if such materials appear in a sensors field-of-view, they will be reflectors and will appear to have similar emissivity as whatever they are reflecting. This is probably high emissivity building material. So, in effect, the sensor is usually viewing high emissivity factors. Of course, a reflecting background would redirect the affected field-of-view.

Some materials are less obvious. Glass for example has a very high transmission in the visible part of the spectrum and thus has very low spectral emissivity at these wavelengths. At 9.5 microns it is opaque with a spectral emissivity of 0.94. This leads to confusion during sensor installation on how to treat glass. What happens if a PIR sensor looks at glass? At IR wavelengths it looks pretty much like the adjacent masonry wall but because it is thin, outside temperature might affect it by conduction, so it might be thermally unstable. Modern multi-layer insulated windows are probably much better. Visible light is transmitted through windows and might be a false alarm source for sensors not sufficiently hardened.

Materials and Emissivity Factors.

500 deg K 0.05
1000 deg K 0.11
2000 deg K 0.26
3000 deg K 0.33
3500 deg K 0.35
bulletPolished silver
bulletPolished aluminum, 300 deg K
bulletPolished aluminum, 1000 deg K
bulletPolished copper
bulletPolished iron
bulletPolished brass
bulletOxidized iron
bulletBlack oxidized copper
bulletAluminum oxide
bulletLaboratory blackbody
0.98 - 0.99

Most thermal radiators are not perfect black bodies. Many are what are termed gray bodies. A gray body is one which emits the same spectral distribution as a blackbody but with less intensity.


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Last modified: October 25, 2009