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What Factors Influence the Emissivity of a Material?

There are several factors which influence the emissivity of a material. We need to be aware of their effect on emissivity values.

Wavelength
Effect of Wavelength - Emissivity will normally vary with wavelength - for example, the emissivity of polished metals tends to decrease as wavelength becomes longer. Non-metallic materials tend to behave differently to metals often showing large variations of emissivity with wavelength. Semi-transparent materials such as plastic film also show strong variations with wavelength and require special consideration. The graph shows the typical emissivity curve for iron (the red curve), along with a “greybody” object which has a constant emissivity across the whole of the infrared spectrum (the blue line).

Effect of Surface Condition -
In the case of metallic materials, emissivity will decrease with polishing and increase with surface roughness and the degree of oxidisation. Metals which have been subject to a high temperature industrial process normally have a heavy oxide layer and have a high and stable emissivity values. Materials which have acquired a thin oxide layer such as bright metals can have an emissivity which depends critically on oxide thickness. 

Viewing Angle GraphEffect of Viewing Angle - The emissivity of most materials is not strongly dependent on viewing angle provided measurement is made within about 45° of normal as per graph). The maximum recommended angle for mounting an infrared thermometer is 45°.

Effect of Temperature - The emissivity of materials does not tend to vary with temperature when using a thermometer which operates over a narrow waveband. Emissivity will usually only change with temperature if the surface properties of the material change, for example if coatings become tarnished or degraded, or for metals such as aluminium where emissivity depends critically on oxide layer structure, which is heavily temperature dependent.  

Emissivity Measurement
We can determine the emissivity of the surface of a target material in various ways:

• Consult AMETEK Land operating instructions, which list typical emissivities. Caution should be used when applying emissivity values from other sources to ensure that the wavelength (and temperature) at which these were determined is the same as the operational wavelength of the instrument in question.

• Use a reference temperature reading to derive emissivity.Typically, thermocouples are used in the industry. A momentary reference measurement taken at the same place and time the infrared device is measuring can be used to calculate the true emissivity of the target surface.The reference probe can then be removed, and the calculated emissivity value used by the infrared instrument thenceforward to ensure correct temperature measurements.This is the easiest and most common procedure to adjust the emissivity in the field.

• Determine the emissivity by a laboratory method. This is usually performed via measurement of reflectivity, by comparison between measurements of the surface reflecting hot and cold secondary targets.

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