Target Surroundings
Infrared Detectors are often used to measure the temperature inside a furnace.  Sighting is on the furnace wall or roof or into a closed tube.  In such cases, the target is considered to be a black body emitter.

When the purpose of the infrared Detector is to measure the temperature of a hot body in cold surroundings, the reading given depends on the particular material viewed.  The property influencing the reading is the emissivity or emittance of the surface.

Emissivity is the ratio of the rate of emission of radiation from a polished, optically flat surface, to the rate of emission from a black body at the same temperature  The property of a particular non-ideal sample or body is defined as emittance, and this term is sometimes encountered.

The emissivity of any sample is affected by a number of factors.  Those having a considerable influence follow.

Sighting Angle

Infrared Detectors measure radiation from a surface through a small solid angle.  The emissivity of a surface is usually a function of the angle between the line of sight and the normal to the surface and these effects become important at angles greater than 60^o. The emissivity of insulating materials is a maximum when viewed normal to the surface.  Infrared Detectors should not be sighted at more than 45^oC to the normal surface.

Wavelength

When measured over all wavelengths of radiation emitted by a material, the term is "total emissivity".  When measured over a small waveband, usually at short wavelengths, the term is "spectral emissivity". 

Many semi conducting materials, including a lot of pure oxides at higher temperatures, are partially transparent.  This results in variations of a spectral emissivity with wave length.  Special care must be taken in choosing an infrared detector for use with such materials.

Temperature
Special emissivity does not vary greatly with temperature though it is usually dependent on wavelength.  for metals and alloys, emissivity is higher at short wavelengths than at long ones so that, as the temperature of a sample increases and more energy is emitted at short wavelengths, total emissivity increases.  non-metals, if they are opaque at all wavelengths, generally have spectral emissivities which are not greatly dependent on wavelength and emissivity does not change with a rise in temperature.  Many pure oxides present special problems.

Surface Conditions
Surface roughness and oxide layers generally increase the emissivity of a sample

The ORO (wavebands 0.3 to 2.6µm and 0.25 to 3.8µm can only be considered as total radiation detectors when most of the incident energy falls below the long wavelength cur-off.  This occurs above 800^oC for the ORO.

Emissivity values are often quoted under two headings, "Hemispherical Emissivity" and "Normal Emissivity".  Hemispherical Emissivity is the properties of a surface over the whole 2π steradians solid angle.  Normal Emissivity is the properties of radiation in a narrow solid angle normal to the surface.

The values given in the following tables are not intended as a source of precise data for practical temperature measurement.  Usually an experimental emissivity check on the surface is necessary.