Principle Analysis: Emissivity of Infrared Thermometers
Emissivity is defined as the ratio of radiant energy emitted by a real object to that of a blackbody at the same temperature under identical conditions. The identical conditions refer to the same geometric parameters (radiation-emitting area, solid angle magnitude and direction for measuring radiant power) and spectral conditions (spectral range of the measured radiant flux). Since emissivity depends on measurement conditions, several definitions of emissivity exist.
Hemispherical Emissivity Hemispherical emissivity is the ratio of radiant exitance (radiant flux emitted per unit area of the radiator into the hemispherical space) to the radiant exitance of a blackbody at the same temperature. It is further divided into total hemispherical emissivity and spectral hemispherical emissivity.
Normal Emissivity Normal emissivity is measured within a small solid angle along the normal direction of the radiating surface. It equals the ratio of the radiance in the normal direction to the radiance of a blackbody at the same temperature. As infrared systems detect radiant energy within a small solid angle along the normal of the target surface, normal emissivity is of great importance.
For a blackbody, all types of emissivity equal 1, while for real objects, all emissivity values are less than 1. The emissivity we generally refer to is the average emissivity.
Emissivity Correction
The emissivity varies with different object surfaces. Emissivity correction is usually required to guarantee temperature measurement accuracy. Thermometers are calibrated against blackbodies, yet the emissivity of any real object surface is lower than that of a blackbody.
The emissivity correction method for infrared thermometers works as follows: adjust the amplification factor of the amplifier according to the emissivity of different objects, so that the signal generated by the radiation of a real object at a given temperature in the system is equal to the signal produced by a blackbody at the same temperature. For example, if an object has an emissivity of 0.8, the amplifier gain needs to be increased to 1/0.8 = 1.25 times the original value. At industrial sites, it is generally difficult to determine the target emissivity because the measured targets differ in material, shape and surface condition. Measurement errors caused by other factors will also lead to discrepancies between measured readings and true values. Adjusting the emissivity parameter can effectively resolve this problem without affecting measurement linearity.
