Wuxi Slossen's principle analysis of the emissivity of infrared thermometers
Emissivity is the ratio of the radiated energy of an actual object to a black body of the same temperature under the same conditions. The so-called same conditions refer to the same geometric conditions (emitting radiation area, solid angle size and direction for measuring radiation power) and spectral conditions (spectral range for measuring radiation flux). Since emissivity is related to measurement conditions, there are several definitions of emissivity.
Hemispheric emissivity Hemispheric emissivity is the ratio of the radiant energy flux (radiation outgoing degree) emitted by the radiator to the hemispheric space per unit area and the radiating outgoing degree of the black body at the same temperature, which is divided into two types: full amount and spectral amount.
normal emissivity
Normal emissivity is the emissivity measured within a small solid angle in the normal direction of the radiating surface. It is the ratio of the radiant brightness in the normal direction to the radiant brightness of a black body at the same temperature. Since infrared systems detect radiated energy within a small solid angle in the normal direction of the target surface, the normal emissivity is very important.
For a black body, all emissivities are equal to 1, while for real objects, the values of various emissivities are less than 1. The emissivity we are talking about so far is the average emissivity.
Regarding emissivity correction:
The emissivity of different object surfaces is different. In order to ensure the accuracy of temperature measurement, emissivity correction is generally required. Since the thermometer is calibrated with a black body, the surface emissivity of any object is smaller than the black body emissivity.
The emissivity correction method of the infrared thermometer is to adjust the amplification factor of the amplifier according to the emissivity of different objects, so that the radiation of an actual object with a certain temperature produces a signal in the system that is the same as that produced by a black body with the same temperature. The signals are the same. For example, if the emissivity of a certain object is 0.8, then the magnification of the amplifier needs to be increased to 1/0.8 = 1.25 times. However, in industrial sites, it is generally difficult to determine the target emissivity parameters due to the different materials, shapes and surface conditions of the measurement targets. There are also measurement errors caused by other factors that will cause the difference between the measured value and the true value. The introduction of emissivity parameter adjustment can solve this problem well without affecting the measurement linearity. You can adjust it based on experience temperature or process temperature according to the following steps:
For example: the measuring range of the thermometer is: 500-1400℃
The real temperature is 1200℃, the measured temperature is 1150℃,
At this time, the emissivity parameter can be adjusted to:
(1150-500)÷(1200-500)=0.928≈0.93
After such adjustment, the measured value will be closer to the real value. You can also refer to the "Material Emissivity Coefficient Table" for adjustment. However, the parameters in this table may not necessarily apply to process needs. It must be clear that the essence of emissivity adjustment is to correct measurement errors.
