Why is the infrared thermometer calibrated with a blackbody furnace?

When using an infrared thermometer to measure the temperature of a target, the infrared radiation of the target within its wavelength range must first be measured, and then the infrared thermometer calculates the temperature of the target. The monochromatic infrared thermometer is proportional to the radiation within the wavelength band; the two-color infrared thermometer is proportional to the ratio of the radiation of the two wavelength bands.

A black body is an idealized radiator that absorbs radiation energy of all wavelengths, has no energy reflection or transmission, and its surface emissivity is 1. It should be pointed out that there is no real black body in nature, but in order to clarify and obtain the law of infrared radiation distribution, a suitable model must be selected in theoretical research. This is the quantized oscillator model of body cavity radiation proposed by Planck, which leads to Planck's law of black body radiation, that is, the black body spectral radiance expressed in wavelength. This is the starting point of all infrared radiation theories, so it is called the black body radiation law.

The influence of object emissivity on radiation temperature measurement: Almost all actual objects in nature are not black bodies. The radiation amount of all real objects depends not only on the radiation wavelength and the temperature of the object, but also on the type of material, preparation method, thermal process, surface state and environmental conditions of the object. Therefore, in order to make the blackbody radiation law applicable to all real objects, a proportional coefficient related to the material properties and surface state must be introduced, namely the emissivity. This coefficient indicates the degree of closeness between the thermal radiation of the real object and the blackbody radiation, and its value is between zero and a value less than 1. According to the radiation law, as long as the emissivity of the material is known, the infrared radiation characteristics of any object can be known.
The main factors affecting the emissivity: material type, surface roughness, physical and chemical structure and material thickness, etc.