Theoretical Principle and Application of Infrared Thermometers
In nature, when an object's temperature is above absolute zero, it continuously emits electromagnetic waves to the surroundings due to internal thermal motion. These waves include infrared radiation in the wavelength range of 0.75 µm to 100 µm. A characteristic property is that, at a given temperature and wavelength, the radiant energy emitted by an object reaches a maximum. Such an ideal substance is called a blackbody, with an emissivity set to 1. All other substances, with emissivity less than 1, are referred to as graybodies.
The spectral radiant power P(λ,T) of a blackbody follows Planck's Law as a function of absolute temperature T. This law describes the radiant power per unit area of a blackbody at wavelength λ and absolute temperature T.
As temperature increases, the radiant energy of an object becomes stronger. This is the foundation of infrared radiation theory and the design basis for single-band infrared thermometers.
As temperature rises, the radiation peak shifts toward shorter wavelengths (to the left), obeying Wien's Displacement Law: the peak wavelength is inversely proportional to the absolute temperature T. The dashed line represents the locus of these peak values. This relationship explains why high-temperature thermometers mostly operate at short wavelengths, while low-temperature thermometers use longer wavelengths.
The rate of change of radiant energy with respect to temperature is greater at shorter wavelengths than at longer ones. In other words, thermometers operating at shorter wavelengths have a higher signal-to-noise ratio (higher sensitivity) and stronger anti-interference performance. Thermometers should ideally be designed to operate near the peak wavelength, especially for low-temperature and small targets, where this factor is particularly critical.
Part Two
An infrared thermometer consists of an optical system, a photoelectric detector, a signal amplifier, signal processing circuitry, and a display output module.Radiation from the measured object and a reference source is modulated by a modulator and then input to the infrared detector.The difference between the two signals is amplified by an inverse amplifier and used to control the temperature of the reference source, so that its spectral radiance matches that of the measured object.The display then indicates the brightness temperature of the measured object.
