How Does an Infrared Thermometer Work?
Understanding the infrared thermometers’ working principle, technical specifications, working conditions, operation and maintenance is the basis for users to correctly select an infrared thermometer or use it.
All objects above absolute zero are radiating infrared energy to the surrounding space. An object’s infrared radiation energy and its wavelength distribution are closely related with its surface temperature. Therefore, the surface temperature of an object can be accurately detected by measuring the infrared energy of the object, which is the objective basis of infrared radiation temperature measurement.
Furthermore, it has to refer to Black-body radiation law. Black body is an ideal radiator. It absorbs the radiation energy of all wavelengths, but reflects or penetrates no energy. The emissivity on its surface is 1. It should be noted that there is no real black body in nature. However, in order to understand and obtain the distribution rule of infrared radiation, a proper model must be selected in theoretical research, which is the quantized oscillator model of body cavity radiation purposed by Planck. On this basis, Planck’s law of black-body radiation is derived, namely, spectral radiance of black body is represented by wavelengths. This is the foundation of all infrared radiation theories, called black-body radiation law.
Nearly all actual objects in nature are not black bodies. Radiation quantity of actual objects not only depends on the radiation wavelength and object’s temperature, it’s also related with the materials, preparation method, thermal process, surface state and environmental conditions of the object. Therefore, to ensure that the black-body radiation law is suitable for all actual objects, a scale coefficient related to material properties and surface states must be introduced, namely the emissivity. This coefficient represents the degree of proximity between the thermal radiation of actual object and black-body radiation, and the value is generally between 0 and 1. According to the radiation law, as long as knowing the material emissivity, infrared radiation characteristics of any object can be obtained. What are main factors affecting the emissivity? There are material type, surface roughness, physicochemical structure, material thickness and so on.
An infrared thermometer are constituted of optical system, photoelectric detector, signal amplifier, signal processing and display output. The optical system of infrared thermometer accumulates the target infrared energy in its field of view. And the size of field of view is determined by the optical components and location of the infrared thermometer. The infrared energy is focused on the photoelectric detector and converted to corresponding electrical signal. The signal is corrected by the amplifier and signal processing circuit according to the algorithm inside the instrument and the target emissivity and then is converted to the temperature value of measured target.