Working Principle and Error Analysis of Infrared Thermometer
Infrared thermometer system composition
Infrared temperature measurement adopts a point-by-point analysis method, that is, the thermal radiation of a local area of the object is focused on a single detector, and the radiation power is converted into temperature through the emissivity of the known object. Due to the different detected objects, measurement ranges and usage occasions, the appearance design and internal structure of infrared thermometers are different, but the basic structure is generally similar, mainly including optical system, photodetector, signal amplifier and signal processing, display output The infrared radiation emitted by the radiator of its basic structure enters the optical system, and the infrared radiation is modulated into alternating radiation by the modulator, which is converted into a corresponding electrical signal by the detector. The signal passes through the amplifier and signal processing circuit, and is converted into the temperature value of the measured target after being corrected according to the algorithm in the instrument and the target emissivity.
Error Analysis of Infrared Temperature Measurement
Since the infrared temperature measurement is non-contact, there will be various errors, and there are many factors that affect the errors. In addition to the factors of the instrument itself, it is mainly manifested in the following aspects.
1. Radiation rate
Emissivity is a physical quantity of an object’s radiation ability relative to a black body. It is not only related to the object’s material shape, surface roughness, unevenness, etc., but also related to the direction of the test. If the object is a smooth surface, its directionality is more sensitive. The emissivity of different substances is different, and the amount of radiation energy received by an infrared thermometer from an object is proportional to its emissivity.
(1) Setting of emissivity
According to Kirchhoff's theorem [2]: the hemispherical monochromatic emissivity (ε) of the object surface is equal to its hemispherical monochromatic absorptivity (α), ε=α. Under thermal equilibrium conditions, the radiation power of an object is equal to its absorbed power, that is, the sum of the absorption rate (α), reflectivity (ρ), and transmittance (γ) is 1, that is, α+ρ+γ=1, and Figure 3 explains the above law. For opaque (or with a certain thickness) object transmittance visible γ=0, only radiation and reflection (α+ρ=1), when the emissivity of the object is higher, the reflectivity is smaller, the influence of background and reflection The smaller the value is, the higher the accuracy of the test will be; on the contrary, the higher the background temperature or the higher the reflectivity, the greater the impact on the test. It can be seen from this that in the actual detection process, we must pay attention to the emissivity corresponding to different objects and thermometers, and set the emissivity as accurately as possible to reduce the error of the measured temperature.
(2) Test angle
The emissivity is related to the test direction. The larger the test angle, the greater the test error. This is easily overlooked when using infrared for temperature measurement. Generally speaking, the test angle is best within 30°C, generally not higher than 45°C, if it has to be tested at a temperature higher than 45°C, the emissivity can be appropriately lowered for correction. If the temperature measurement data of two identical objects are to be judged and analyzed, then the test angle must be the same during the test, so that it is more comparable.
