The main factors affecting the temperature measurement of the infrared camera

The main factors affecting the temperature measurement of the infrared camera

The detector in the thermal imaging camera receives not the real temperature of the object, but the radiation temperature. Although the radiation temperature has been corrected by the atmospheric transmission factor, there is still a certain difference between it and the true temperature of the surface of the object. Only by correcting these differences can the true temperature of the object be obtained. The influencing factors in the actual temperature measurement process include emissivity, scattering and absorption on the optical path, background noise, and the stability of the infrared camera. The degree of influence of these factors is different depending on the measurement conditions, and must be accurately calibrated to ensure the reliability of the measurement.

1. Emissivity

The emissivity is the biggest uncertainty factor that affects the temperature measurement accuracy of the infrared camera. To get the true temperature of an object, the emissivity value of the object must be accurately set. The emissivity is affected by factors such as material properties, surface state, and temperature.

Material properties include not only differences in the chemical composition and chemical properties of the material, but also differences in the internal structure (such as surface layer structure and crystalline state, etc.) and physical properties of the material. The properties of the materials are different, and the materials have different emission properties, radiation absorption or transmission properties. The emissivity of most non-metallic materials in the infrared spectral region is relatively high, while the emissivity of most pure metal surfaces is very low. When the temperature is lower than 300K, the emissivity of metal oxides generally exceeds 0.8.

The effect of the surface state is that any actual object has different surface roughness, and it will always appear as an uneven irregular shape, and there is no absolutely smooth object surface. Different surface morphologies affect reflectivity first, and thus emissivity. The type and roughness of the material directly affect the emissivity.

The relationship between temperature and emissivity is difficult to be quantitatively summarized by a unified analytical expression, because different materials have different emissivity changes in different wavelengths and temperature ranges. Although in many cases it is believed that the emissivity changes with temperature, but the emission It is not specified how the rate changes with temperature. General experiments show that the emissivity of most pure metal materials increases approximately proportionally with the Kelvin temperature, but the proportionality factor is related to the resistivity of metals; the emissivity of most non-metallic materials decreases with increasing temperature .

2. Effect of background noise

When using an infrared thermal imager for radiation temperature measurement, the signal is very small and often overwhelmed by background noise. Therefore, the temperature measurement below normal temperature must consider the influence of background noise, because it is greatly affected by background noise. When measuring indoors, the reflected light of surrounding high-temperature objects will also affect the measurement results of the temperature of the object to be measured; the main background noise outdoors is direct radiation, refraction and spatial scattering of sunlight. Therefore, various influencing factors must be considered when measuring temperature. The basic countermeasures adopted are as follows:

(1) Set up a shield near the object to be measured to reduce the interference of the external environment.

(2) Accurately align the focal length to prevent the radiation energy of non-tested objects from entering the test angle.

(3) When measuring outdoors, choose night or cloudy weather to exclude the influence of sunlight.

(4) Improve the emissivity by making small holes or using high emissivity paints to make it close to 1.

3. The effect of absorption on the optical path

Water, carbon dioxide, ozone, carbon monoxide, etc. in the air all absorb infrared light. According to the adaptability of the instrument itself and the actual working environment, the influence of water vapor on the temperature measurement accuracy is mainly considered. In the case of high wind force, the temperature of the measured object will drop. Because of the influence of wind speed cooling convection, it also affects the accuracy of temperature measurement. The Swedish National Electric Power Bureau has defined a correction formula for wind influence.

4. The influence of the stability of the thermal imager

In actual temperature measurement, the infrared thermal imager is different from other instruments, and is greatly affected by the ambient temperature to a large extent. When the temperature to be measured is lower than normal temperature, the infrared lens itself has some inevitable influencing factors, so that the influence of environmental temperature change is even greater than the influence of signal change. Although some compensation measures have been taken in the design of the instrument, when the ambient temperature is higher than the specified value, the instrument must be cooled when it is used to maintain a constant temperature.

5. Compensation for the radiation emitted by the thermal imager itself

A well-designed thermal imager can automatically compensate for the radiation from the thermal imager itself and its optical components, but few thermal imagers can properly compensate. Therefore, the temperature of the measured target depends on the temperature of the thermal imager. Due to non-100% reflection or transmission, the radiation to the thermal imager itself is mainly caused by the attenuation of the radiation by optical elements (such as plane mirrors and lenses).