What Is the Working Principle of Infrared Thermometers?
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An infrared thermometer is a non-contact temperature measurement device that uses infrared radiation energy to detect the surface temperature of a target object. Many people think that the working principle of infrared thermometers does not seem complicated. They only need to "align, press keys, and read" to complete the temperature measurement. But in fact, its operating mechanism involves principles from multiple disciplines such as thermodynamics, optics, electronic engineering, and signal processing. In order to accurately understand how it completes temperature detection, we need to analyze its working principle layer by layer from the nature of infrared radiation, the collection mechanism of the optical system, the response characteristics of the sensor to the signal processing conversion process.
All objects whose temperature is higher than absolute zero (-273.15°C) have the microscopic particles (atoms, molecules) that make up the matter in a state of continuous thermal motion. This thermal movement causes the object to continuously emit electromagnetic waves outward, a process called thermal radiation. Thermal radiation has a wide range of electromagnetic spectrum, including infrared radiation. There is a certain physical relationship between the temperature of an object and the characteristics of the infrared radiation it emits: Generally, the higher the temperature of an object, the stronger the total infrared radiation energy emitted per unit surface area per unit time, and the wavelength with the strongest radiation energy will also move toward the shortwave direction. Infrared temperature measurement technology is based on measuring the infrared radiation emitted by an object and then calculating the surface temperature of the object.
Optical systems collect energy
This optical system is usually composed of optical components such as infrared lenses, concave mirrors and filters. First of all, the infrared lens is used to define the measurement range of the thermometer. It can collect the infrared radiation emitted by the target surface within the field of view and initially concentrate the energy. Then, the concave reflector is responsible for further reflecting and focusing these infrared rays, making the originally randomly diffused energy paths consistent, thereby increasing the energy density of infrared radiation per unit area, so that it can effectively hit the sensing surface of the detector. This process is like a magnifying glass focusing sunlight to ignite paper, concentrating scattered energy into a stronger signal. At the same time, the filter is responsible for screening the infrared band. It can filter out stray light or other electromagnetic interference in non-target bands, and only retain infrared radiation within a specific wavelength range to enter the system, improving the purity of the measurement from the source.
Detectors Convert Energy Into Electrical Signals
Infrared detector is the core sensing element of infrared thermometer. When infrared radiation concentrated by the optical system strikes the sensing area of the detector, the detector material absorbs this radiation energy. Absorbing energy causes a measurable change in some physical property of the detector itself. For example, in a thermopile detector, the absorption of infrared radiation will cause the temperature of the detector's sensing node to rise. According to the thermoelectric effect, the temperature rise will generate a small voltage signal proportional to the absorbed energy. In other types of detectors, such as pyroelectric detectors, changes in temperature can cause changes in the surface charge of the material, producing a current or voltage signal. The key is that regardless of the specific mechanism, the function of the detector is to convert the received infrared radiation power into an electrical signal that corresponds in magnitude.
Signal Processing And Computing
The electrical signal generated by the sensor is a raw analog signal and cannot directly reflect the temperature value. This signal must be calculated and converted by a complete set of signal processing modules before it can finally present a specific temperature reading on the display. The signal processing process includes: amplifying weak signals, filtering noise interference, performing A/D (analog/digital) conversion, calculating based on the nonlinear correction model of the relationship between radiation intensity and temperature, applying compensation algorithms to correct environmental temperature changes, etc.
First, the signal amplification circuit boosts the weak signal to a sufficient amplitude for subsequent processing. Next, a filter may be needed to remove noise signals that are not relevant to the temperature measurement. Among them, the most critical step is ambient temperature compensation: there will be a sensor inside the thermometer to measure the working environment temperature of the instrument itself. The electronic system uses this information to correct the output signal of the detector to reduce the impact of the temperature change of the instrument itself on the measurement results. In addition, the infrared emissivity of the object surface (i.e., a parameter of the object's ability to radiate infrared) has a significant impact on the radiation intensity, and the electronic system corrects the signal based on the preset or user-entered emissivity value. Finally, the processor calculates and converts the amplified, compensated and corrected electrical signal values into corresponding temperature readings based on the calibration data and built-in algorithms (these algorithms are based on physical principles such as Planck's law and Stefan-Boltzmann law).
Display Of Temperature Data
After processing and calculation by the electronic system, the final temperature value obtained will be transmitted to the display unit of the instrument. This display unit is usually a liquid crystal display (LCD) or similar device, which clearly displays the calculated temperature to the user in digital form (for example, degrees Celsius or Fahrenheit). The user can know the surface temperature of the target object being measured by reading the numbers on the display.
