What Are the Principles and Classifications of Infrared Thermometers
1. Infrared Principle
Any object with a temperature above absolute zero (-273°C) emits thermal radiation outward. Differences in object temperature lead to variations in radiated energy and radiation wavelength, with infrared radiation always included in the spectrum. For objects below 1000°C, infrared waves constitute the dominant electromagnetic waves in thermal radiation. Therefore, measuring an object's inherent infrared radiation enables accurate detection of its surface temperature, which forms the objective foundation and core principle of infrared temperature measurement.
A blackbody is an ideal radiator that absorbs all incident radiant energy across wavelengths without reflection or transmission, with a surface emissivity of 1. However, virtually all natural objects are non-blackbody. To clarify and quantify the laws of infrared radiation distribution, a theoretical model is required for research. Planck proposed the quantized oscillator model for cavity radiation and derived Planck's law of blackbody radiation, which defines the spectral radiance of blackbodies by wavelength. Serving as the cornerstone of all infrared radiation theories, this is also known as the blackbody radiation law.
In addition to radiation wavelength and object temperature, the radiant output of real objects is affected by material type, processing methods, thermal history, surface conditions and environmental factors. To apply the blackbody radiation law to practical objects, a proportional coefficient related to material properties and surface conditions is introduced, namely emissivity. This coefficient indicates how closely the thermal radiation of a real object approximates that of a blackbody, ranging between 0 and 1. In accordance with radiation laws, the infrared radiation characteristics of any material can be determined once its emissivity is known. Key factors influencing emissivity include material type, surface roughness, physicochemical structure and material thickness.
2. Working Principle and Structure of Infrared Thermometers
In nature, all objects above absolute zero continuously emit infrared radiation into the surrounding space. The magnitude and wavelength distribution of an object's infrared radiation are closely correlated with its surface temperature. Thus, precise surface temperature measurement can be achieved by capturing an object's self-emitted infrared energy, which is the fundamental basis of infrared radiation thermometry.
The working principle of infrared thermometers is to convert the infrared radiant energy emitted by measured objects (e.g., molten steel) into electrical signals. The intensity of infrared radiation corresponds directly to the object's temperature, and the measured temperature is determined based on the converted electrical signal strength.
An infrared thermometer consists of an optical system, photoelectric detector, signal amplifier, signal processing unit and display output module. The optical system collects infrared radiant energy from the target within the field of view, whose coverage is determined by the thermometer's optical components and layout. Focused infrared energy is converted into proportional electrical signals by the photoelectric detector. After amplification and processing by dedicated circuits, the signals are calibrated via built-in algorithms and target emissivity parameters to output the final measured temperature value.
When measuring target temperature with an infrared thermometer, the device first captures infrared radiation within the set spectral band and calculates the target temperature accordingly. Based on working principles, infrared thermometers are classified into single-color thermometers and two-color thermometers (ratio radiation thermometers). Single-color thermometers measure temperature proportional to radiation intensity within a single band, while two-color thermometers calculate temperature based on the radiation ratio of two separate spectral bands.
3. Development and Classification of Infrared Thermometers
Modern infrared temperature measurement technology supports scanning thermal detection on variable-temperature surfaces to generate temperature distribution images and identify hidden temperature differences efficiently, a function realized by infrared thermal imagers.
Infrared thermal imaging technology was initially applied to military equipment. Texas Instruments (TI) developed the world's first infrared scanning detection system. Since then, Western countries have widely adopted infrared thermal imaging technology for military platforms including aircraft, tanks and warships as thermal targeting systems, greatly enhancing target search and strike capabilities.
Infrared temperature measurement equipment is generally categorized into three types:
(1) Infrared spot thermometers: including portable and fixed mounting models;
(2) Infrared scanners;
(3) Infrared thermal imagers.
