Basic knowledge of thermal imaging infrared camera(01)

01 thermal imaging principle

In fact, all objects in nature, as long as its temperature is higher than the absolute temperature (-273 °C), there will be irregular movement of molecules and atoms, and its surface will continue to radiate infrared light.

In thermal imaging, infrared light in the thermal infrared band (8 μm-14 μm) is mainly collected to detect the thermal radiation emitted by the object. After the heat radiation is detected, it is converted into a gray value and the difference in gray values of each object is used for imaging to find and identify the target.

As shown below, the left is a photo of thermal image and right is visible light . It can be seen that the thermographic image presents different surface temperatures through different grayscale differences.

Figure 1: Comparison of visible and thermal imaging

02 thermal imaging camera introduction

Based on the above principles of thermal imaging, thermal imaging cameras are used. Similar to the principle of ordinary cameras, thermal radiation is equivalent to visible light. It passes through the lens to the detector, processes the image, and finally outputs the codestream to form a video image that can be observed by the human eye, as shown in the following figure:

Figure 2: Working principle of thermal imaging

Thermal imaging lenses and infrared detectors are important components of thermal imaging cameras throughout the imaging process.

Thermal imaging lens:

The thermal imaging lens adopt germanium lens. The high-purity germanium single crystal has a high refractive index and can block the visible light and ultraviolet light from the outside, allowing only infrared light to pass through.

Infrared Detectors:

Thermal imaging detectors are classified into cooled detectors and uncooled detectors.

The cooled equipment has high sensitivity, fast response, and clear images, but requires low-temperature refrigeration, resulting in large and expensive. Generally used in the military industry.

Uncooled devices have a slightly lower sensitivity, but they are basically in line with civilian requirements and do not require cryogenic refrigeration. The price is only one-tenth to a fraction of that of an cooled thermal device. At present, mainstream uncooled detectors generally use vanadium oxide or polysilicon.

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