Main solutions of passive infrared imaging in video surveillance

The application premise of passive infrared imaging technology is that the light capture device must be able to collect infrared signals in addition to visible light signals, and the signal processing can process the original infrared signal into a grayscale signal (commonly known as a black and white signal). Black and white cameras can achieve this function and have very high sensitivity. At present, all cameras based on digital processing technology can also complete this task, but due to the color imaging under visible light environment, contradictions have begun to appear. When processing color signals, because DSP processing requires the video signal to be separated into grayscale signals and chrominance (or color difference) signals for separate processing, and the infrared signal itself is invisible to the human eye, but after being collected by the light capture device and processed by DSP, it has become a grayscale signal that can be recognized by the human eye. The superposition of the two grayscale signals (visible light and infrared light) will inevitably make the image unable to be synthesized according to the ideal situation when synthesizing grayscale and chrominance, which will cause the grayscale and chrominance of the image to be distorted. The most typical example is that if the infrared light is too strong, the entire image will be gray. At present, there are four main solutions for passive infrared imaging in video surveillance:

1. Pure color camera: This solution is to prevent passive infrared imaging, that is, to prevent infrared rays from entering. It uses an optical low-pass filter (OPLF: Optical Low Filter, commonly known as a low-pass filter. Figure 1 shows the relationship between its light transmittance and wavelength. It can be seen that it basically does not absorb and reflect visible light, but basically completely isolates infrared light) to directly block infrared rays, so that the image will be basically unaffected by infrared signals. The purpose of this is to avoid infrared imaging.

2. Completely ignore the impact of infrared signals on the color of the image and use a color filter that can pass a large amount of infrared light. This is a low-cost solution that works well in passive infrared imaging, but it is easy to have the chromaticity and grayscale distortion problems mentioned above in color mode.

3. Only specific infrared light, such as 850nm, is allowed to pass, while most other infrared light cannot pass. This is the principle of single-filter sensing infrared camera. Its main technical basis is the use of color filters different from those of color cameras.

This solution can solve the color cast problem to a certain extent, and can also be used in situations where infrared imaging is used without visible light. However, this solution also has some problems. When there is sufficient sunlight, the infrared signal is very rich, which will cause image color and grayscale distortion. When using infrared imaging, because only infrared light with a very narrow frequency can pass through, the imaging is not sensitive, so this type of camera is generally used in areas within a diameter of 30 meters indoors.

4. When visible light is strong, OPLF is used to block most of the infrared light to ensure the color restoration and authenticity of the grayscale signal. When visible light is weak, OPLF is not used, but a high-pass filter that allows most visible light and infrared light to pass is used. Because imaging mainly relies on infrared light, which is a grayscale signal, the color component is generally removed at this time and only the grayscale signal is retained, so what you see is only a grayscale image, which is what we call a black and white image.

This solution actually combines the advantages of color cameras in good visible light conditions and black and white cameras in low illumination conditions. It can be said to be the best solution for all-weather monitoring in environments with large light changes. The true color reproduction under natural light and the highly sensitive passive infrared imaging under no visible light, its static effect can even be comparable to some all-in-one cameras that use the same technology and add 4x DSS photosensitivity technology.

Although the use of passive infrared imaging can better solve the contradiction between the absence of visible light and monitoring, there are also some problems that need to be solved due to the optical differences between infrared and visible light. The different refractive indices of the main optical medium glass for the two light waves will cause different focal lengths of the optical components, so it is easy to cause focusing problems. However, these problems have been solved accordingly through continuous use and improvement. With the continuous maturity and widespread application of passive infrared imaging technology, the monitoring efficiency has been greatly improved, and people's lives will be more reliably protected.