Night vision technology based on low light and infrared

The low-light-level night vision technology, which began in the 1960s, relies on natural light to illuminate the scene at night, works in a passive way, and has good concealment. It has been widely used in military, security, transportation and other fields. In recent years, low-light-level night vision technology has developed rapidly. On the basis of the first, second and third generations, the fourth generation of technology has emerged. The infrared thermal imaging technology, which began in the 1950s, has also gone through three generations. It detects by receiving infrared rays radiated by various parts of the scene itself. Compared with low-light imaging technology, it has the characteristics of strong ability to penetrate smoke and dust, can identify false targets, and can work day and night. It can be said that low-light imaging technology and infrared thermal imaging technology have become the two mainstays of night vision technology.

2 Low-light-level night vision technology and its development

2.1 The first generation of low-light-level night vision technology

In the early 1960s, based on the invention of multi-alkali photocathode (Sb-Na-K-Cs), optical fiber panel and the improvement of concentric sphere electron optical system design theory, these three technologies were engineered to develop the first generation of micro-light tubes. Its first-stage single tube can achieve a brightness gain of about 50 times, and through a three-stage cascade, the gain can reach 5x104~105 times. The first generation of low-light night vision technology is a passive observation method, which is characterized by good concealment, small size, light weight, high yield rate, and easy mass production; technically, it takes into account and solves the contradiction between the flat image field of the optical system and the concentric sphere electron optical system's requirement for a spherical object (image) surface, and the imaging quality is significantly improved. Its disadvantage is that it is afraid of strong light and has a halo phenomenon.

2.2 Second generation of low-light night vision technology

The main feature of the second-generation low-light-level night vision device is the invention of the microchannel plate electron multiplier (MCP) and its introduction into a single-stage low-light-level tube. A single-stage low-light-level tube equipped with one MCP can achieve a brightness gain of 104-105, thus replacing the original bulky and heavy three-stage cascaded first-generation low-light-level tube; at the same time, the inner wall of the MCP microchannel plate is actually a continuous dynode with a fixed plate resistance. Therefore, under a constant operating voltage, when there is a strong current input, there is a self-saturation effect of a constant output current. This effect just overcomes the halo phenomenon of the low-light-level tube; coupled with its smaller size and lighter weight, the second-generation low-light-level night vision device is currently the main body of domestic low-light-level night vision equipment.

2.3 Third-generation low-light-level night vision technology

The main feature of the third generation of low-light-level night vision devices is the introduction of transmissive GaAs photocathode and MCP with Al2O3 and ion barrier membrane into the close-fitting low-light tube. Compared with the second generation of low-light devices, the sensitivity of the third generation of low-light devices has increased by 4 to 8 times, reaching 800μA/Im~2600μA/Im, and the life span has been extended by 3 times. The utilization rate of the night sky light spectrum has been significantly improved, and the target viewing distance in the dark (10-4lx) night has been extended by 50%-100%. The process basis of the third generation of low-light devices is ultra-high vacuum, NEA surface activation, double close-fitting, double indium sealing, surface physics, surface chemistry and long life, high gain MCP technology, etc., which has created good conditions for the development of high-tech products such as the fourth generation of low-light tubes and long-wave infrared photocathode image intensifiers.

Figure 1 shows the images obtained by three generations of low-light night vision devices under the same conditions. It can be clearly seen from the figure that the third generation is better than the second generation, and the second generation is far better than the first generation.

2.4 Development Trend of Low-Light-Level Night Vision Technology

The research direction of low-light-level night vision devices is to improve the performance of several generations of existing products, reduce costs, expand equipment, and further extend the infrared response of new-generation products and improve the sensitivity of devices.

2.4.1 Super Second-Generation Low-Light-Level Night Vision Technology

The second-generation micro-light tube uses the same technology as the third-generation micro-light close-up tube. The main technical feature is the introduction of a highly sensitive multi-alkali photocathode into the second-generation micro-light tube, and the use of the third-generation micro-light MCP, tube structure, integrated power supply, crystallography, semiconductor body characteristics and other mechanism and process research results. Its imaging quality has been greatly improved. Due to its relatively simple process and relatively low price, it has become the current mainstream product.

2.4.2 The fourth-generation micro-light night vision technology

Recently, designers of micro-light tubes have removed the ion barrier membrane from the MCP to obtain a film-free micro-light tube, while adding an automatic gate switch power supply to control the switching speed of the photocathode voltage, and improved the low-halo imaging technology, which helps to enhance the visual performance under strong light. In 1998, Litton first successfully developed an imaging tube without a film MCP, which greatly improved the target detection distance and resolution, especially under extremely low illumination conditions. Its key technologies involve new high-performance film-free MCP, automatic pulse gating power supply used between the photocathode and MCP, and no-halo imaging technology. Although this film-free BCG-MCPIV generation micro-light tube technology has just started, its good performance will inevitably make it a new hot spot in the field of low-light image enhancement technology in this century.

3 Infrared imaging technology and its development

3.1 The first generation of infrared thermal imaging technology

The development of thermal imaging technology began in the 1950s. At first, only thermal imagers based on unit devices could be developed, with low field frequency and limited to small-scale applications. It was not until the 1970s, when the long-wave mercury cadmium telluride (MCT) material and photoconductive multi-element linear array device technology matured, that thermal imagers began to be mass-produced and equipped to the military. There are many types of thermal imagers, which can be roughly divided into two categories: one is a general-purpose component-based thermal imager; the other is a thermal imager designed according to special requirements.

The United States has developed a universal component thermal imaging system with 60-yuan, 120-yuan and 180-yuan optical conductor array devices and parallel scanning. Their frame rate is compatible with television and is also an interlaced scanning system. Each field has only 60 lines, 120 lines and 180 lines, and each frame of the image is displayed by a synchronously scanned 60-yuan, 120-yuan and 180-yuan light-emitting diode. In Europe, the British thermal imager is a representative example of a serial-parallel scanning system. It is based on the swept-area photoconductive MCT detector and constitutes the second type of universal component thermal imager in the UK. This is a thermal imager that is fully compatible with television and has the same resolution as ordinary televisions. Regardless of whether it is a serial scanning, parallel scanning or serial-parallel scanning system, thermal imagers require optical mechanical scanning. Therefore, this type of thermal imager is collectively referred to as the first generation of thermal imagers.

3.2 Second generation infrared thermal imaging technology

Recently, thermal imagers that use infrared focal plane array (IRFPA) devices for imaging instead of optical scanning are being vigorously developed. Since optical scanning is eliminated, this sensor that uses large-scale focal plane imaging is called a staring sensor. It is small in size, light in weight, and highly reliable. There can be a detector array of more than hundreds of yuan in the pitch direction, which can obtain a larger angular field of view. A special scanning mechanism can also be used to complete 360-degree scanning at a much slower scanning speed than a general thermal imager to maintain high sensitivity. This type of device mainly includes InSb IRFPA, HgCdTeIRFPA, SBDFPA, uncooled IRFPA, and multi-quantum well IRFPA. This type of thermal imager is called a second-generation thermal imager.

3.3 Third-generation infrared thermal imaging technology

The number of infrared focal plane detectors used in the third generation of infrared thermal imaging technology has reached 320x240 units or higher (i.e. 105-106), and its performance has improved by nearly 3 orders of magnitude. At present, the unit sensitivity of 3μm-5μm focal plane detectors is about 2 to 3 times higher than that of 8μm-14μm detectors. Therefore, the overall performance indicators of medium-wave and long-wave thermal imagers based on 320x240 units are not much different, so 3μm-5μm focal plane detectors are particularly important in the third generation of focal plane thermal imaging technology. In the long run, HgCdTe focal plane detectors with high quantum efficiency, high sensitivity, and covering medium and long waves are still the first choice for the development of focal plane devices.

3.4 Development trend of infrared technology
The development of infrared technology is marked by the development of infrared detectors, and its development trend can be inferred from the development of infrared detectors.

(1) Infrared focal plane devices have developed to high density, fast response, and the number of units has reached 106-10. (1) Large-scale integrated devices with more than 100 elements have developed from two-dimensional to three-dimensional multi-level structures, which can realize high-definition thermal imagers in applications, greatly reducing the size of the whole machine and enhancing its functions.

(2) The development of two-color and multi-color infrared devices enables the whole machine to simultaneously realize multi-spectral imaging detection of different wavelengths, exponentially expanding the amount of system information and becoming an effective means of target identification and optoelectronic confrontation.

(3) The detector realizes the neural network function on the focal plane and performs logical processing according to the program, making the infrared whole machine intelligent.

(4) Increasing the working temperature of the detector. High-performance room-temperature infrared detectors and focal plane devices are one of the development focuses. They do not require a refrigerator, which will make the whole machine more sophisticated and reliable, thus achieving full solid-state.

(5) Improving the yield rate and reducing the price.

4 Future Development of Night Vision Technology
4.1 Comparison between Infrared Thermal Imaging Technology and Low-light Imaging Technology
Due to their different working principles, infrared thermal imaging technology and low-light imaging technology have their own advantages and disadvantages.

(1) Unlike low-light night vision devices, infrared thermal imaging systems do not rely on night light, but rely on the radiation of the target and the background to produce scene images. Therefore, infrared thermal imaging systems can work 24 hours a day.

(2) With the development of computer technology, many infrared thermal imaging systems have complete software systems to realize image processing, image calculation and other functions, and the image quality is greatly improved.

(3) Infrared radiation has a stronger ability to penetrate fog, haze, rain and snow than low-light light radiation, so the infrared thermal imaging system has a longer range.

(4) Infrared thermal imaging can detect hidden thermal targets through camouflage, and can even identify the heat traces left by aircraft and tanks that have just left.

(5) Low-light night vision devices have clear images, small size, light weight, low price, easy use and maintenance, and are not easily detected and interfered by electronic means, so they have a wide range of applications.

(6) Low-light night vision devices have a fast response speed, and high-speed photography can be achieved using photocathode image tubes.

(7) Generally, the low-light imaging surface is a continuous target surface, and the resolution is very high. The highest resolution is currently 90lp/IIHn. It is equivalent to more than 1,600 TV lines.

(8) The spectrum response of low-light night vision has great potential to expand to the short-wave range. The detection and imaging of high-energy ions, x-rays, ultraviolet rays, and blue-green light scenes are basically based on the principles of low-light imaging technology such as external photoelectric conversion, enhancement, processing, and display.

From the perspective of discipline and technological development, infrared technology has certain advantages. The existence of visible light is conditional, and any object is an infrared source and is constantly radiating infrared rays, so the application of infrared technology will be everywhere. At present, in terms of close-range night vision, low-light night vision devices still occupy a dominant position due to their low price and good image quality. As the price of infrared devices decreases, infrared thermal imagers will surely play a big role. In terms of long-range night vision, the role of infrared thermal imagers is more prominent.

4.2 Fusion of low-light images and infrared images
In the period when low-light and infrared technologies are constantly progressing, considering the complementarity of the two, how to fuse low-light images with infrared images to obtain better observation effects without increasing the difficulty of existing technologies has become one of the hot research topics in the current development of night vision technology.

Low-light images have poor contrast, limited grayscale, poor instantaneous dynamic range, flickering at high gain, and are only sensitive to the reflection of the target scene, regardless of the thermal contrast of the target scene. Infrared images have poor contrast and large dynamic range, but they are only sensitive to the radiation of the target scene and are not sensitive to changes in the brightness of the scene. Both have shortcomings. With the development of low-light and infrared imaging technology, it has become an effective technical means to integrate and explore the characteristic information of low-light and infrared images and fuse them into a more comprehensive image. Night vision image fusion can enhance scene understanding and highlight targets, which is conducive to faster and more accurate detection of targets in hidden, camouflaged and confusing military backgrounds. Displaying the fused image in a natural form suitable for human eye observation can significantly improve the recognition performance of the human eye and reduce operator fatigue.