With the advent of balloons, planes and satellites, humans not only flew into the blue sky, but also flew higher and higher. At present, commonly used technical means include optical imaging, infrared imaging, and radar imaging.

With the advent of balloons, planes and satellite , humans not only flew into the blue sky, but also flew higher and higher. A question arises: What can the sky bring to mankind?

Life experience tells us that you can see far and wide by climbing high. However, there are two problems with the naked eye: one is that the observation results are inconvenient to share, and the other is that the observation distance is limited. So there is a new demand: use technical means to overlook the earth. At present, commonly used technical means include optical imaging, infrared imaging, , and radar imaging. These methods have their own advantages and can not only be used in the military field, but also in civilian fields such as environmental monitoring, geodetic mapping, and geological exploration.

1. Different functions

Satellite optical photos of the deforestation case in Dunhuang

The earliest appearance was optical imaging, which is commonly known as aerial photography. Optical imaging, similar to the human eye, is essentially visible light imaging. Objectively record the ground scenery or battlefield scenes that can be seen by human eyes and restore them realistically for people to analyze and judge. Here we emphasize the ground scenery or scene. With the development of optical imaging technology, balloons and planes are slow to talk about. Even in space hundreds of kilometers away from the ground, satellites can take clear ground photos. The above picture is two satellite photos from the "Dunhuang Deforestation Case" exposed in 2021. Through comparison, it can be seen that a large area of ​​protective forest was illegally cut down and turned into crop arable land.

sniper lurking in the grass

infrared imaging photos of soldiers

optical imaging is powerless for soldiers lurking in the grass, weapons and equipment hidden in the woods, and military operations covered by night. This leaves room for development for infrared imaging. Infrared imaging, also known as thermal imaging, uses infrared sensor to sense the infrared radiation characteristics of the observed object to form an infrared image for people to analyze and judge. Here we target hidden targets on the ground. The temperatures of personnel and weapons and equipment in operation are higher than the ambient temperature. The higher the temperature, the more obvious the infrared radiation characteristics, so that the target can be extracted from the background. This is what makes infrared imaging unique. On March 2, 2021, the Russian Satellite Communications reported a news report that during a combat operation in Syria, US snipers hid among piles of rocks and avoided the search by the Russian patrol, but did not escape the Russian thermal imager.

Optical imaging photos of Xinpu Shipyard

Radar image photos of Xinpu Shipyard

What is the magical use of radar imaging? Above is a set of photos of North Korea's Shinpo Shipyard circulating on the Internet. The picture above is an optical photo of the dock blocked by canopy . The picture below is the situation in the dock seen through the canopy by radar imaging, including the submarine under construction, barge , and the bracket supporting the canopy. Radar imaging is the target image formed by using synthesis of aperture radar based on radar echo. Radar electromagnetic wave has strong penetration and can penetrate 30 meters for anhydrous soil. It is suitable for searching and monitoring of shading targets or underground hidden targets.

2. The essence is different and the same destination

At present, commonly used imaging technologies include optical imaging, infrared imaging and radar imaging. With the development of related technologies in the future, some new imaging technology methods will emerge. Putting aside the specific technical details of these methods, the common problem of physical imaging is the visualization of information and the digitization of images.

electromagnetic spectrum diagram

information visualization is a figurative and intuitive concrete expression of abstract data, such as graphics, tables, maps, etc., to help people better perceive and understand abstract data. In the electromagnetic spectrum, visible light only accounts for a small part. infrared and radar microwaves are invisible to humans, and there is no image at all. However, in order to enable people to perceive their existence and feel the difference between them, with the help of the concept of "image", different grayscales or different colors are used to distinguish different signals, and artificially convert the target infrared radiation distribution or radar echo distribution into grayscale images or color images visible to humans.

infrared imaging photos of fighter takeoff

Xi'an South Gate Radar Imaging Photos of haze

Image digitization is the process of converting continuously distributed simulated images into digital images that a computer can process. It is generally divided into three steps: sampling, quantization, and encoding.

image pixels and resolution

sampling, the core is to use finite pixel points to describe an image. Taking two-dimensional continuous images as an example, they are divided into rectangular mesh structures at equal intervals in horizontal and vertical directions, and each square is called a pixel point. Sampling is to create a collection of all pixel points of an image. The sampling quality can be measured by the number of pixels per unit length of the image, which is called image resolution, and the unit is DPI, the abbreviation of Dots Per Inch in English, that is, the number of pixels per inch of the image.

pixel color value example

quantization refers to using a certain range of numerical values ​​to represent each pixel point after image sampling, including grayscale value or color value. For grayscale graphs, the grayscale range is generally from 0 to 255, white is 255, and black is 0. For color charts, in RGB (three primary colors) mode, three sets of values ​​from 0 to 255 are used to represent the color value of a certain color. For example, the color value of red is 255, 0, 0; green is 0, 255, 0; blue is 0, 0, 255. The three-color combination can form 16 million colors. The quantization results are reflected as the total number of colors contained in the image. The more colors the better the sampling quality, but the larger the amount of data processed in the image.

compression encoding is to solve the problem of image data volume. Obviously, the data volume of images obtained after digitization is huge, and the amount of information must be compressed by encoding technology. The specific content will not be expanded anymore.

3. Pursuing endless

If you use four words to describe the current development status and trends in the field of physical imaging, the rise may be the best choice. It is highlighted in three aspects:

, one is the development new field. Focusing on the electromagnetic spectrum, there is a relatively special and undeveloped terahertz frequency band between microwave and infrared (0.1~10THz, 1THz=1´1012Hz). With the development of the terahertz band, terahertz imaging has become a new field of physical imaging. The terahertz frequency band coincides with millimeter wave and infrared light at both the low and high frequencies, covering the characteristic spectrum of semiconductors, plasma , organisms and biological macromolecular matter. It is the transition zone between macro classical theory and micro quantum theory , and it is also the transition zone between electronics and photonics . It is called the "terahertz gap" of the electromagnetic spectrum. terahertz imaging has the characteristics of strong penetration, high degree of automation, non-contact, and non-destructiveness. It is expected to make new breakthroughs in internal information acquisition, hidden object identification, biomedical imaging, non-destructive material testing, cultural relics geological exploration, anti-terrorism security inspection and target radar imaging.

The second is to promote new hot spots. Physical imaging technology originated in the military field and is mainly used in the military field. With the maturity of related imaging technologies, the spread of imaging technology into the civil field has been promoted, and more, better, more practical and more convenient civil products have been developed, and leveraging the civil market with strong demand, which has become a new hot spot in the development of imaging technology at home and abroad. Among them, infrared imaging focuses on non-refrigeration products, mainly focusing on security, assisted driving, police fire protection, industrial monitoring, power testing, medical quarantine, personal consumption, etc. The application scenarios of radar imaging are more macro, mainly aimed at government departments and related industries, such as geodesic mapping, crop census, soil moisture analysis, geological disaster monitoring, , underground/underwater archaeological survey, urban construction planning, etc.

The third is to seek new breakthroughs. The life cycle curve is also applicable to technology, products and markets. As technology, products and markets mature, it will inevitably require products to be miniaturized, serialized, and scaled, so as to continuously improve functions, performance and efficiency, and gradually reduce prices. More, better and more practical products will continue to meet the diversified needs of the market, and innovation and breakthroughs will never end. The same is true in the field of physical imaging, especially in the civilian field.