What bands are commonly used in remote sensing technology in electromagnetic spectrum?

Experiments show that radio waves, microwaves, infrared rays, visible light, ultraviolet rays and gamma rays are all electromagnetic waves, but different wave sources have different wavelengths (or frequencies). The chart (Figure 2-2) in which the wavelengths (or frequencies) of various electromagnetic waves in vacuum are arranged in sequence is called electromagnetic spectrum.

In the electromagnetic spectrum, the longest wavelength is radio wave, which is divided into long wave, medium wave, short wave, ultrashort wave and microwave according to different wavelengths. Followed by infrared, visible light, ultraviolet and x-ray. The shortest wavelength is gamma rays. The whole electromagnetic spectrum forms a complete and continuous spectrum. The different wavelengths (or frequencies) of various electromagnetic waves are due to the different wave sources that generate electromagnetic waves. For example, radio waves are emitted through electromagnetic oscillation, microwaves are excited and emitted through resonant cavities and waveguides, and are emitted into space through microwave antennas; Infrared radiation is caused by molecular vibration and rotational energy level transition; The transition of outer electrons in atoms and molecules produces visible light and near ultraviolet radiation; Ultraviolet, X-ray and γ-ray are produced by electronic transition and state change inside the nucleus. Cosmic rays come from space.

In the electromagnetic spectrum, due to the different wavelengths (or frequencies), the properties of various electromagnetic waves are very different (such as propagation directivity, penetration, visibility, color and so on). For example, visible light, the human eye can directly feel and see objects of various colors; Infrared rays can overcome obstacles at night; Microwave can penetrate clouds, fog, smoke, rain, etc. But they also have something in common:

1. All kinds of electromagnetic waves travel at the same speed in vacuum (or air), which is equal to the speed of light:

C = 3×1010cm/sec.

2. Obey the same laws of reflection, refraction, interference, diffraction and polarization.

At present, the electromagnetic waves used in remote sensing technology are concentrated in the spectral bands from ultraviolet, visible light, infrared to microwave, and the boundaries of each spectral band in different data are slightly different. This book adopts the wavelength range listed in Table 2- 1.

Table 2- 1 Classification name and wavelength range of electromagnetic waves used by remote sensing technology

The main features of spectral bands commonly used in remote sensing are as follows:

The ultraviolet wavelength range is 0.01-0.4 μ m. Solar radiation contains ultraviolet rays. When it passes through the atmosphere, almost all ultraviolet rays with a wavelength less than 0.3μm are absorbed, and only ultraviolet rays with a wavelength of 0.3-0.4 μ m can pass through the atmosphere and reach the ground, with little energy, which can make the silver bromide film sensitive. The application of ultraviolet band in remote sensing is later than other bands. At present, it is mainly used to detect the distribution of carbonate rocks. The reflection of carbonate rocks on ultraviolet rays in the short wave region below 0.4μm is stronger than other types of rocks. In addition, the oil film floating on the water surface reflects stronger ultraviolet rays than the surrounding water surface, so it can be used to monitor oil pollution. However, the detectable height of ultraviolet band from the air is roughly below 2000m, which is not suitable for high-altitude remote sensing.

Visible light in the electromagnetic spectrum, visible light only occupies a very narrow range, the wavelength range is 0.4-0.76 micron ... It is composed of red, orange, yellow, green, cyan, blue and violet light. The human eye can directly feel visible light, not only for the panchromatic light of visible light, but also for the monochromatic light of different bands. Therefore, visible light is the main band to identify the characteristics of substances. In remote sensing technology, optical photography is often used to receive and record the reflection characteristics of visible light by ground objects. Visible light can also be divided into several bands, and the same scene can be photographed simultaneously at the same time to obtain photos of different bands; The reflection characteristics of visible light can also be received and recorded by scanning. Visible light is the most commonly used band in remote sensing.

The infrared wavelength range is 0.76- 1000μ m, which can be divided into near infrared (0.76-3.0μ m), middle infrared (3.0-6.0μ m), far infrared (6.0- 15.0μ m) and far infrared (15) for practical convenience.

Near infrared is similar to visible light in nature, so it is also called optical infrared. Because it mainly reflects the infrared radiation of the sun on the surface, it is also called reflected infrared. In remote sensing technology, photography and scanning are used to receive and record the infrared reflection of solar radiation from ground objects. At present, due to the limitation of the sensitivity of photosensitive materials, only the wavelength range of 0.76- 1.3 micron can be sensed. Near infrared band is also a common band in remote sensing technology.

Mid-infrared, far-infrared and ultra-far-infrared are the causes of thermal sensation, so they are also called thermal infrared. Any object in nature can radiate infrared rays when the temperature is higher than the absolute temperature (-273. 15℃). At room temperature, the wavelength of infrared rays emitted by objects is mostly between 3 and 4 microns, while the ultra-far infrared rays above 15μm are easily absorbed by the atmosphere and water molecules, so the remote sensing technology mainly uses 3- 15 μ m bands, and 3-5 μ m bands and 8- 14 μ m bands are widely used. Infrared remote sensing uses thermal induction to detect the radiation of ground objects (such as thermal pollution, volcanoes, forest fires, etc.). ), so it can be carried out not only during the day, but also at night, and all-day remote sensing can be carried out.

The wavelength range of microwave is1mm-1m. Microwave can be divided into millimeter wave, centimeter wave and decimeter wave, as shown in Table 2- 1. Both microwave radiation and infrared radiation have thermal radiation properties. Because the wavelength of microwave is longer than visible light and infrared light, it can penetrate clouds without being affected by the weather, so it can carry out all-weather remote sensing detection. Microwave remote sensing can adopt active or passive imaging. In addition, microwave has a certain penetration ability to some substances, and can directly penetrate vegetation, ice, snow, soil and other surface coverings. Therefore, microwave is a potential remote sensing band in remote sensing technology.

In different bands of electromagnetic spectrum, the usual wavelength units are also different. The wavelength unit of radio band is kilometers or meters, and the wavelength unit of microwave band is centimeters or millimeters. In infrared band, the common unit is micron (micron), and in visible light and ultraviolet light, the common unit is nanometer (nm) or micron. The conversion of wavelength units is as follows:

1nm = 10-3 μm = 10-7cm = 10-9m

1 μm = 10-3mm = 10-4cm = 10-6m

In addition to wavelength, electromagnetic waves can also be expressed by frequency. For example, the common unit of radio waves is GHz. It is customary to use wavelength to represent short waves (such as gamma rays, x rays, ultraviolet rays, visible light, infrared rays, etc.). ) and frequency to represent long waves (such as radio waves, microwaves, etc.). ).