Sunlight passes through the atmosphere, will be affected by the atmosphere of the Sunlight Effect of absorption and scattering, so that through the atmosphere attenuation of solar energy by. But the effect of the absorption and scattering of light from the sun's atmosphere with the solar light Wavelength And change.
Atmospheric window(atmospheric window)
Of a celestial body radiation can penetrate the atmosphere of some Band . Because of absorption and Reflection on the radiation of various particles in the earth's atmosphere, only certain spectral range to reach the ground objects radiation. According to the different scope is divided into The optical window , infrared window and Radio window .
Visible light Wavelength About 3000 ~ 7000. For wavelengths shorter than 3000 Angstroms objects Ultraviolet radiation On the ground, almost not observed, because 2000 ~ 3000 Angstroms ultraviolet radiation by the atmosphere Ozone layer Absorption, can penetrate to the height of about 50 km; 1000 ~ 2000 angstroms far ultraviolet radiation Molecular oxygen Absorption, can only reach a height of about 100 kilometers; and oxygen in the atmosphere Atomic , molecular oxygen, nitrogen atom, nitrogen molecules absorbed radiation wavelengths shorter than 1000 angstroms. 3000 - 7000 angstrom by radiation absorption is very small, mainly due to Atmospheric scattering While weakening.
The molecules of water vapor is the main absorber of infrared radiation. A strong absorption band of water vapor is 0.71 ~ 0.735 μ (micron), 0.81 ~ 0.84, 0.89 ~ 0.99, 1.07 ~ 1.20, 1.3 ~ 1.5, 1.7 ~ 2, 2.4 ~ 3.3, 4.8 ~ 8. In 13.5 ～ 17 μ. Carbon dioxide The absorption band. These absorption pore belt formed between some infrared window. One of the most wide infrared window in 8 ～ 13 μ Office (with 9.5 μ near the absorption of ozone). 17 ~ 22 μ is translucent window. 22 μ until 1 Millimeter wave Because of the serious strengths, moisture absorbing, on ground observers fully opaque. But in the high altitude, air dry place, 24.5 ~ 42 μ radiation transmittance of 30 ~ 60-. At an altitude of 3.5 km height, can be observed in 330 ~ 380, 420 ~ 490, 580 ~ 670 μ (transmittance of about 30-) radiation, can also be observed in the 670 to 780 mu (about 70-) and 800 ～ 910 μ (about 85-) of radiation.
The upper bound of the band varies from 15 to 200 meters, as The ionosphere The density, the observation point location and the solar activity conditions (see radio window of atmosphere).
Common atmospheric window
The effect of the atmosphere on the factors of electromagnetic wave scattering and absorption, so that part Band Solar radiation at The atmosphere The transmittance is small or simply can not get through. In atmosphere transmittance higher band called the atmospheric window of electromagnetic wave radiation. In order to use ground target reflectivity or emissivity. Electromagnetic wave Information on the imaging, remote sensing Objects Characteristics of electromagnetic wave "channel" detection should be chosen in the atmospheric window. At present in the Remote sensing Some of the atmospheric window used for:
1, 0.3 ～ 1.155 μ m, including UV, all Visible light And part of Near infrared Namely, ultraviolet, visible, near infrared band. This band is the best band tomography imaging, but also many common band satellite remote sensor imaging. For example, the Landsat satellite TM band 1 ~ 4; SPOT satellite HRV band etc.. Of which: 0.3 ~ 0.4 μ m, transmittance was about 70-; 0.4~0.7 μ m, transmittance greater than 95%; 0.7~1.1 μ m, transmittance was about 80-.
2, 1.4~1.9 m, Near infrared window , transmittance of 60-~95%, 1.55~1.75 μ m high transmittance. The Band When the sunshine during the day and good imaging used these bands. For example, TM 5, 7b band and is used to detect the plant water content and cloud, snow or used in geology Drafting Etc..
3, 2.0~2.5 m, Near infrared window About 80-, transmittance.
4, 3.5~5.0 m, In the infrared window , transmittance of 60-~70-. The band body Thermal radiation Strong. This interval apart from ground objects Reflected solar radiation Outside, the ground objects also have their own Long wave radiation . For example, the NOVV satellite AVHRR Remote sensing Use the 3.55~3.93 μ M detection The sea surface temperature For day and night, cloud.
5, 8.0~14.0 μ m, the thermal infrared window, transmittance of about 80-. Mainly from the objects' radiation Energy For the night, Imaging Temperature measurement, object detection.
6, 1.0~1.8mm, Microwave window , transmittance was about 35-~40-.
7, 2.0~5.0mm, Microwave window , transmittance was about 50-~70-.
8, 8.0~1000.0mm, Microwave window About 100%, transmittance. Because microwave has the characteristic through the clouds and fog, so has the characteristics of all-weather, all-time. But from the front of the Passive remote sensing The transition to the microwave band Active remote sensing Band.