Optical remote sensing
In optical remote sensing , the sensors can only record the radiation in visible bands, near infrared and short wave infrared bands. Different features radiate differentially which are absorbed by the sensors of optical remote sensing . Sun is the solar source of optical remote sensing . Wavelengths of bands in a optical remote sensing system ranges from 0.30mm to 15.0.
Depending on the number of spectral bands used, optical remote sensing can be classified into following divisions:
• Panchromatic Imaging System: A single channel sensor is used to detect radiation within a broad wavelength range. The range of wavelength and the visible range can become same. If this happens then the imagery will appear as a black and white photograph taken from space. The approximate brightness of the targets will measure the physical quantity. The color of the target is not available. Examples of panchromatic imaging system are SPOT HRV-PAN and IKONOS PAN.
• Multispectral imaging system: The difference between a panchromatic imaging system and multispectral imaging system is that the multispectral imaging system uses a multichannel detector and records radiation within a narrow range of wavelength. Both brightness and color informations are available on the image. Examples are LANDSAT MSS, LANDSAT TM, SPOT HRV-XS and IKONOS MS.
• Superspectral Imaging Systems: It consists of more than ten spectral channels. Band widths are narrow which helps to capture the finer spectral characteristics of the features that are captured by the sensors. Examples of this kind of optical remote sensing system are MODIS and MERIS.
• Hyderspectral Imaging Systems or Imaging Spectrometer: This is a more advanced optical remote sensing technique which records more than hundred spectral bands. Multi spectral bands helps to record information in minute details like agricultural crops, their maturity, moisture level ,coastal management etc. Example of this type of system are Hyperion.
Source of illumination of optical remote sensing :
Sun is the only source of illumination. Radiation from a black body with a temperature of 5900K can simulate the solar irradiation spectrum above the spectrum. Sensors mounted on space and ground can measure the solar irradiation physically.
The wavelength of a optical remote sensing region:
|
Optical Wavelengths |
Wavelength (mm) |
|
Portion Visible (Reflective) |
0.38-0.72 |
|
Near Infrared (Reflective) |
0.72-1.30 |
|
Middle Infrared (Reflective) |
1.30-3.00 |
|
Far Infrared (Thermal, Emissive ) |
7.00-15.0 |
Spectral Reflectance of different land use and land cover in optical remote sensing sensors:
• Water: Clear water has a low reflectance than turbid water. So clear water appears blue whereas turbid water appears yellowish or greenish. Turbid water has a lot of suspended solid matter which has a high reflectance.
• Bare soil: The reflectance from the bare soil depends upon its composition. So the color of bare soil in the imagery is not possible to tell without knowing the composition.
• Vegetation: The reflectance is minimum in blue and red areas of the spectrum because of the absorption by chlorophyll. Green region has the maximum peak.
Thus optical remote sensing is useful to identify features along withtheir characteristics on the earth surface.
