In optical remote sensing the source of energy is the Sun. The sensors measure the electromagnetic radiation reflected from the target in the following bands:
- Portion Visible (Reflective)
- Near Infrared (Reflective)
- Middle Infrared (Reflective)
- Far Infrared (Thermal, Emissive )
Optical systems may be distinguished according to the spectral sampling (number of spectral bands used in the process):
- Panchromatic: measured in one spectral band
- Multispectral: measured in 2-10 spectral bands
- Hyperspectral: measured in 10-200 typically contiguous spectral bands
The narrower the bands i.e. a smaller wavelength range, the more precise will be the survey. But in this way the instrument will collect less specific energy, increasing at the same time the "noise" associated with the information.
In the resultant image we are able to identify objects by their spectral signature, i.e. their response to the different wavelengths.
The solar radiation spectrum is an excellent approximation of a blackbody spectrum.
A blackbody is a theoretical object that absorbs all radiation incident upon it and radiates it as a continuous spectrum. A blackbody is at constant temperature and its spectrum is determined by the temperature alone. By temperature we mean the surface temperature. Hotter objects emit more total energy at all the wavelengths.
We have seen that the electromagnetic radiation travels from the Sun to the Earth (target) and then to the sensors trough the atmosphere. The atmospheric constituents cause absorption and scattering of radiation.
When you are comparing emission and absorption spectra you refer to the spectral lines generated by a transparent gas in the foreground of a blackbody. If the transparent gas is colder than the blackbody the lines will appear dark, i.e. absorption lines. If the transparent gas is warmer the lines will appear in emission.
Absorption reduces the solar radiation within the absorption bands of the gases. In remote sensing we want to minimize this effect, considering wavelength regions outside the main absorption bands of the gases in the atmosphere.
Applications of optical remote sensing include meteorology, oceanography, global vegetation monitoring, atmospheric composition, land cover and changes maps, disaster monitoring, cartography and hydrology. We will go soon trough practical examples.