2.3 Radiation Laws
At the reflective spectral region, we are more concerned about the reflective properties of an object. But in the thermal spectral region, we have to rely on the emittance of an object. This is because most matters at the conventional temperature (temperature of our environment) emit energy that can be measured. Therefore, we introduce some basics of the radiation theory.
The first theory treats electromagnetic radiation as many discrete particles called photons or quanta (terms in Physics). The energy of a quantum is given by
E = hv
E energy of a quantum (Joules)
h = 6.626 x 10-34 (Planck's constant)
Energy (or radiation) of a quantum is inversely proportional to the wavelength. The longer the wavelength of a quantum, the smaller is its energy. (The shorter the wavelength, the stronger is its energy.) Thus, the energy of a very short wavelength (UV and shorter) is dangerous to human health. If we want to sense emittance from objects at longer wavelength, we will have to either use very sensitive devices or use less sensitive device to view a larger area to get sufficient amount of energy.
This has implications to remote sensing sensor design. To use the available sensing technology at hand, we will have to balance between wavelength and spatial resolution. If we wish to make our sensor to have higher spatial resolution, we may have to use short wavelength regions.
The second radiation theory is Stefan-Boltzmann Law:
M: total radiant existence for a surface of a material watts/m2
: Stefan-Boltzmann constant, 5.6697 x 10-8 Wm-2 °K-4
T: absolute temperature, K
This means that any material with a temperature greater than 0 K will emit energy. The total energy emitted from a surface is proportional to T4 .
This law is expressed for an energy source that behaves as a blackbody - a hypothetical, ideal radiator that absorbs and re-emits all energy incident upon it. Actual matters are not perfect blackbody. For any matter, we can measure its emitting energy (M), and compare it with the energy emitted from a blackbody at the same temperature (Mb) by:
"" is the emissivity of the matter. A perfect reflector will have nothing to emit. Therefore, its e will be "0". A true blackbody has an of 1. Most other matters fall in between these two extremes.
The third theory is Wien's displacement law which specifies the relationship between the peak wavelength of emittance and the temperature of a matter.
max = 2897.8/T
As the temperature of a blackbody gets higher, the wavelength at which the blackbody emits its maximum energy becomes shorter.
Figure 2.2. Blackbody radiation.
Figure 2.2 shows blackbody radiation curves for temperature levels of the Sun, a candescent lamp and the Earth. During the day time we can see the energy from the sun is overwhelming. During the night, however, we can use the spectral region between 3 µm and 16 µm to observe the emittance properties of the earth surface.
At wavelengths longer than the thermal infrared region, i.e. at the microwave region, the energy (radiation) level is very low. Therefore, we often use human-made energy source to illuminate the target (such as Radar) and to collect the backscatter from the target. A remote sensing system relying on human-made energy source is called an "active" remote sensing system. Remote sensing relying on energy sources which is not human-made is called "passive" remote sensing.