Sources of Energy in Remote Sensing
1. Solar Energy:
❑ The primary source of energy for most remote sensing systems is the Sun.
❑ It emits electromagnetic radiation in various wavelengths, including visible
light, infrared, and ultraviolet.
❑ This energy interacts with objects on Earth's surface, and the reflected or
emitted radiation is captured by sensors on satellites or aircraft.
2. Thermal Energy:
❑ Some remote sensing systems use thermal radiation emitted by objects
based on their temperature.
❑ This is particularly useful for measuring heat emissions, such as in thermal
infrared remote sensing.
❑ Helps monitor temperature changes and detect heat sources like fires.
3. Artificial Sources of Energy:
❑ Sometimes, remote sensing systems use artificial light sources like radar or
lasers.
❑ Radar (Radio Detection and Ranging) uses microwave energy to detect
objects and measure distances.
❑ Lidar (Light Detection and Ranging) uses laser beams to measure distance by
calculating how long it takes for the light to return.
Type of energy flow
1. Reflected Energy
❑ Reflected energy is energy that bounces off a surface after hitting it.
❑ like light or sound)
❑ The angle at which the energy hits the surface is the same as the angle at which
it bounces off (this is called the "angle of incidence" and "angle of reflection").
❑ Examples:
• Light reflecting off a mirror: This is how mirrors work.
• Echo: Sound reflecting off a wall or building creates an echo.
2. Emitted Energy
❑ Emitted energy is energy that is released or given off by an
object.
❑ Emission occurs when an object releases energy, usually in the
form of light or heat.
❑ Heat emission is what happens when an object heats up and
releases energy (like a stove or a heated pan).
❑ Examples:
• The Sun emitting light and heat: The Sun emits light
energy that reaches us and provides warmth.
• A lightbulb emitting light: The energy from the electricity
makes the filament glow, giving off light.
3. Back Scattered Energy
❑ Back scattered energy refers to energy that bounces back
toward the source, or from where it came.
❑ How does it work?
When energy hits an object or surface, some of it can
scatter in many directions. Back scattering is when
the energy goes in the opposite direction and returns
to the original source or near it.
❑ Examples:
• Back scattered light: When sunlight hits a surface like water
or a wall, part of the light bounces back toward where it came
from.
• Radar: When radar waves hit an object (like a plane), some
of the energy bounces back to the radar system.
BLACK BODY
A blackbody is a hypothetical object that absorbs all radiation falling on it and reflects and transmits none. A blackbody is
also a perfect emitter of radiation. It emits radiation across all wavelengths – the phenomenon known as blackbody
radiation. The thermal energy spectrum of a blackbody shows the radiation intensity over a range of wavelengths or
frequencies.
Black Body Radiation
To stay in thermal equilibrium, a black body must emit radiation at the same rate as it absorbs, so it must also be a good
emitter of radiation, emitting electromagnetic waves of as many frequencies as it can absorb, i.e. all the frequencies. The
radiation emitted by the blackbody is known as blackbody radiation.
The characteristics of the blackbody radiation are explained with the help of the following laws:
Wien’s displacement law
Planck’s law
Stefan-Boltzmann law
λmax=b/T
λmax: Wavelength at which the radiation intensity is maximum,
known as peak wavelength
b: A constant called Wien’s constant, whose value is
2.897 x 10-3 m·K (2898)
Stars are often approximated as blackbodies. The temperature of a star can be calculated from its peak
wavelength in the radiation curve. For example, the hottest stars with a surface temperature of 10,000 K will
emit radiation in the ultraviolet range. Relatively cooler stars like Sun, whose surface temperature is 6000 K,
emit radiation in the visible range. Much cooler stars emit radiation in the infrared range. All cold objects,
including humans, emit radiation in the infrared range. Neither ultraviolet nor infrared radiation is visible to
the human eye. Therefore, special telescopes are used to detect and study the radiation emitted by these
stars.
1. Planck’s Law
Planck’s law gives the amount of radiation emitted per unit solid angle in terms of
wavelength or frequency and equilibrium temperature. According to Planck’s law, the
spectral radiance at a given temperature is given by
Where
B: Spectral radiance
λ: Wavelength of radiation
T: Absolute temperature
c: Speed of light
h: Planck’s constant
Wien’s Law or Wien’s Displacement Law
Wien’s Law states that the wavelength at which the emission of a blackbody is maximized is inversely proportional to its
temperature. In other words, as the temperature of an object increases, the wavelength of the peak emission decreases.
Where
λmax: Wavelength at which the radiation
intensity is maximum, known as peak
wavelength
b: A constant called Wien’s constant, whose
value is 2.897 x 10-3 m·K
T: Absolute temperature
Stefan-Boltzmann’s Law
Stefan-Boltzmann’s law states that the total radiant power emitted by a surface across all wavelengths is proportional to
the fourth power of its absolute temperature(0 K (kelvin) or –273.15 °C.). Therefore, the total energy radiated by a
blackbody depends on its temperature.
