chapter 3: Radiative transfer Flashcards
Define remote sensing
- The science of acquiring information about the earth’s surface without actually being in contact with it
How is remote sensing done?
- Sensing and recording reflected and emitted energy and
- Processing
- Analyzing
- Applying that information
Information received from remote sensing comes in the form of
- Electromagnetic radiation
Electromagnetic radiation consist of:
- Alternating
- Electric field and
- Magnetic field
- The electric field vector is perpendicular to the magnetic field vector
- The direction of propagation is perpendicular to both
Radiation is often specified by:
- Its wavelength
- It is the distance between crests of the electric or magnetic field
Alternate way to describe radiation
- Give its frequency
- It is the rate at which the electric or magnetic field oscillates when observed at a point
What is the unit of frequency?
- Hz or
- One cycle per second
Give the equation of finding the frequency
- V=c/lamda
- C is the speed with which electromagnetic radiation travels and is known as the speed of light
- In vacume c is 299,792 km s-1
- In the atmosphere c is slower 299,703 km s-1 due to interaction with air molecules
Radiation is specified by
- Wavenumber k
- The recoprical of the wavelength
- Inversely proportional to wavelength
- Directly proportional to frequency
Property of electromagnetic radiation:
- Can transport energy
- Many of the units used to quantify EM radiation are based on energy
Unit of radiant energy
- Joule
Radiant flux
- Radiant energy per unit time
Radiant flux unit:
- Watts
- Joules per second
Radiant flux depend on
- Area
- It is usually normalized by surface area
Define radiant flux density
- Radiant flux crossing a unit area
Radiant flux density unit:
- Watts per square meter
Why is radiant flux density subdivided?
- To indicate which way the energy is traveling
Radiant flux density is subdivided to
- Irradiance (E)
- Radiant flux density incident on an area
- Radiant exitance
- Radiant flux density emerging from an area
In nature radiation is a function of
- Direction
- The direction dependence is taken into account by employing the solid angle
Solid angle:
- If one draws lines from the center of the unit sphere to every point on the surface of an object, the area of the projection on the unit sphere is the solid angle
Unit of solid angle:
- Steradians (sr)
Equation of solid angle:
- Ω = A/r2
- Object with cross sectional area
- A << r2
- R is the radius of the sphere
In a sphere of one foot radius a steradian would correspond to a solid angle that
- Subtended an area of one square foot on the surface of the sphere. Since the total area of a sphere is 2pi r2 there are 4 pi steradians in a sphere
Define radiance
- Radiant flux density per unit solid angle leaving (or incident on) the surface perpendicular to the beam
