GEOG 474
Energy Sources and Radiometric Principles
Introduction
4 basic components of a remote sensing system
- energy source
- transmission path
- target
- sensor
Medium for transmitting information from target to sensor - electromagnetic energy
Discuss
- basic definition of electromagnetic energy
- sources of electromagnetic energy
- models and characteristics of electromagnetic waves
Electromagnetic Radiation (emr)
Electromagnetic energy (radiation) is one of many forms of energy (such as chemical, electrical, kinetic, magnetic, nuclear, or thermal).
EMR is the source of signals collected by most remote sensing instruments
The source of this energy varies depending on the sensor characteristics
Most systems rely on the sun to generate all the EM energy needed to image earth's atmosphere and land surfaces. These systems are called passive sensors. Other sensors generate their own energy, called active sensors, transmits energy in a certain direction and records the portion reflected back by features within the signal path.
Electromagnetic energy can be generated by changes in the energy levels of electrons, acceleration of electrical charges, decay of radioactive substances, and the thermal motion of atoms and molecules. Nuclear reactions within the sun produce a full spectrum of EM radiation which is transmitted through space without major changes in its character until it reaches the atmosphere.
Emr consists of an electrical and magnetic field that varies in magnitude in a direction perpendicular to the direction of propagation.
Electromagnetic Spectrum (ems)
EMS represents the continuum of electromagnetic energy from extremely short wavelengths (cosmic and gamma rays) to extremely long wavelengths (microwaves). Spectrum is arbitrarily segmented into major divisions. There are no natural breaks in the ems. These separations are made by us for our convenience.
UV - 3 nanometers - .4 micrometers
Causes fluorescence and is good in some geological and vegetation applications.
Big sagebrush (artemisia spp.) fluorecess under ultra-violet light. Some flowers also fluoress under UV light allowing insects to locate nectar reservoirs.
Not much is done with UV for remote sensing since these shorter wavelengths are easily scattered by the atmosphere making spaceborne and some airborne sensors impractical.
VISIBLE - small portion of the EMS that humans are sensitive to
BLUE (.4-.5 micrometers)
GREEN (.5-.6 micrometers)
RED (.6-.73 micrometers)
INFRARED SPECTRUM - .72 - 15 micrometers
- There are three logical zones in the IR spectrum:
NEAR INFRARED - reflected, can be recorded on film emulsions. (0.7 - 1.3 micrometers)
MID INFRARED - reflected, can be detected using electro-optical sensors. (1.3 - 3.0 micrometers)
THERMAL INFRARED - emitted, can only be detected using electro-optical sensors. (3.0 - 5.0 and 8 - 14 micrometers)
MICROWAVE - Radar sensors, wavelengths range from 1mm to 1m
Units of Wavelength
The basic unit in which wavelengths are measured in the meter (m). In remote sensing, most energy in the visible and infrared portions of the electromagnetic spectrum is measured in micrometers (10-6 m). However, some wavelengths (such as radio and microwaves) are too long for the micrometer to be a convenient unit of measure. For example, while the wavelength of blue light is approximately 0.4-0.5 micrometers, a radio wave is in the neighborhood of 100,000,000 micrometers long (100 m)! You should be aware that visible wavelengths (including ultraviolet, visible, and near infrared) are frequently referred to in units other than the micrometer. Astronomers use a unit called angstrom (10-10 m) to measure these wavelengths. One micrometer equals 10,000 angstroms. Occasionally you may run across this unit when reading satellite documentation from NASA, although most of the information they have for remote sensing audiences uses micrometers. Also, some of the older literature in remote sensing refers to micrometers as microns, and many of the biological sciences still use "micron". One micron equals one micrometer.
Basic Principles of Electromagnetic Energy
Modern physics view EMR as having dual nature, enabling it to be independently described as a wave or a particle.
Wave Model (basic wave theory - Maxwell's equations)
Shows EMR carried by a series of continuous waves that are equally and repetitively spaced in time (harmonic waves)
Wave pattern is in the form of 2 fluctuating fields - one electric and the other magnetic. Each has a sinusoidal shape because their plots resemble sine curves.
Paired fields are perpendicular to each other, and both are perpendicular to direction of wave propagation (transverse waves)
Wave nature of EMR is characterized by:
- wavelength
- frequency
Wavelength (lambda) - linear distance between 2 successive wave crests or troughs
Frequency (v or f) - # of wave crests or troughs (cycles) that pass a fixed point per second
Wavelength and frequency are related to the velocity of an electromagnetic wave (speed of light) -
speed of light (c) = frequency (f) x wavelength (lambda) (1)
- frequency and wavelength are directly proportional to velocity which is essentially a constant
- electromagnetic energy travels at the speed of light 2.99983x108 (3x108 ) ms-1 (186,000 miles s-1)
- wavelength and frequency have an inverse relationship
Particle Model
Emphasizes behavior of EMR as if EMR were composed of a collection of discrete, particle-like objects called quanta or photons, in which electromagnetic energy is transferred at the speed of light.
Energy of a quantum is given as:
Q = h f = (h c) / lambda (2)
Q - energy of quantum [Joules - J]
h - Plank's constant [6.26x10-34 J s]
- direct relationship between frequency and energy (energy of a photon varies directly with frequency)
- inverse relationship between wavelength and energy (energy of a photon varies inversely with wavelength)
Relate wave model and quantum model of emr (Equation 1 and 2)
2. substitutingintoQ=hf / yielding
This equation shows that the shorter the wavelength, the higher the energy.
For this reason, shorter wavelengths are easier to sense than very long ones such as passive terrestrial microwave emissions
Summary
Remote sensing is concerned with the measurement of EMR returned by the Earth's natural and man-made features that first receive energy from the sun or an artificial source such as a radar transmitter.
Different objects return different types and amounts of EMR.
Objective of remote sensing is to detect these differences with the appropriate instruments.
Differences make it possible to identify and assess a broad range of surface features and their conditions
Energy Sources
Electromagnetic waves are radiated through space from some source.
When the energy encounters an object, even a very tiny one like a molecule of air, one of three reactions occurs.
- reflected off the object,
- absorbed by the object, or
- transmitted through the object.
Total amount of radiation that strikes an object is incident radiation -
reflected radiation + absorbed radiation + transmitted radiation (3)
In remote sensing, we are largely concerned with REFLECTED RADIATION.Reflected radiation causes our eyes to see colors, causes infrared film to record vegetation, and allows radar images of the earth to be created. Source of a vast majority of this reflected radiation is the sun.
While the sun is the most obvious source of the electromagnetic energy measured in remote sensing, it is not the only energy source one might encounter. This is because all matter at temperatures greater than absolute zero (0 Kelvin) continuously emits electromagnetic radiation. Generally, the hotter an object is, the more it radiates, but all objects with even the slightest sub-molecular motion radiate some energy.
Remote Sensing uses electromagnetic energy from both natural and man-made sources.
Blackbody Model
Blackbody
- is a perfect absorber and emitter of radiation
- all radiation incident on a blackbody object is re-emitted
- emittance is a function only of temperature.
In nature, true blackbodies do not exist. However, many objects approximate blackbodies.
Blackbody curves in figure
- show the amount of energy radiated at each wavelength for blackbodies of various temperatures
- red line is the blackbody curve for the earth, whose ambient temperature (the "average" temperature given off by the soil, water, vegetation, and built environment) is about 300 Kelvin
- area under this curve is the total energy emitted across all wavelengths by the earth. (Stefan-Boltzmann Law).
There is a direct relationship between the temperature of a blackbody and the amount of electromagnetic energy it emits. The hotter the object, the more energy it gives off.
Even though a perfect blackbody is only a theoretical construct, most objects in nature behave like "imperfect" blackbodies. Consider the theoretical curve created by a perfect blackbody and the actual curve created by the Sun.
Blackbody curves for hotter temperatures "peak" at lower wavelengths than do the curves for cooler temperature objects (Wien's Displacement Law)
- wavelength at which the largest portion of energy is emitted depends on temperature.
Hotter objects emit more energy at lower wavelengths than do cooler objects
- 6000 K curve (Sun) peaks at 0.5 micrometers
- 300 K curve (Earth) peaks at about 9 micrometers
Radiation Laws
Plank's Radiation Law for Blackbodies gives the spectral radiance of an object as a function of its temperature.
Wien's Displacement law
If we differentiate Plank's Radiation Law for blackbodies and set it equal to zero, we arrive at a formula which gives the wavelength of maximum radiance for a blackbody of a given temperature. This formula is referred to as Wien's Displacement Law.
Finally, if a blackbody is acting as a perfect emitter, the total emitted energy over the whole spectrum is given by the
Stefan-Boltzmann law:
Source materials:Remote Sensing Core Curriculum and Utah State Geography Department Remote Sensing Lecture Materials
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Last revised on September 22, 1999 by Tracy DeLiberty.