Chapter Contents
Electromagnetic energy sources and radiation
Energy Interactions in the Atmosphere
Energy Interactions with the Earth Surface Features
All electromagnetic radiation has fundamental properties and behaves in predictable ways according to the basics of wave theory. In this chapter, we will look at Electromagnetic energy sources and radiation principles, Energy interactions in the atmosphere, and Energy interactions with earth surface features.
Electromagnetic energy sources and radiation
Many Kinds of Energy
In physics, energy means the capacity for doing work. It may exist in potential, kinetic, thermal, electrical, chemical, nuclear, or other various forms. Potential energy is the stored energy that depends upon the relative position of various parts of a system.
The form of energy can be converted from one to another. For example, kinetic energy is converted to electrical energy by the hydroelectric power generator. Even though energies can be converted from one to another, the total energy in a system does not change. This concept, known as the conservation of energy, constitutes one of the basic principles of classical physics. This principle, along with the parallel principle of conservation of matter, holds true only for phenomena involving velocities that are small compared with the velocity of light. At higher velocities close to that of light, as in nuclear reactions, energy and matter are inter-convertible. In modern physics the two concepts, the conservation of energy and of mass, are thus unified.
Remote sensing uses electromagnetic energy to identify features with various devices such as cameras. Electromagnetic energy, more specifically electromagnetic radiation is the energy waves that are produced by the oscillation or acceleration of an electric charge. Electromagnetic waves have both electric and magnetic components. Electromagnetic radiation can be arranged in a spectrum that extends from waves of extremely high frequency and short wavelength to extremely low frequency and long wavelength.
Electromagnetic Energy
Every object of which absolute temperature is above zero degrees Kelvin, radiates electromagnetic energy. Visible light, heat, radio waves, ultraviolet rays, X-rays, and gamma-rays are some examples of electromagnetic energy.
There are two theories on the form of electromagnetic energy. They are the wave theory laid out by a British physicist, James Maxwell in the 1860s, and the quantum theory developed by a German scientist, Max Planck in 1900. Both theories help our understanding of electromagnetic energy, because electromagnetic radiation can sometimes behave like a particle, and sometimes behave like a wave.
Wave Theory
Figure 1. Electromagnetic Wave Theory
In the wave theory, energy is described as electromagnetic waves in which the vibrations move in a direction perpendicular to the direction of the advancing wave front. According to wave theory, electromagnetic radiation consists of an electrical field and a magnetic field. The electrical field varies in magnitude in a direction perpendicular to the direction in which the radiation is traveling. The magnetic field is oriented at right angles to the electrical field. Both these fields travel at the speed of light.
Two characteristics of electromagnetic radiation are particularly important for understanding remote sensing. These are the wavelength and frequency. The wavelength, as shown in Figure 1, is the length of one wave cycle, which can be measured as the distance between successive wave crests. Wavelength is usually represented by the Greek letter Lambda. Frequency is the number of wave cycles per unit time, which is frequently measured in cycles per second. Frequency and wavelength are inversely related to each other, as shown in the equation. The speed of light is the product of frequency and wavelength. The shorter the wavelength, the higher the frequency. The longer the wavelength, the lower the frequency.
Metric Units
Figure 2. Metric Units
In remote sensing, metric units are frequently used. The standard unit of length in the metric system is the meter. The prefixes kilo, mega, giga, and tera represent 10 to the 3rd, 6th, 9th, and 12th power, respectively. In the other direction, the prefixes mili, micro, nano, and pico represents 10 to the negative 3rd, 6th, 9th, and 12th power, respectively. The micro and nano meter units are very frequently used in remote sensing. You need to become familiar with the symbols representing them, i.e. µm and nm.
Wavelengths and Their Names
Figure 3. Spectral Bands
Different sizes of waves can be arranged along the electromagnetic spectrum. The electromagnetic spectrum ranges from the shorter wavelengths to the longer wavelengths. There are several regions of the electromagnetic spectrum that are important to remote sensing.
The ultraviolet or UV portion of the spectrum is just beyond the violet portion of the visible wavelengths. Some Earth surface materials, primarily rocks and minerals, fluoresce or emit visible light when illuminated by UV radiation.
The portion which our eyes can detect is the visible light spectrum. The range of visible wavelengths is about 400 to 700 nanometers. The shortest visible wavelength color is violet, and the longest is red.
The next portion of the spectrum of interest is the infrared region which covers the wavelength range of approximately 700 nanometers to 100 micrometers. The infrared region can be divided into two categories based on radiation properties. One is the near-infrared, also known as the reflected infrared region. The other is the emitted or thermal infrared region. Radiation in the reflected infrared region is used for remote sensing purposes in ways very similar to radiation in the visible portion. The reflected infrared covers wavelengths from approximately 700 nanometers to 3 micrometers. The thermal infrared region is quite different than the reflected infrared portions, as this energy is essentially the radiation that is emitted from an object's surface in the form of heat. The thermal infrared covers wavelengths from approximately 3 to 100 micrometers.
The next portion of the spectrum is the microwave region that ranges from about 1 millimeter to 1 meter. Because microwaves can penetrate clouds and smoke, microwaves are important for disaster monitoring and military applications.
A specific range of the spectrum is called a band. For example, the blue band ranges from approximately 400 nanometers to 500 nanometers.
Quantum Theory
Figure 4. Quantum theory
In 1900, Planck discovered that energy is radiated in small, discrete units, which he called quanta. Developing his quantum theory further, he discovered a universal constant of nature, which came to be known as Planck's constant. Planck’s constant is 6.626E-34 J sec. Planck's law states that the energy of each quantum is equal to the frequency of the radiation multiplied by the universal constant. Planck's discoveries became the basis of an entirely new field of physics, known as quantum mechanics, and provided a foundation for research in such fields as atomic energy. Planck’s theory cast a significant insight on remote sensing too. Because there is less energy as wavelength increases, it is more difficult to use the longer wavelength energies for remote sensing.
Object Temperature and Energy Radiation
Every object of which absolute temperature is above zero degrees Kelvin radiates electromagnetic energy. The amount of radiated energy is difficult to measure accurately because it is mixed with reflected and transmitted energies. An imaginary object that does not reflect or transmit energy is called, a blackbody. A blackbody absorbs all energy that falls upon it and radiates all of that energy.
Figure 5. Stefan-Boltzmann Law
According to Stefan-Boltzmann, the amount of blackbody radiation is a function of temperature. Total radiation is equal to the Stefan-Boltzmann Constant, i.e. 5.6697E-8 W m-2, multiplied by the temperature to the 4th power. This function is called the Stefan-Boltzmann Law. The temperature is measured in degrees Kelvin. To convert a Celsius degree to Kelvin, add 273.15. The Stefan-Boltzmann Law tells us that radiation increases exponentially as temperature increases.
Wavelength of Maximum Intensity Radiation
The energy radiated by an object usually covers a wide range of wavelengths. Typically, certain wavelength energy predominates over other wavelengths. Wien’s displacement law explains which wavelength energy dominates. According to the law, the wavelength of the maximum intensity radiation is a function of the temperature of the object. The wavelength can be calculated with the Wien’s displacement constant (2,897,768 nanometers) divided by an object’s temperature in degree Kelvin.
Figure 6. The Wien’s displacement law
For example, the Sun’s temperature is about 6000K; therefore, the wavelength of maximum intensity radiation is about 483 nanometers which is within the blue band. Our eyes are sensitive to this range of wavelength. In the case of the Earth, its temperature is about 300K; therefore, the wavelength of maximum intensity radiation is about 9.7 micrometers which is beyond the wavelength range that our eyes can sense.
Energy Interactions in the Atmosphere
Atmosphere
Particles and gases in the atmosphere can affect the travel of light and radiation. The atmosphere consists of gases, called “air”. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide and smaller amounts of various other gases as well as varying amounts of water vapor. There are five distinct layers in the atmosphere:
1. Troposphere:
· 0 – 10 km above sea level.
· Contains most of the air and cloud.
· Most weather events occur.
· Temperature decreases as altitude increases.
2. Stratosphere:
· 10 – 50 km above sea level.
· Temperature increases as altitude increases.
· Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence that is common in the troposphere.
· The ozone layer exists between 20-30 km altitude.
3. Mesosphere:
· 50 – 85 km above sea level.
· Temperature decreases as altitude increases.
· Very cold. The average temperature is around -120°F.
· Most meteors burn up in the mesosphere.
· The highest elevation at which a cloud can form.
4. Thermosphere:
· 85 – 600 km above sea level.
· Temperature increases as altitude increases. Temperature may reach to 4500 °F.
· Auroras occur.
· Absorbs UV and X-ray.
5. Exosphere:
· 600 – 10,000 km above sea level.
· Thin air
· Many satellites fly in this zone because of little friction. Ex. Earth-observing satellites.
Atmosphere and Energy Travel
All radiation detected by remote sensors that observe the Earth must first pass through some distance of atmosphere. The atmosphere transmits, absorbs, and scatters the electromagnetic energy. Energy that passes through the atmosphere with little loss of intensity is considered to be transmitted. Atmospheric absorption and scattering, however, reduce the energy transmitted from the target through the atmosphere that becomes available for detection by remote sensing. Atmospheric absorption is the loss of energy to constituents of the atmosphere. Atmospheric scattering is the random redirection of electromagnetic energy by particles suspended in the atmosphere or by large molecules of atmospheric gases.
The amount of scattering and absorption that occur depends upon the size and abundance of atmospheric particles, the wavelength of the radiation, and the distance the energy must travel through the atmosphere to reach the sensor.
Figure 7. Travel of energy through the atmosphere.
Scattering
Scattering means the diffusion of radiation by particles in the atmosphere. There are two major types of scattering. One is Rayleigh scattering and the other is Mie scattering.
Rayleigh scattering is caused by tiny particles such as air molecules and gases. Particularly, it happens when particles are smaller than the wavelength. Rayleigh scattering causes the daytime sky to be blue instead of black because the shorter blue wavelengths of sunlight are scattered more than the longer green and red wavelengths. Pictures taken from the moon show a black sky because there is no atmosphere to scatter the incoming solar radiation. During sunrise and sunset on the Earth, the sun's rays travel a longer path through the atmosphere to reach an observer on the ground. The longer orange and red wavelengths are scattered significantly less than other visible wavelengths, causing the sunset sky to appear orange and red, especially when the concentration of atmospheric particles is high. Rayleigh scattering also brings bluish-gray haze in high altitude aerial images.
When the particle diameter is about the same or larger than the wavelength, all wavelength energies are scattered, producing white-to-gray tones. We call this kind of scattering Mie scattering. The particles that cause Mie scattering include moisture, pollen, smoke, and dust. Mie scattering explains the reason why cloud and smoke colors are white to gray instead of being blueish or reddish.
Atmospheric Absorption
Another important role of the atmosphere is to absorb electromagnetic energy. Just as our body absorbs heat from a fireplace, the atmosphere absorbs certain bands of solar energy. Major absorbers are water vapor, carbon dioxide, and ozone. The amount of energy absorption is dependent on the wavelength. For example, ozone absorbs the wavelength energy around 10 micrometers, but it does not when wavelengths are around 10 centimeters or longer. Remember that according to Wien’s Law, the 10-micrometer region is close to the peak radiance emitted by the Earth. These wavelengths are trapped by the ozone layer, contributing to the greenhouse effect.
A wavelength range that is not absorbed by the atmosphere is called atmospheric window. Within the atmospheric windows, electromagnetic energy passes through so that the transmittance level is high. As shown in Figure 8, there are many atmospheric windows before and after the 20 through 600-micrometer range. However, all electromagnetic energies within that range are blocked by water. It is very important to consider the atmospheric window during sensor design, because, for example, it is nonsense to design a space-borne sensor that detects ground objects with the electromagnetic energy of around 100 micrometers.
Figure 8. Atmospheric window
Energy Interactions with the Earth Surface Features
Radiation that is not absorbed or scattered in the atmosphere can reach and interact with the Earth's surface. There are three forms of interaction that can take place when energy is incident upon the surface. These are absorption, transmission, and reflection. The total incident energy will interact with the surface in one or more of these three ways. The proportions of each will depend on the wavelength of the energy and the material and condition of the feature. It is important to understand how features reflect the incident energy because many remote sensing sensors detect reflected energy.
Figure 9. Energy interactions with an object
Reflection Patterns
When an object receives an incident energy, the reflection of the energy, i.e. back-scattering, can be either specular, diffusive, or in-between, as shown in Figure 10. When a surface is smooth we get a specular or mirror-like reflection in which almost all of the energy is directed away from the surface in a single direction. Diffusive reflection occurs when the surface is rough and the energy is reflected almost uniformly in all directions. Most Earth's surface features lie somewhere between perfectly specular or perfectly diffusive. Whether a particular target reflects specularly or diffusely, or somewhere in between, depends on the surface roughness of the feature in comparison to the wavelength of the incoming radiation. If the wavelengths are much smaller than the surface variations or the particle sizes that make up the surface, the diffuse reflection will dominate. For example, fine-grained sand would appear fairly smooth to long-wavelength microwaves but would appear quite rough to the visible wavelengths. Figure 11 shows oil spills in dark tones because of the specular reflection on smooth oil surfaces. Some airplanes apply both specular and diffusive reflections to achieve the maximum stealth from radar detectors.
Figure 10. Reflection, a.k.a. back-scattering, patterns.
Figure 11. Oil spills appear in dark tones because of the specular back-scattering of radar energy, which makes a sensor receive no back-scattered energy.
Objects and Their Reflectance Characteristics
For any given material, the amount of reflectance varies with wavelength. When the reflectance coming from the material is plotted over a range of wavelengths, the connected points produce a curve called the material's spectral response curve, a.k.a. spectral signature. In the spectral response curve, the X-axis represents wavelengths and the Y-axis represents the amount of reflectance either in percentage or ratio to the total incident energy amount.
Because different objects reflect incident energy differently, spectral response curves are different from one object to another. As shown in Figure 12, the spectral response of a pine tree is significantly different from the response of dry grass. Analyzing the spectral responses allows us to identify objects, which is one of the primary operations in remote sensing. For example, pine trees reflect about 70% (= 0.7) at about 0.9 µm, while dry grasses reflect about 34% (= 0.34) at the same wavelength.
Figure 12. Spectral response curve. (R. N. Clark, et al., 2003)
Factors Affecting Objects’ Reflectance
It is also very important to know that the actual reflectance patterns in the real world are different from the patterns that are acquired from a laboratory environment. Various factors cause the difference.
One is the spatial effects, which means that spectral response curves are affected by the spatial location in which the object is located. For example, pine trees in the north-facing slope will reflect differently from the ones in the south-facing slope.
Another is the temporal effects which mean that age, time and season affect the spectral response curves. For example, a young pine tree’s reflectance will be different from an old pine tree’s reflectance.
Another is the atmospheric effects. As discussed before, the atmosphere attenuates the incident energy and also acts as a reflector itself, resulting in distortion of the spectral response curve. Particularly, high altitude imaging is more susceptible to atmosphere effects.
References
R. N. Clark, G. A. Swayze, R. Wise, K. E. Livo, T. M. Hoefen, R. F. Kokaly, and S. J. Sutley, 2003. USGS Digital Spectral Library splib05a, U.S. Geological Survey, Open File Report 03-395. https://pubs.usgs.gov/of/2003/ofr-03-395/datatable.html