Wave Model of Electromagnetic Energy

1. Wave Model of Electromagnetic Energy An electromagnetic wave is composed of electric and magnetic vectors that are orthogonal to one another and travel from the source at the speed of light (3 x 108 m s-1). Jensen, 2000

2. Spectral Bandwidths of Landsat and SPOT Sensor Systems Jensen, 2000

3. Blackbody Radiation Curves Blackbody radiation curves for several objects including the Sun and the Earth which approximate 6,000 ˚K and 300 ˚K blackbodies, respectively. Notice that as the temperature of the object increases, its dominant wavelength shifts toward the short wavelength portion of the spectrum. Jensen, 2000

4. Radiant Intensity of the Sun The Sun approximates a 6,000 K blackbody with a dominant wavelength of 0.5 m (green light). Earth approximates a 300 K blackbody with a dominant wavelength of 9.7 m . The 6,000 K Sun produces 41% of its energy in the visible region from 0.4 - 0.7 m (blue, green, and red light). The other 59% of the energy is in wavelengths shorter than blue light (<0.4 m) and longer than red light (>0.7 m). Eyes are only sensitive to light from the 0.4 to 0.7 m. Remote sensor detectors can be made sensitive to energy in the non-visible regions of the spectrum. Jensen, 2000

5. Electromagnetic Spectrum The Sun produces a continuous spectrum of energy from gamma rays to radio waves. The visible portion of the spectrum may be measured using wavelength (measured in nanometers or micrometers, i.e. nm or m) or electron volts (eV) terminology. All units are interchangeable. Jensen, 2000

6. Particle Model of Electromagnetic Energy Niels Bohr (1885-1962) and Max Planck recognized the discrete nature of exchanges of radiant energy and proposed the quantum theory of electromagnetic radiation. This theory states that energy is transferred in discrete packets called quanta or photons as discussed. The relationship between the frequency of radiation expressed by wave theory and the quantum is: Q = h  where Q is the energy of a quantum measured in Joules (J), h is the Planck constant (6.626 x 10-34 J s-1), and  is the frequency of the radiation.

7. Particle Model of Electromagnetic Energy Referring to the previous formulas, we can multiply the equation by h/h, or 1, without changing its value:  = h c h  By substituting Q for h , we can express the wavelength associated with a quantum of energy as:  = h c / Q, or Q = h c /  Thus, the energy of a quantum is inversely proportional to its wavelength, i.e. the longer the wavelength involved, the lower its energy content.

8. The Energy of Quanta (Photons) The energy of quanta (photons) ranging from gamma rays to radio waves in the electromagnetic spectrum.

9. (a-c) A photon of electromagnetic energy is emitted when an electron in an atom or molecule drops from a higher energy state to a lower energy state. The type of light emitted (i.e. its wavelength) is a function of the changes in the energy levels of the outer, valence electron. (d) Matter can also be subjected to such high temperatures that electrons that normally move in captured, nonradiating orbits are broken free. When this happens, the atom remains with a positive charge equal to the negatively charged electron which escaped. The electron becomes a free electron and the atom is called an ion. If another free electron fills the vacant energy level created by the free electron, then radiation from all wavelengths is produced, i.e. a continuous spectrum of energy. The intense heat at the surface of the Sun produces a continuous spectrum in this manner. The Creation of Light

10. Creation of Light After being energized by several thousand volts of electricity, the outermost electron in each energized atom of sodium vapor climbs to a high rung on the energy latter and then returns down the ladder in a predictable fashion. The last two rungs in the descent are 2.1 eV apart. This produces a photon of yellow light which has 2.1 eV of energy.

11. Atmospheric Scattering The type of scattering is a function of: • the wavelength of the incident radiant energy, and • the size of the gas molecule, dust particle, or water vapor droplet encountered.

12. Rayleigh Scattering The intensity of Rayleigh scattering varies inversely with the fourth power of the wavelength (-4).

13. Atmospheric Refraction Refraction in three non-turbulent atmospheric layers. The incident radiant energy is bent from its normal trajectory as it travels from one atmospheric layer to another. Snell's law can be used to predict how much bending will take place based on a knowledge of the angle of incidence and the optical density of each atmospheric level.

14. Absorption • Absorption is the process by which radiant energy is absorbed and converted into other forms of energy. An absorption band is a range of wavelengths (or frequencies) in the electromagnetic spectrum within which radiant energy is absorbed by substances such as water (H2O), carbon dioxide (CO2), oxygen (O2), ozone (O3), and nitrous oxide (N2O). • The cumulative effect of the absorption by the various constituents can cause the atmosphere to close down in certain regions of the spectrum. This is bad for remote sensing because no energy is available to be sensed.

15. Absorption • In certain parts of the spectrum such as the visible region (0.4 - 0.7 m), the atmosphere does not absorb all of the incident energy but transmits it effectively. Parts of the spectrum that transmit energy effectively are called “atmospheric windows”. •Absorption occurs when energy of the same frequency as the resonant frequency of an atom or molecule is absorbed, producing an excited state. If, instead of re-radiating a photon of the same wavelength, the energy is transformed into heat motion and is reradiated at a longer wavelength, absorption occurs. When dealing with a medium like air, absorption and scattering are frequently combined into an extinction coefficient. • Transmission is inversely related to the extinction coefficient times the thickness of the layer. Certain wavelengths of radiation are affected far more by absorption than by scattering. This is particularly true of infrared and wavelengths shorter than visible light.

16. Absorption of the Sun's Incident Electromagnetic Energy in the Region from 0.1 to 30 m by Various Atmospheric Gases Jensen, 2000

17. Reflectance Reflectance is the process whereby radiation “bounces off” an object like a cloud or the terrain. Actually, the process is more complicated, involving reradiation of photons in unison by atoms or molecules in a layer one-half wavelength deep. • Reflection exhibits fundamental characteristics that are important in remote sensing. First, the incident radiation, the reflected radiation, and a vertical to the surface from which the angles of incidence and reflection are measured all lie in the same plane. Second, the angle of incidence and the angle of reflection are equal.

18. Reflectance There are various types of reflecting surfaces: • When specular reflection occurs, the surface from which the radiation is reflected is essentially smooth (i.e. the average surface profile is several times smaller than the wavelength of radiation striking the surface). • If the surface is rough, the reflected rays go in many directions, depending on the orientation of the smaller reflecting surfaces. This diffuse reflection does not yield a mirror image, but instead produces diffused radiation. White paper, white powders and other materials reflect visible light in this diffuse manner. • If the surface is so rough that there are no individual reflecting surfaces, then scattering may occur. Lambert defined a perfectly diffuse surface; hence the commonly designated Lambertian surface is one for which the radiant flux leaving the surface is constant for any angle of reflectance to the surface normal.

19. Reflectance

20. Spectral Reflectance Curves of Selected Materials Jensen, 2000

21. The concept of radiant flux density for an area on the surface of the earth. Irradiance is a measure of the amount of incoming energy in Watts m-2.Exitance is a measure of the amount of energy leaving in Watts m-2.

22. The concept of radiance leaving a specific projected source area on the ground, in a specific direction, and within a specific solid angle.This is the most precise radiometric measurement used in remote sensing.

23. Nominal Spatial and Temporal Resolution of Remote Sensor Data Required to Extract Urban/Suburban Infrastructure Information Jensen, 2000