Lecture 2: principles of electromagnetic radiation

Transcription

1 Remote sensing for agricultural applications: principles and methods Lecture 2: principles of electromagnetic radiation Instructed by Prof. Tao Cheng Nanjing Agricultural University March Crop 11, Circles 2014 in Kansas Lansat-7

2 Outline Electromagnetic radiation models Atmospheric energy-matter interaction Terrain energy-matter interactions Hemispherical reflectance, absorptance, and transmittance Radiant flux density Atmospheric energy-matter interaction again Energy-matter interactions at the sensor Target and path radiance Bring up your knowledge of high-school physics. We need it today! 2

3 Electromagnetic Energy Interactions Sensor The sun Light speed Atmosphere The earth Energy radiated from the sun interacts with: the Earth s atmosphere the Earth s surface the Earth s atmosphere again and finally reaches the sensor. 3

4 In detail Energy-matter interactions in the atmosphere, at the study area, and at the remote sensor detector. Jensen (2006) 4

5 How is energy transferred? Jensen (2006) Energy may be transferred three ways: Conduction: conducted directly from one object to another in direct physical contact Convection: convectional currents in the atmosphere Radiation: transmitted in the form of electromagnetic (EM) waves 5

6 Electromagnetic radiation models We can better understand the process of EM radiation using two models: the wave model (e.g., wavelength) the particle model (e.g., photons interact with leaves) High school physics: wave-particle duality 6

7 The wave model of EM energy Jensen (2006) λ = c ν ν = c λ Key concepts: Wavelength (λ) The mean distance between two consecutive maximums Normally measured in um or nm Frequency (ν) The number of wavelengths that pass a point per unit time Normally measured in Hz λ is inversely proportional to ν. c is the speed of light. 7

8 The EM spectrum Figure from Wikipedia ( 8

9 Stefan-Boltzmann law Every object above absolute zero emits EM energy, e.g., water, vegetation, and the surface of the Sun. How different are the emitted energies? The total emitted radiation from a blackbody (M λ ) is: M λ = σt 4 σ = Wm 2 K 4 (the Stefan-Boltzmann constant). T is temperature. Jensen (2006) 9

10 Wien s displacement law λ max = k T λ max : the dominant wavelength, K = 2898 um K, T is the absolute temperature. The Sun: λ max = 2898 μm K 6000 K = 0.48 μm The Earth: λ max = 2898 μm K 300K = 9.66 μm 10

11 Blackbody radiation curves The area under each curve is the total radiated energy for each object. The sun produces much EM energy than the Earth because of higher temperature. 11

12 Our eyes are only sensitive to light in the visible region. Our remote sensors can be sensitive to light in the infrared and ultraviolet regions. 12

13 Particle model of EM energy We can also describe EM energy in terms of particle-like properties using the quantum theory: Q = hν Q is the energy of a quantum h is the Planck constant ν is the frequency of the radiation. λ = hc hν = hc Q Note: the longer the wavelength, the lower its energy content. In remote sensing: it is more difficult to detect longerwavelength energy emitted at thermal infrared wavelengths than those at shorter wavelengths. 13

14 EM energy and atmosphere interactions The EM radiation generated by the Sun interacts with the Earth s atmosphere before it reaches the Earth s surface. Refraction, scattering, absorption, reflectance Refraction: Occurs due to variation in the speed of EM radiation from one medium to another. Medium 1 Medium 2 14

15 Atmospheric scattering Scattering A very serious effect of atmosphere Different from reflection, because the direction of scattering is unpredictable. Accomplished by absorption and re-emission of EM radiation in unpredictable directions. Three types of scattering: Rayleigh scattering Mie scattering Nonselective scattering Type of scattering is a function of: The wavelength of the incident radiation The size of atmospheric particle 15

16 (Diameter << wavelength) (Diameter wavelength) Incident EM radiation (Diameter > 10 wavelength) Jensen (2006) Rayleigh scattering: Molecular scattering the amount of scattering is inversely proportional to the fourth power of the radiation s wavelength Mostly occurs 2-8 km asl. Is responsible for the blue sky on a cloudless day and red sunsets. Mie scattering Non-molecular or aerosol particle scattering Occurs below 4.5 km. Non-selective scattering All wavelengths of light are scattered, by clouds or fogbanks. Occurs in the lowest portions of the atmosphere. Equal for visible wavelengths. Atmospheric scattering is a very important factor to be considered in remote sensing investigations. Correction for this effect may be necessary. 16

17 Absorption N 2 O O 2 & O 3 In the um region: H2O is the primary absorber O2 & O3 have strong absorptions around 0.7 um Jensen (2006) The absorption of the EM energy from the Sun by various gases in the atmosphere. If the energy at a wavelength is strongly absorbed, then we cannot use it for remote sensing. The white portions of the spectrum are called atmospheric windows. Through these windows, we can sense features on the Earth. 17

18 Reflectance Various types of reflections Specular reflection: smooth surfaces, e.g., calm water bodies Diffuse reflection: rough surfaces. Radiation can be bounced off in many directions Lambertian surface: perfectly diffuse surface. 18 Figure from

19 Terrain energy-matter interactions In remote sensing, we focus on the radiant flux incident from the Sun and how it changes after the energy interacts with the terrain. Radiation budget equation: incident radiant flux = reflected + absorbed + transmitted Φ iλ = Φ rλ + Φ aλ + Φ tλ 1 = Φ r λ Φ iλ + Φ a λ Φ iλ + Φ t λ Φ iλ 1 = ρ λ + τ λ + α λ Hemispherical absorptance Hemispherical transmittance Hemispherical reflectance 19

20 Spectral reflectance of selected materials 20

21 Radiant flux density Irradiance E λ : the amount of radiant flux incident upon a surface per unit area E λ = Φ λ A (W m 2 ) L λ = Φ λ Ω Acosθ (W m 2 sr 1 ) Solid angle can be thought of a 3-D cone that funnels radiant flux. Jensen (2006) 21

22 From solar irradiance to radiance at the sensor Path 1:spectral solar irradiance reaching the target area = E Oλ T θo cosθ O Path 2: upward spectral diffuse sky irradiance E duλ Path 3: downward spectral diffuse sky irradiance E ddλ path 4 & path 5: reflected or scattered spectral irradiance from nearby terrain. Jensen (2006) 22

23 From solar irradiance to radiance at the sensor The total incident solar irradiance: λ 2 E gλ = (E Oλ T θo cosθ O + E duλ + E ddλ )dλ λ 1 The total radiance exiting from the target area: λ 2 L T = 1 (E π Oλ T θo cosθ O + E λ duλ + E ddλ )ρ 1 λ T θv dλ The total radiance recorded by the sensor: L S = L T +L P (Wm 2 sr 1 ) L P : path radiance to be removed using atmospheric radiative transfer models. Methods for atmospheric correction: MODTRAN Second Simulation of the Satellite Signal in the Solar Spectrum (6S) ACORN ATCOR Jensen (2006) 23

24 Paper review assignment Song, C., Woodcock, C. E., Seto, K. C., Lenney, M. P., & Macomber, S. A. (2001). Classification and change detection using Landsat TM data: when and how to correct atmospheric effects? Remote Sensing of Environment, 75, Citation report from Web of Science, accessed March 10, Submission deadline: March 18, 2014, 1:00 pm. 24