Understanding Multispectral Reflectance

1. Understanding Multispectral Reflectance • Remote sensing measures reflected “light” (EMR) • Different materials reflect EMR differently • Basis for distinguishing materials

2. Reflectance

3. Learning Objectives • Be able to define reflectance qualitatively and quantitatively. • Understand the terms that describe the path of light from the sun to the earth and up to the satellite. • Understand the equations that define the amount of light at various places on the light path.

4. Learning Objectives (cont.) • What are reflectance spectra and what do they look like for common materials? • What leaf characteristics control the shape of its spectral reflectance curve?

5. Spectral Reflectance • Definition: The amount of reflected radiation divided by that amount of incoming radiationin a particular wavelength range % Reflectance =100 x reflected/incoming What are the units of reflectance?

6. Demo Mirror and Flashlight

7. Types of Reflectance • Specular • Mirrors or surfaces of lakes, for example • Angle of incidence = angle of reflection • Diffuse (aka Lambertian) • Reflects equally in all directions • Usually we assume Lambertian reflectance for natural surfaces • Idealized—not really found in nature but often close

9. Reflectance • Varies with wavelength • Varies with geometry • Diagnostic of different materials

10. Radiant Flux Rate of flow of energy onto, off of, or through a surface Φλ (Watts)

11. Some Important Terms • Irradiance (Eλ) (Incoming light from sun) • Radiance (L λ ) (Light received at satellite) L λ Eλ

12. Irradiance Radiant flux incident per unit area of a surface Eλ = Φλ/A (radiant flux/area in a particular wavelength) (So…what would the units be?)

13. Radiance (Lλ) Radiant flux per unit solid angle arriving at a satellitefrom a given direction per unit area. What are the units??

14. Radiance vs. Reflectance • Satellites measure radiance (What are the units??) • Objects on the ground are often characterized by their reflectance (What are the units??) • Often we want reflectance but we measure radiance. How do we deal with this?

15. RADIANCE!!! Top of Atmosphere IRRADIANCE REFLECTANCE!!

16. Top of Atmosphere (TOA) Irradiance • Incoming radiation from the sun (E0) • Amount is described by Stefan-Boltzmann Law which relates the absolute temperature of an object to amount of energy it gives off • Peak wavelength is determined by Wien’s Displacement Law which relates object’s temperature to emitted wavelength

17. Top of Atmosphere Irradiance • Top of atmosphere irradiance varies over time. Why? • Is top of atmosphere irradiance equal to ground irradiance? Why or why not?

18. Stefan-Boltzmann Law Q = εσ*T4 Where: Q = amount of energy radiated ε= emissivity σ= Stefan Boltzmann constant (5.6697 x 10-8 Wm-2K-4) T = Temperature (oK) What does this equation tell you?

19. Wien’s Displacement Law Peak λ(µm) = 2898/T(oK) Sun is 5778 oK So peak λ= 2898/5778 = 0.5 µm (green) What does this equation tell you about temperature and the energy of emitted light?

20. Which is hotter???

21. Wien’s Displacement Law Demo from UC Boulder http://phet.colorado.edu/sims/blackbody-spectrum/blackbody-spectrum_en.html

22. Calculating Irradiance at Ground Eλ = E0tmcosθ Eλ = Irradiance on ground for a particular wavelength (W/m2) E0 = Irradiance at top of atmosphere for that wavelength (W/m2) t = atmospheric transmittance (fraction) m = relative air mass (fraction) θ = angle of incidence (degrees or radians)

23. Angle of Incidence (θ) • Depends on slope of ground • Depends on sun altitude • Angle of sun above the horizon • Depends on time of year and time of day • Depends on sun azimuth • The compass direction of sun • Depends on time of year and time of day

24. Zenith Angle Solar Angle We use the zenith angle for angle of incidence in the irradiance equation.

25. Calculating Radiance from Reflectance Lλ = Etmr/π (+ Lp) Lλ = Radiance at satellite (W/m2/sr) (not the same as DN – must convert!) Eλ= Irradiance on ground t λ= Atmospheric transmittance m = Relative air mass r λ= Reflectance of object π = 3.1416 Lp accounts for atmospheric scattering; will discuss later in semester.

26. Reflectance Spectra • Reflectance spectra are graphs of reflectance vs. wavelength. • Understanding reflectance spectra is fundamental to using remote sensing

28. Spectral Properties of Vegetation • Unlike minerals, vegetation is composed of a limited set of spectrally active compounds • Relative abundance of compounds, including water, indicates veg. condition • Vegetation structure has significant influence on reflectance. • Spatial scale of reflectance measurement is critical.

29. Vegetation Reflectance

30. Plant Pigments • Chlorophyll A (green) • Chlorophyll B (green) • Others: e.g., β – carotene (yellow) and Xanthophylls (red)

31. Leaf Structure

32. Cell Structure

33. Lots of palisade mesophyll = low NIR reflectance • Lots of spongy mesophyll = higher NIR reflectance

34. Leaf Water Content

36. The Red Edge The Red Edge

37. Multiple Leaf Layers • Reflectance increases with the number of leaf layers in a non-linear fashion • Eventually, with enough layers, the reflectance “saturates”

41. How do you get spectra? • Measure in the field with field spectroradiometers • Measure in the lab • Collect from image data • Look at spectral libraries: http://speclab.cr.usgs.gov/spectral-lib.html)

42. Spectral Investigations

43. Applications • Precision Agriculture • Mapping vegetation communities • Prospecting for oil • Prospecting for other minerals • Mapping invasive plants • Measuring nutrient content of plants • Etc., etc.