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 the amount of incoming radiation in a particular wavelength range
% Reflectance =100 x reflected/incoming
What are the units of reflectance?

6. Types of Reflectance Specular Diffuse (aka Lambertian)
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 (most materials reflect light in many directions, but not equally)

8. Reflectance of Materials
Varies with wavelength
Varies with geometry
Diagnostic of different materials
What kinds of reflectance do you see here?
Why do the different ponchos look different (e.g., pink vs. green)?

9. Important Terms and Equations
Radiant Flux
Rate of flow of energy onto, off of, or through a surface
Φλ (Watts)

10. Some Important Terms Irradiance (Eλ) (Incoming light from sun)
Radiance (L λ ) (Light Received at satellite)
L λ Eλ

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

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

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

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

15. 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 its emitted wavelength distribution

16. 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?

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

18. 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?

19. Which is hotter???

20. Wien’s Displacement Law
Demo from UC Boulder

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

22. 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

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

24. Calculating Radiance 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 =
Lp (called path radiance) accounts for atmospheric scattering; will discuss later in semester.

25. Reflectance Spectra
Reflectance spectra are graphs of reflectance (y-axis) vs. wavelength (x-axis).
Understanding reflectance spectra is fundamental to using remote sensing

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

30. Leaf Structure

31. Cell Structure

32. Relatively speaking… Lots of palisade mesophyll = low NIR reflectance
Lots of spongy mesophyll = higher NIR reflectance

33. Leaf Water Content

35. The Red Edge
The Red Edge

36. Multiple Leaf Layers
Reflectance increases with the number of leaf layers in a non-linear fashion
Eventually, with enough layers, the reflectance “saturates” (stops increasing)

40. How do you get spectra?
Measure in the field with field spectroradiometers
Measure in the lab
Collect from image data
Look at spectral libraries:

41. Spectral Investigations