Understanding Multispectral
Reflectance
 Remote
sensing measures reflected “light”
(EMR)
 Different materials reflect EMR differently
 Basis for distinguishing materials
Reflectance
Learning Objectives
1.
2.
3.
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.
Learning Objectives (cont.)
4.
5.
What are reflectance spectra and what do
they look like for common materials?
What leaf characteristics control the
shape of its spectral reflectance curve?
Spectral Reflectance
 Definition:
The amount of reflected radiation
divided by that amount of incoming radiation in
a particular wavelength range
% Reflectance =100 x reflected/incoming
What are the units of reflectance?
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
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)?
Basic equations describing light
Radiant Flux
Rate of flow of energy onto, off of, or through a surface
Φλ (Watts)
Some Important Terms
 Irradiance
(Eλ) (Incoming light from sun)
 Radiance (L λ ) (Light received at satellite)
Lλ
Eλ
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?)
Radiance (Lλ)
Radiant flux per unit solid angle arriving at a
satellite from a given direction per unit area.
What are the units??
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?
RADIANCE!!!
Top of Atmosphere
IRRADIANCE
REFLECTANCE!!
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
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?
Stefan-Boltzmann Law
Q=
4
єσ*T
Where:
Q = amount of energy radiated
є = emissivity
σ = Stefan Boltzmann constant (5.6697 x 10-8
Wm-2K-4)
T = Temperature (oK)
What does this equation tell you?
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?
Which is hotter???
Wien’s Displacement Law
Demo from UC Boulder
http://phet.colorado.edu/sims/blackbody-spectrum/blackbodyspectrum_en.html
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
wavelength (W/m2)
t = atmospheric transmittance (fraction)
m = relative air mass (fraction)
θ = angle of incidence (degrees or radians)
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
Zenith Angle
Solar Angle
We use the zenith angle for angle of incidence in
the irradiance equation.
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 (called path radiance) accounts for
atmospheric scattering; will discuss later in
semester.
Reflectance Spectra
 Reflectance
spectra are graphs of
reflectance (y-axis) vs. wavelength (x-axis).
 Understanding
reflectance spectra is
fundamental to using remote sensing
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.
Plant Pigments
 Chlorophyll
A (green)
 Chlorophyll B (green)
 Others: e.g., β – carotene (yellow) and
Xanthophylls (red)
Leaf Structure
Cell Structure
Relatively speaking…
 Lots
of palisade mesophyll = low NIR
reflectance
 Lots of spongy mesophyll = higher NIR
reflectance
Leaf Water Content
The Red Edge
The Red Edge
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)
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)
Spectral Investigations