Understanding Multispectral Reflectance

Remote sensing measures reflected “light” (EMR)

Different materials reflect EMR differently Basis for distinguishing materials

Reflectance

Learning Objectives

1. Be able to define reflectance qualitatively and quantitatively.

2. Understand the terms that describe the path of light from the sun to the earth and up to the satellite.

3. Understand the equations that define the amount of light at various places on the light path.

Learning Objectives (cont.)

4. What are reflectance spectra and what do they look like for common materials?

5. 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?

Demo

Mirror and Flashlight

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

Varies with wavelength Varies with geometry Diagnostic of different materials

Radiant FluxRate 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)

Eλ

L λ

IrradianceRadiant 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 AtmosphereIRRADIANCE

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 = єσ*T4

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

http://phet.colorado.edu/sims/blackbody-spectrum/blackbody-spectrum_en.html

Demo from UC Boulder

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)

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

Solar Angle

Zenith 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 groundt λ = Atmospheric transmittancem = Relative air massr λ = Reflectance of objectπ = 3.1416

Lp accounts for atmospheric scattering; will discuss later in semester.

Reflectance Spectra

Reflectance spectra are graphs of reflectance vs. wavelength.

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.

Vegetation Reflectance

Plant Pigments

Chlorophyll A (green) Chlorophyll B (green) Others: e.g., β – carotene (yellow) and

Xanthophylls (red)

Leaf Structure

Cell Structure

Lots of palisade mesophyll = low NIR reflectance

Lots of spongy mesophyll = higher NIR reflectance

Relatively speaking…

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”

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

Applications

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