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

Reflectance

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.

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?

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?

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)

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)?

Important Terms and Equations 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) L λ Eλ

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?)

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??) Are radiance and reflectance the same?? 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 its emitted wavelength distribution

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 Demo from UC Boulder http://phet.colorado.edu/sims/blackbody-spectrum/blackbody-spectrum_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 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

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

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 = 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