Spectral Reflectance Curves Lecture 5

When specular reflection occurs, the surface from which the radiation is reflected is essentially smooth (i.e. the average surface profile is several times smaller than the wavelength of radiation striking the surface). If the surface is rough, the reflected rays go in many directions, depending on the orientation of the smaller reflecting surfaces. This diffuse reflection does not yield a mirror image, but instead produces diffused radiation. White paper, white powders and other materials reflect visible light in this diffuse manner. If the surface is so rough that there are no individual reflecting surfaces, then scattering may occur. Lambert defined a perfectly diffuse surface; hence the commonly designated Lambertian surface is one for which the radiant flux leaving the surface is constant for any angle of reflectance to the surface normal. Reflectance Specular reflection Lambertian reflection

There are no perfect specular and Lambertian surfaces in nature – most bodies approximate these two end members. Diffuse reflections contain spectral information about the “color” of reflecting surfaces, while specular reflections generally lack this information. Hence in remote sensing, we are mostly interested in measuring the diffuse reflection properties of terrestrial objects. Reflectance

Radiation budget equation The total amount of radiant flux in a specific wavelength ( ) incident to the terrain ( ) must be accounted for by evaluating the amount of radiant flux reflected from the surface ( ), the amount of radiant flux absorbed by the surface ( ), and the amount of radiant flux transmitted through the surface ( ): Or,

More terminology : Hemispherical reflectance, absorption, and transmittance The Hemispherical reflectance (  ) is defined as the dimensionless ratio of the radiant flux reflected from a surface to the radiant flux incident to it: Hemispherical transmittance (  ) is defined as the dimensionless ratio of the radiant flux transmitted through a surface to the radiant flux incident to it: Hemispherical absorption (  ) is defined by the dimensionless relationship:

Percent reflectance, absorption, and transmittance These radiometric quantities are useful for producing general statements about the spectral reflectance, absorption, and transmittance characteristics of terrain features. If we take the simple hemispherical reflectance (or absorption/transmittance) equation and multiply it by 100, we obtain an expression for percent reflectance ( ):

Spectral reflectance curves Spectral reflectance curve is a graph of the spectral reflectance of an object as a function of wavelength. Configuration of the spectral reflectance curves gives us insights into the spectral characteristics of an object. Spectral reflectance curves guide us in selecting wavelengths region(s) in which remote sensing data should be acquired for the given science goal.

Spectral reflectance curves: Vegetation

Spectral reflectance curves Red=0.65μm, G=0.55 μm, B=0.45 μm Red=0.9μm, G=0.55 μm, B=0.45 μm Spectral reflectance (%) Wave length (μm) 1.0

B -2, G -3, R - 4 B -1, G -2, R - 3 Dr. Nicholas Short, Remote Sensing Tutorial Spectral reflectance curves Bigleaf Maple Douglas Fir nm Band nm Band nm Band nm Band nm Band nm Band 7 LANDSAT ETM+ BANDS Coniferous – dark Deciduous - bright

Spectral reflectance curves: Soil a: Organic dominated b: Fresh soil (unaltered) c: iron altered d: organic affected e: Iron rich

Spectral reflectance curves: Water A: a: Ocean water b: Turbid water c: Water in leaf

Red shades : vegetation (forest, valley, crops) Medium greyish-browns - barren soil + dry grass Deep blues/black - ocean – (lighter where sediments are present) Blue and white patterns – breakers Bluish tones + streets - Towns B -1, G -2, R - 4 Spectral reflectance curves

Spectral signatures and response Spectral responses measured by remote sensors is often diagnostic of the type and/or condition of objects – hence these are called spectral signatures However, a preferable term could be spectral response patterns, because signature is a pattern that is absolute and unique. Spectral response patterns are influenced by: Atmospheric effects Temporal effects Spatial effects Geometric effects Pixel purity