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Define the term albedo.
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How do the terms albedo and reflectance differ?
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In what units is albedo expressed?
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What is planetary albedo?
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What is the approximate value of the planetary albedo of the Earth?
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The albedo of water at high sun angles is very low, but at low sun angles it is very high. Why?
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State Lambert's Cosine Law.
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What is the principal characteristic of a Lambertian surface?
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The radiant flux of a Lambertian surface reduces with the viewing angle. How can the radiance of such a surface be constant?
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What is spectral albedo?
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What is the BRDF?
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Give some of the parameters that determine the BRDF.
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Albedo is the ratio of the amount of energy reflected by an object to the amount incident upon it.
They are essentially the same, but albedo is sometimes restricted to reflectance at visible wavelengths.
Albedo can be expressed either as a percentage or as an index in the range 0..1. Albedo is dimensionless.
A planetary albedo is the albedo of an entire planet, averaged over all its surface.
About 30%.
As a result of geometric reflections at low sun angles. Undisturbed water surfaces absorb solar radiation from high incident angles almost completely. At lower angles of incidence light is reflected geometrically. This phenomenon is called sun glint.
The radiant flux per unit solid angle in any direction from an element of a perfectly diffusing surface (a 'Lambertian surface') varies as the cosine of the angle between that direction and the normal of the surface.
J = J0 cos(Φ)
The radiant flux of a Lambertian surface is isotropic. When an area on the surface is viewed from any angle, it has the same radiance.
Because the area viewed (and therefore the solid angle) also reduces with the viewing angle. Since radiance is radiant flux per unit solid angle, it stays constant when both the radiant flux and the solid angle are changed at the same rate (as the cosine of the viewing angle).
Spectral albedo is the reflectance from an object in terms of a particular wavelength of incident radiation or over a range of wavelengths.
BRDF stands for Bidirectional Reflectance Distribution Function. It describes the angular distribution of reflected solar radiation as a function of viewing direction. Different surfaces reflect in different directional patterns.
The BRDF is wavelength specific. It depends on the structural and optical properties of the surface, such as
- shadow-casting,
- multiple scattering,
- mutual shadowing,
- transmission, reflection, absorption, emission,
- facet orientation, facet distribution and facet density.
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