Definitions
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Radiant flux (W)
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the rate at which light energy is emitted
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Steradian (sr)
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a unit of solid (3D) angle (there are 4 p
steradians in a sphere)
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Radiant Intensity I (W/sr)
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the rate that light energy is radiated through a given solid angle
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Radiance L(W/(sr m2))
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the rate of energy radiated through a given solid angle as seen reflected
from a surface (i.e. the hemisphere is projected onto the surface)
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Irradiance (W/m2)
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the rate of incident or incoming energy at a surface point per unit
surface area.
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Hyperphysics : http://hyperphysics.phy-astr.gsu.edu/hbase/vision/photomcon.html