Radiometry is the field of measurement of any electromagnetic radiations, in particular light.
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Foreshortening is a phenomenon in which a source or a patch of surface which is tilted with respect to the direction in which the illumination is travelling, it “looks smaller” to a patch or the source respectively.
Radiance is the amount of energy travelling at some point in a specified direction, per unit time, per unit area perpendicular to the direction of travel, per unit solid angle. It is a function of position and direction. It is constant along a straight line.