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Radiometry evolved over a long period of time around rather in-coherent sources of thermal nature. Only during the last few years have the effects of coherence begun to be taken into account in radiometric considerations of light sources. In this review article the fundamental concepts of conventional radiometry and of the theory of partial coherence will be first briefly recalled. The basic radiometric quantities, namely the radiance, the radiant emittance, and the radiant intensity, associated with a planar source of any state of coherence will then be introduced. It will be pointed out that the radiant intensity, representing the primary measurable quantity, obeys in all circumstances the usual postulates of conventional radiometry, whereas the radiance and the ra-diant emittance turn out to be much more elusive concepts. The radiometric characteristics of light from incoherent and coherent sources as well as from a certain type of partially coherent source, viz., the so-called quasihomogeneous source, will be analyzed. Quasihomogeneous sources are useful models for radiation sources that are usually found in nature. Lambertian sources will be discussed as examples.
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