2. Radiometric and photometric quantities

2.1 Flow

Flux Φ is the instantaneous value of a radiation flow. It may be all the radiation emitted by a source, or only that carried by the beam leaving an instrument, or even that received by a detector.

Flux can be measured by the power (in watts: system of energy units) it carries, or by the number of photons it delivers per unit of time (in s ^–1 : system of photon units).

2.2 Geometric extent

Consider a source surface S illuminating...