Radiative flux, often referred to as radiant flux, is a measure of the amount of radiant energy (such as light or thermal radiation) that passes through a given surface area per unit time. It is typically expressed in watts (W), where one watt equals one joule of energy per second.
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Radiative flux is the first angular moment of specific intensity and measures net radiant power crossing unit area. Integrating over frequency gives bolometric radiative flux. The receiving surface's orientation matters through ; observed radiative flux from a compact distant source conventionally refers to an area perpendicular to its line of sight.