Radiant Flux (Φ) is energy per unit time (dQ/dt) that is radiated from a source over optical wavelengths, which are defined to be from 3x1011 and 3x1016 Hz. This range is approximately equivalent to wavelengths from .01 to 1000 μm and includes the regions of the electromagnetic spectrum commonly referred to as Ultra Violet (UV), Visible, and Infra Red (IR).
The flux is measured in units of Joules per second (J/s), or Watts (W). A radiant flux of 1 W means that a source produces 1 Joule every second. If we integrate radiant flux over time we obtain the total Energy (Q) output by the source.
Most of the time when we deal with light sources, we do not really need to qualify optical radiation. For instance, if we look at the standard incandescent lamp, we know that the flux output below .01 μm is negligible, and that there is also very little energy output in the microwave and radio regions of the EM spectrum. So we can reasonably assume that the Flux output for a standard lamp is restricted to be in the optical range, and in doing so we may take the nominal wattage as our radiant flux. That is, a 60 W incandescent lamp would have a radiant flux of 60 W, and a 250 W Mercury vapor lamp would have a radiant flux of 250 W.