4. Radiant Power Density

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This page in essence brings together the ideas of the previous pages about light, which are all factors that are used to calculate the radiant power density. First of all, it is important to avoid confusion by addressing that light intensity (measured in Watts per meter squared, or power per unit area) decreases with distance from the source (see Solar Radiation Outside the Earth’s Atmosphere), however radiant power density (measured in the same units) can refer to either the source's surface or the receiving surface (see below). We use the idea of radiant power density to describe the spectrum of light radiated by a given object, as well as the spectrum of light that reaches a surface (these would be two separate power densities).

The main points regarding radiant power density:

Radiant power density (also known as radiant flux density) is the power per unit area of the light either emitted by an object or received on a surface, and is determined by both the photon flux (the number of photons passing through a given area in a given time) and the amount of photons for each given wavelength/energy (taken into account in spectral irradiance).
The intensity or irradiance is also confusingly called radiant flux/power density, but in solar physics usually refers to light received on a surface at some distance from the source. Irradiance is also defined as power per unit area.
The radiant emittance or radiant exitance is specifically the radiant flux/power density emitted by an object, or the intensity of light at the source. Thus, when we speak of radiant power density in solar applications, we are probably referring to the power density incident to our Earth's surface that comes from the sun.

The equation for radiant power density is given by

where F(λ) is the spectral irradiance and dλ is an infinitessimally small interval of wavelengths¹. This equation just means that if you sum up all the energy contributions from all the photons passing through a surface based on their particular wavelengths, you obtain a number that gives the radiant power density on that surface.

References

1. “Radiant Power Density”. pveducation.org. <www.pveducation.org/pvcdrom/p...ral-irradiance>