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It's the number of photons that are incident on a particular area. As opposed to spectral density which is the power or energy per unit time, per unit area.
You mean lecture 2 in the TU Delft Solar cells course, right?
i think photo photon flux density is photons incident per unit area per unit time expressed in lumen or candela
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Photon flux density is defined as the number of photons incident on an unit area during an unit time interval and within a particular energy range.
To be simple, Photon flux density is also called as "luminance" and is expressed in cd/m2.
The photon flux density is the number of photons per unit area, per unit time, and
per unit wavelength. It helps us to have better knowledge of the spectral distribution of the solar radiation since it goes further than concept has irradiance and spectral power density.
Irradiance is the total power from a radiant source falling on a unit area. Spectral power density goes further as it associates the wavelength and helps determining each particular power of irradiance.
Photon is the quantum of irradiance. Associating photons to wavelength help us determining to which energy can be linked each particular photon.