Question

What is the difference Photon flux density and irradiance?

Answer 1

Photon flux density is the photometric light intensity illuminating a surface. It is the photons per unit area per unit time typically in candelas or lumens as the unit of photons, and usually termed "luminance".

Irradiance is the energy of those photons per unit area per unit time, or power (e.g., Watts) per unit area, illuminating the surface

Answer 2

A photon is the quantum of light and all other forms of electromagnetic radiation. Photons are characterized by energy.

The photon flux density is the number of photons per unit area, per unit time, and per unit wavelength while irradiance is the total power from a radiant source falling on a unit area.

Irradiation is irradiance integrated over a period of time.