The definition of photon flux is the number of photons per second per a unit area.
It is important in identifying the number of generated electrons, and hence the produced current from a solar cell. The energy or wavelength of the photons in the light source must be specified because it gives no information about the energy or the wavelength of the photons.
The amount of radiation can be measured in numerous ways. The two most common ways for photovoltaics are:

Energy flux or Irradiance (light intensity), which is the amount of energy per area per time, in units of W/m2 or mW/cm2

photon flux, which is calculated by the number of photons per area per time, in units of cm–2s–1

Calculations

Power density for photons at a particular wavelength can be calculated using the combination of the photon wavelength or energy and the photon flux at that that wavelength. Power density can be calculated by multiplying the photon flux by the single photon energy.
Since it explains the number of photons hitting a surface in a given time frame, multiplying by the amount of energy of the photons forming the photon flux provides the energy hitting the surface per unit time, which is equivalent to a power density.
To calculate power density in units of Watt per m², the photons energy should be in Joules. Below are the equations:
where
Φ = photon fluxq = electronic charge value ( 1.6 x10^{-19} )
The above equation gives an implication that for the photon flux of higher energy (or shorter wavelength), number of photons required to give certain radiant power density will be less than the number of photons in photon flux of lower energy (or longer wavelength) needed to give an equal radiant power density.
NP which is the number of photons per second per unit area can be calculated from Irradiance I by the following equation:
Np= I/E_{p}= I•((λ•10^{-9})/h•c)
Photon flux can also be calculated by converting photon numbers to µmoles of photons

Photon counting

A technique used for counting individual photons is used which is called SPD (single-photon detector) . The efficiency of single-photon detector’s efficiency is defined by the quantum efficiency and all electronic losses that are present in such system.
It is becoming more important in photovoltaics and solar panels to calculate the amount of radiation and properly design the required systems to produce all required energy and be properly installed and setup. It also has other numerous uses in biological and chemical processes