[2.23]

The number of photons, dN_p, emitted, transmitted, or received in an element of time, dt, divided by that element of time:

Units: s^-1

Note 1: The photon flux emitted by a source in the wavelength interval from λ₁ to λ₂ can be calculated from the spectral radiant flux:

where:

h is Planck’s constant

c is the speed of light

\Phi_\lambda(\lambda) is the spectral radiant flux, measured in W/nm

Note 2: To convert to moles of photons, divide by Avogadro’s number (N_A):

 The unit for photon flux, Φ_p, is mol/s and is usually expressed in micromoles per second (μmol/s).

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