What is the difference between PPF and PPFD?

PPF stands for Photosynthetic photon flux and ppfd for the density. PPF with the unit µmol/s (micromole per second) describes the total amount of photons emitted by a light-source. PPFD measures the light intensity on a surface. PPFD is PPF divided by the Area. (PPF/A |  µmol/m²s)