How many light quanta or photons are required for the liberation of one oxygen molecule during the process of photosynthesis?

Total 8 photons are required for evolving one molecule of oxygen and this is known as quantum requirement. For the liberation of one oxygen molecule, there must be involvement of 2 water molecules. These water molecules can liberate 4 electrons, 4 protons and a molecule of oxygen. The electrons are moved ultimately to NADP+ to form NADPH during which 4 photons are used in PSII and 4 photons in PSI. Hence there is requirement of 8 photons for the liberation of a molecule of oxygen.