The quantum yield of photosynthesis, which is also the photochemical quenching of fluorescence, is calculated through the following equation:
Φ = (F-F)/F = F/F
F is the low fluorescence intensity, which is measured by a short light flash that is not strong enough to cause photochemistry, and thus induces fluorescence. F is the maximum fluorescence that can be obtained from a sample by measuring the highest intensity of fluorescence after a saturating flash. The difference between the measured values is the variable fluorescence F.