is variable fluorescence. Calculated as  =  - .
 is the ratio of variable fluorescence to maximal fluorescence. Calculated as . This is a measure of the maximum efficiency of PSII (the efficiency if all PSII centres were open).  can be used to estimate the potential efficiency of PSII by taking dark-adapted measurements.
 measures the efficiency of Photosystem II. Calculated as  = . This parameter measures the proportion of light absorbed by PSII that is used in photochemistry. As such, it can give a measure of the rate of linear electron transport and so indicates overall photosynthesis.
 (photochemical quenching). Calculated as . This parameter approximates the proportion of PSII reaction centres that are open.
Whilst  gives an estimation of the efficiency,  and  tell us which processes which have altered the efficiency. Closure of reaction centers as a result of a high intensity light will alter the value of . Changes in the efficiency of non-photochemical quenching will alter the ratio .