Most light meters measure the instantaneous photosynthetic photon flux in μmol m^-2 s^-1 and yet plant growth is determined by the integrated daily photosynthetic photon flux in mol m^-2 d^-1.

Here is how to convert from μmol m^-2 s^-1 to mol m^-2 d^-1 when using electric lights.

There are 3600 seconds in an hour. If the electric light output is constant, the instantaneous PPF can be added up (integrated) over an hour.

Therefore 1 μmol m^-2 s^-1 = 3600 μmol m^-2 h^-1.

This must be divided by a million(106) to convert from micromoles to moles:      3600 μmol m^-2 h^-1 = 0.0036 mol m^-2 h^-1

If the electric lights are on continuously (24 hours) the hourly PPF must be multiplied by 24 to get the daily PPF:      0.0036 mol m^-2 h^-1 = 0.0864 mol m^-2 d^-1

There are 86,400 seconds in a day (3600 x 24 hours) so the daily PPF can be calculated directly from the instantaneous PPF from the following equation: μmol m^-2 s^-1 multiplied by 86,400 and divided by 106 = mol m^-2 d^-1

So:   1 μmol m^-2 s^-1 = 0.0864 mol m^-2 d^-1 with continuous light

These conversions are accurate only under electric lights. The integrated daily PPF from sunlight changes during the day so a single measurement cannot be used to estimate daily PPF.

Daily PPF Reference Table