In my research i want to determine LAI in above plant species,so for that i found method Using lux meter,and using those lux values i have to determine it
Photosynthesis is a dynamic process. If you want to get an average one should take a series of measurements in a day and take the average. Also it varies with the leaf age. I think it would be better to take measurements on number of leaves (green) with different age.
Rate of photosynthesis can be only evaluated through perfroming gas exchange analysis (CO2). Also, more indirectly, photochemical activity of PSII can be measured by the means of chlorophyll a fluorescence analysis. Measuring illuminance or irradiance (after recalculation) does not aid much to understanding of photosynthesis, for this you need to measure photon flux density of photosynthetically active radiation.