That's the title of my column in the September/October issue of GCM China. I wrote about light:

"It isn’t that difficult once one understands the terminology and the units of measurement. There are just three technical terms to learn, and after that, it is all quite clear."

Those terms are:

• the light grass can use is photosynthetically active radiation (PAR)
• to find out what PAR is at any moment, one measures the photosynethetic photon flux density (PPFD)
• to measure PAR for a day, add up all the PPFD from sunrise to sunset to get the daily light integral (DLI)

Visit the GCM China website to read the full issue, or download PDF versions of the article in English or in Chinese.