Light is a key component of Photosynthesis - put simply no light, no photosynthesis. During the winter months with shorter days, even though we have temperatures suitable for growth, it's very likely we'll go through periods when light is the limiting factor.
I've grabbed a few slides from a recent presentation myself and Dr Andy Owen did to try to explain some of the units used for measuring light. The easiest way I've found to explain them is to compare them to rainfall - which we are all far too used to measuring!
Micromoles per meter squared a second - This is comparable to saying it's drizzling or raining cats and dogs - it's a measurement of how much light is hitting the ground NOW.
Daily Light Integral (DLI)- Take all of those seconds of light hitting the ground and bundle them up into one figure - like a rain gauge measuring how much rain did we had yesterday.
7 day accumulated figure - This is the useful one - add all those days DLI's over the last 7 days and that's your accumulated DLI. A bit like saying we've had 72mm of rain in the last 7 days.
This for me it's the useful one - after all one day of no light won't impact a grass plant but 7 days of no light will.
