From the camp of Bruce!
This is a really great tool to calculate sunlight PPF (aka PPFD aka PAR) at any time of day and any day per year and any location and elevation. The only trick is the sky has to be clear, as in no clouds, etc., for the calculated PPF to be the actual PPF.
The PPF value provided is very useful for a few different reasons. Including planning a greenhouse (and whether it will need supplemental lighting), forecasting when it may be too bright outside for the plants so you can plan on how to help them, note effects of PPF on plants without an expensive quantum sensor, etc. Also, this tool is used to check the accuracy of quantum sensors, for example LI-COR LI-190 and Apogee ‘sunlight’ calibrated quantum sensors.
Make sure to read the “INPUT AND OUTPUT DEFINITIONS” section to know how to use this tool:
http://clearskycalculator.com/quantumsensor.htm