Hey Mike,
I'm sorry to contradict you, but LED's do transmit and absorb in a specific bandwidth... This is why I got interested in using them in the first place...
[quote author=Atmospheric Monitoring with Arduino]
Mims reasoned that since LEDs take in
electricity and emit light (along that narrow emission band, remember), they
should also take in light and give off electricity. A series of experiments proved
him right, and by 1973 he had formalized what is now known as the Mims
Effect: LEDs will absorb light along a relatively narrow band of color, and emit
a small amount of electricity.[/quote]
So when one uses different kinds of led each with their specific bandwidth, one can make a photometer to measure them.
The next thing is to figure out how to extrapolate the actual PAR value from the total of all the leds, but I know someone who uses a PAR meter professionally and he's willing to let me use it to calibrate my device to his...But first need to get mine working 
Erni:
BillieBricksI corrected the schematic, but you are right: the gain is 10/0.1 = 100 , which is too much when using my photodiode, it should be about 15, so a 1.5 Mohm resistor should be suitable.
Again. Thank you for taking the time and even test the theory/schematic.
I know I've got lots to learn, but you make it fun!!! 