Somebody finally got it right ;-)
PAR is the reference to the measurement of photosynthetically available radiation (400-700nm). Which can be measured as energy (Watts/m^2) or in terms of photons.
So when you say PAR you know nothing about the way it is being measured or the units used. When using the Term PPFD (Photosynthetic Photon Flux Density) it is clear that PAR is being expressed in terms of photons, and the units for PPFD are micromoles/m^2/sec.
As you can see from the units, the measurement is a function of the amount of photons falling on a given area (1 m2). As you change the distance between the light source and the area, you also change the number of photons falling on the same area. Try this with your flashlight by shining it on the wall and moving away. As you move away the light spot will get less intense, because you are now spreading the available photons over a larger area.
For a approximate point source of light, this relationship usually follows the inverse square law. ie. if I double the distance, the light intensity will drop to a fourth of what it originally was.
JB NY tested his lamps at a different distance than I did, with a different PAR meter. There are some small difference in the PAR meter, but lets ignore that for a moment. Since they are at different distances the data cannot be directly compared. However, using the inverse square law, the data can be scaled for comparison.
PAR values have no meaning, unless the distance at which the measurements are taken is known.
sanjay.
