directly from http://en.wikipedia.org/wiki/Photosy...tive_radiation

PAR measurement is used in agriculture, forestry and oceanography. One of the requirements for productive farmland is adequate PAR, so PAR is used to evaluate agricultural investment potential.

The expression Photosynthetically Active Radiation, often abbreviated PAR, designates the spectral range of solar light from 400 to 700 nanometers that is useful to terrestrial plants in the process of photosynthesis.

don't tell me about what is important, you don't even seem to know that light IS radiation.