Photosynthetic Active Radiation (PAR) was derived from the Mcree Curve. It is a TOTAL count of light energy (in photons) between
400nm to 700nm. The PAR measurement is fast becoming a popular metric of the growing power of a light source.
However, the PAR measurement has two fundamental flaws. First, the wavelengths between 380nm to 400nm and 700nm to 880nm are excluded from the PAR measurement. Second, all photons are weighted equally regardless of wavelength.
The Mcree Curve clearly shows plants respond to energy outside the 400nm to 700nm PAR range. Furthermore, plants respond differently to energy within the PAR range.
A PAR meter only measures photons between 400nm and 700nm. As you can see in the example above PAR does not distinguish which photons of light are present, rather it only counts the total amount of photons present in those nanometers.