Photosynthetic Active Radiation (PAR) was derived from the McCree Curve. It is a total count of light energy (in photons) between 400nm to 700nm. PAR is the defined range of light energy that most effectively stimulates photosynthesis in plants.

The PAR measurement (PPF) has quickly become a popular metric of the growing power of a light source. However, the PAR measurement has two fundamental flaws.

  1. Wavelengths between 300nm to 400nm and 700nm to 800nm are excluded from the PAR measurement.
  2. All photons are weighted equally regardless of wavelength.

The McCree Curve clearly shows plants respond to energy outside the PAR range. Additionally, weighing all photons equally does not accurately represent the output.

As you can see in the example above, PAR does not distinguish which photons of light are present; it only counts the total amount of photons present in those nanometers. Using a PAR meter will only measure the photons between 400nm and 700nm and display them as equals.