Photosynthetic Photon Flux Density (PPFD) is the metric used to measure PAR over a square meter. It is a total count of light energy (in photons) from 400nm to 700nm that reaches a particular point on a plane. 

Photobiological Photon Flux Density (PBFD) is the metric used to measure PBAR over a square meter. It is a total count of light energy (in photons) from 350nm to 800nm (or the PBAR range used) that reaches a particular point on a plane. 

PPFD and PBFD are both expressed as μmol/m2-sec. See how PPFD and PBFD are measured below.

Measuring PPFD and PBFD

PPFD and PBFD are measured using a light meter. A light fixture will be placed a distance above a plane and measurements will be taken at points within a specified coverage area.

The output of light from a fixture will vary based on the position of the light meter. For example, a light meter placed directly underneath the center of a fixture may reveal a higher PPFD/PBFD number than when placed in the corner of a 4ft x 4ft square. This is due to the reflector and the properties of its design.