YPFD (yield photon flux density) is a photon flux in which each wavelength is weighted by its relative quantum efficiency for photosynthesis, based on McCree's measurements on crop leaves in 1972. Red photons around 600–650 nm get the highest weight; blue and green photons count for less. It is reported in µmol/m²/s like PPFD, but the value is always lower than the unweighted PPFD for the same light.
In practice
Few hobby meters report YPFD; spectroradiometers do. It is useful for comparing spectra on paper: two fixtures with identical PPFD can differ by several percent in YPFD because one has more blue. For daily growing, PPFD remains the working number, and the difference between spectra is small compared with the difference between too little and enough light.
The McCree curve was measured on single leaves under short exposures. It ignores far-red (above 700 nm), which contributes to whole-canopy photosynthesis when mixed with other light, and it does not capture the shape and leaf-thickness effects of blue light. Treat YPFD as a refinement, not a replacement for PPFD.
Don't confuse with
- PPFD: every photon from 400 to 700 nm counts equally.
- ePAR: extends the counted band to 750 nm but does not weight wavelengths.
