Yield Photon Flux (YPF) is a measure of light intensity, weighted based on the light wavelength's usefulness to plants when it is the only color of light given to a plant. Unlike PAR lighting, which considers only photons in the 400-700nm visible light spectrum and weights each photon equally, YPF considers photons from 360-760nm (ultraviolet through near-infrared) and weights each photon based on the plant's photosynthetic response to the particular wavelength of light.

The weighting employed by YPF measurements eliminates some of the shortcomings associated with PAR measurements. For example, 100 photons of pure-green light have the same PAR value as 100 photons of red light, even though most of the green photons will be reflected by the plant while most of the red photons will be absorbed. YPF accounts for this and 100 photons of green light have a lower YPF than 100 photons of red light.

Unfortunately, YPF still has shortcomings when used as a measure of how well a particular light will grow plants. YPF does not account for the fact that different wavelengths of light are used by plants to initiate different biochemical reactions and that a wide spectrum is necessary to grow plants to their maximum potential. An all-red light will have a higher YPF score than a broader-spectrum light containing all the wavelengths of light plants require for photomorphogenesis (creation of secondary metabolites such as pigmentation, flavonoids, THC and CBD).

Like PAR, Yield Photon Flux levels are also measured at a single point, which does not indicate how well plants will grow over the entire footprint of an artificial light source. If light is being focused into a narrow beam by secondary lenses, the YPF score in the center will increase, even though plants can only be grown directly under the light. The inverse square law of light means that YPF measurements will decrease by the square of the distance from the source-- so if a YPF measurement is 100 at 1 inch from the fixture, it will be 25 at 2 inches, and 11.1 at 3 inches. As with PAR, it's easy to claim a high YPF reading for a light fixture if the measurement is taken close to and directly below it.

YPF is a better measurement of how useful light is to plants than PAR, but still shares most of the shortcomings of PAR. A red laser pointer has an amazingly high YPF score but can't be used to grow plants.

Only by considering multiple Yield Photon Flux measurements taken over the entire footprint of the light, at the recommended hanging distance above the plants, and considering the entire spectrum, can useful comparisons be made between grow lights.