PAR (Photosynthetically Active Radiation)
PAR looks at the wavelength range of 400nm to 700nm.
In plants, the light is reacting with Chlorophyll a and b.
Photosynthetic Photon Flux (PPF)
Neither it tells us something on the composition of the photons and wavelength of these, having a big impact on the efficiency used by crops.
Is current PPF PAR measurement realistic and relevant?
Today’s method of weighing the spectrum is not really adequate.
The whole spectrum is weighed equally by counting the photons in the photosynthetically active region (PAR).
More realistic approach
Weighting the emission spectrum of the light source with plants’ spectral sensitivity curve (“plm/W”)This curve is derived from the chlorophyll absorption spectrum taking into account internal energy transfer processes of the plant / leaves.
Photosynthetic action spectra for the green alga Ulva (two cell layers) and higher plants (multiple cell layers).
Keep also in mind that wavelengths above 700nm which have a major impact on. Phytochrome Pfr don’t count in the PAR numbers, neither do UV and wavelengths below 400nm.
Yield Photon Flux (YPF)
However, because short-wavelength photons carry more energy per photon, the maximum amount of photosynthesis per incident unit of energy is at a longer wavelength, around 660 nm, what is also called deep red.
Some longer-term studies with whole plants in higher light indicate that light quality may have a smaller effect on plant growth rate than light quantity.
This law explains us why photons in the red spectrum have a higher impact on plant photosynthesis than for example a blue photon.
Or simply expressed as a rule of thumb, the energy of a photon µmol in J ≈ 120/ λ where λ = the wavelength of the photon in nm.
Maximum energy of a blue 450 photon ≈ 450nm/ 120 ≈ 3.75µmol/J
Photosynthetic Photon Flux Density (PPFD)
The amount of light that actually reaches your plants and algae within the PAR region or the number of photosynthetically active photons that fall on a given surface each second.
Expressed in μmol/s.m².
This also explains why LED grow lights foreseen from advanced optics outperform those without optical controls. Similar to white light applications, a LED emitter has a rather large light distribution – without corrections by optics controls a big piece of the emitted PPF energy doesn’t land on the canopy where you want it.
Optical controls by TIR (Totally Internal Reflection) lenses also improves the leaf canopy penetration in a similar way a diffuse greenhouse glass creates more light scattering and a better homogeneous light distribution over the leaves.
Light is more homogeneously distributed under diffuse light (B) compared with direct light (A) where many sun flecks in the middle and lower part of the canopy are seen. (Li et al., 2014a/b, photo courtesy of Wageningen UR Greenhouse Horticulture, Bleiswijk)
Greenhouse light spill leading to lower PPFD values and enormous energy waste. Light spill is defined as all the energy which is created and is not absorbed by the leaf.
Daily Light Integral (DLI)Daily Light Integral (DLI) measures the total amount of light that is delivered to a plant every day.
DLI is a cumulative measurement of the total number of photons that reach the plants and algae during the daily photoperiod.
DLI measures the number of “moles” of photons per square meter per day and is expressed as mol/d.m².
Photon EfficacyPhoton Efficacy refers to how efficient a horticulture lighting system is at converting electrical energy into photons of PAR.
With the PPF and the input wattage, you can calculate the efficiency.
Expressed in µmol/J.The higher the number, the more efficient a lighting system is at converting electrical energy into photons of PAR.
But remember this number doesn’t tell us anything about the effectiveness of the light on your crops and doesn’t count the light frequencies above 700nm.