PAR (Photosynthetically Active Radiation) is NOT a measurement or “metric” like feet, inches, or kilos.
Rather, it defines the type of light on spectrum of light where plants respond the best by photosynthesis.
Through photosynthesis, plants convert light energy into chemical energy, which is the food they use to grow and thrive.
In plants, the light is mainly reacting with Chlorophyll a and b.
PAR looks typically at the wavelength range of 400nm to 700nm.
In the PAR zone we measure the light which falls on the crop, which is expressed as PPFD or Photosynthetic Photon Flux Density in micromoles per meter per second µmol/sm² – see definition below
In the past we always expressed the light levels in greenhouses in Lux.
Lux or Lumens is a measure of perceived light by the human eyes which typically react on white light.
Our eyes have 3 receptors for light, the S, M and L receptors, which are reacting mainly on blue, green and yellow light.
The curve where lumens and lux are measured is illustrated above – as you can see the start and end point is the same as the PAR zone, but blue light and red light are not taken into account as much as when we measure plant lighting.
Above illustration shows the difference between the light sensitivity of human eyes and the sensitivity of plants.
As plants chlorophyll production is the most effective with blue and red light photons, we can’t express the light levels of LED grow lights any more in Lux like we used to do.
Therefore the new measure is expressed in micromoles per meter per second and includes all the light photons from 400 nanometer up to 700 nanometer.
The PPF or Photosynthetic Photon Flux is the total amount of light in the PAR zone that is produced by a light source each second.
So PPF measures the “photosynthetically active photons emitted by a lighting system per second”.
Expressed in μmol/second.
With the PPF of a grow light, you can calculate or estimate how many lamps you are going to need per area to reach your required light level on the plants.
Take as an example a strawberry grower who is going to do a winter cultivation with a light level of 200µmol/sm².
With an 8 meter cap and 5 meter pole distances each growing area is 40m².
So we need in total 40m² x 200µmol/sm² = 8000µmol of light for this area.
If a single LED grow light produces a PPF of 2000µmol/s, you require 4 lamps per growing area.
The PPF does not tell us how much of the measured light actually lands on the plants or any other surface.
So keep in mind not every lamp with the same PPF is as effective in bringing that light down to the crops.
Neither it tells us something on the spectrum of the light and the wavelength of these photons.
More about that you can discover below.
Is current PPF PAR measurement realistic and relevant?
Today all photons in the whole spectrum are weighed equally, what makes that the values are somewhat difficult to compare.
Take for example the PAR value of HPS SON-T from recent generations – the PAR value you get can get as high as 2.1µmol/J while this value doesn’t tell you anything about how efficient these photons are for your plants to make photosynthesis.
A LED grow light from the same performance (while most recent luminaires produce a much higher efficiency) but with the photons wavelength matching your crop and growth stage, might give you a 25% improvement in growth.
So today’s method of weighing all photons equal in the PAR spectrum is not really adequate.
Situation Today
The whole spectrum is weighed equally by counting the photons in the photosynthetically active region (PAR).
More realistic approach
A more realistic approach would be weighing the photons of the lamps according the 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 and leaves.
Photosynthetic action spectra for the green alga Ulva (two cell layers) and higher plants (multiple cell layers).
Today this method isn’t taken into account as there are too big differences from plant to plant in how they absorb light and on which color spectrum they react the best.
Therefore this method would make it too complex as it is
But keep in mind from what you learned here that just looking at the PPF of a lamp doesn’t mean anything as you should look per crop to the plant sensitivity curve and produce as many light as possible with the energy you have available matching this sensitivity curve.
Keep also in mind that wavelengths above 700nm which have a major impact on the Phytochrome Pfr don’t count in the PAR numbers, neither do UV and wavelengths below 400nm.
Light photons above 700 nanometer, typically called NIR or Far Red, can have some major benefits on some crops and growth stages.
A good example you see from cucumber cultivation, where projects with extra Far Red light show up to 24% higher yields.
The Photosynthetic Photon Flux Density (PPFD) measures the light that actually arrives at the crop canopy in the Par zone.
The amount of light that actually reaches your plants within the PAR region or the number of photosynthetically active photons that fall on a given surface each second.
The PPFD is expressed in μmol/s.m².
The PPFD as of today is the best available measure to compare grow lights in the market.
On lamp level the manufacturer specifies the PPF or light output of the grow light in the PAR zone.
Together with the light distribution of the lamp and the physical placement of the lamps on the project, the PPFD can be simulated.
You can easy see that lamps with a similar light level sometimes don’t give similar results on PPFD on the crops, which mainly comes from the differences in light distribution.
Example of a strawberry project with 2 CoolStack Boost lamps placed per 40m² under the trellis.
In the first place the light needs to be distributed equally over the plants.
But of course every project is different – we see greenhouses with low poles of 3.5 meters as well new projects with high poles of 7 meters.
The distance from the lamps to the crop are as thus totally different.
When we would use the same wide light beam angle on both project of course the light will be well distributed, but with the high poles a lot of light would be thrown to the sides of the greenhouse or would have to travel a very long distance to reach the plants.
So the art of correct lighting is to make sure the light is in the first place well distributed with a minimum of light fluctuations on the crop, but in the second place to ensure a maximum of light falling as direct as possible on the plants with a minimum of light spill.
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 hom*ogeneous light distribution over the leaves.
 |  |
Light is more hom*ogeneously 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.
The Yield Photon Flux YPF weights photons in the range from 360 to 760nm based on plant's photosynthetic response.
So it goes further than the PAR region of 400 to 700nm and also extrapolates the photons to the plant sensitivity curve per crop.
When the exact spectrum of the grow light is known, the Photosynthetic Photon Flux (PPF) values in μmol/s can be modified by applying different weighting factor to different wavelengths and colors.
This results in a quantity called the Yield Photon Flux (YPF).
The red curve in the graph shows that photons around 610 nm (orange-red) have the highest amount of photosynthesis per photon.
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.
The YPF curve as shown below was developed from short-term measurements made on single leaves in low light.
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.
Still it is very much crop depending and light system depending.
With supplemental grow lighting in a greenhouse there is still a large portion of the daily light perceived from the sun.
In that case we mainly focus on supplementing extra light which directly generates chlorophyll production, so Red and blue.
The tendency is to add also extra white LEDs on it so that the overall perception of the light is more neutral to work in.
This is why you see many times in early installations this purple color in greenhouses with LED grow lights.
The relation between light and the energy it can carry is described in the Law of Planck.
This law explains us why for the same amount of energy photons in the red spectrum have a higher impact on plant photosynthesis than for example a blue photon.
Energy of a Photonλ = hc/ λ = 2x10-25 (J m)/ λ
Energy of a µmol (≈ 6x1017) Photonλ ≈ 12x10-8 (J m)/ λ
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.
From this relation between the light wave length and the energy, we can conclude that red photons in the 660nm bandwidth can carry much more energy than longer wavelengths like blue 450nm.
Maximum energy of a deep red 660nm photon ≈ 660nm/ 120 ≈ 5.5µmol/J
Maximum energy of a blue 450 photon ≈ 450nm/ 120 ≈ 3.75µmol/J
Of course above values are absolute maximum ratings.
At this moment the highest efficacy what has been reached in a 660nm wavelength LED package is 4.2µmol/J (Osram Oslon Square V4 – Q1 2020).
The most efficient LED grow lights today produce an efficiency around 3.5µmol/J.
This also indicates that in the future still major improvements on efficacy can be expected.
The 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.
The DLI measures the number of “moles” of photons in the Par region per square meter per day and is expressed as mol/d.m².
The DLI is a good way to implement in the light strategy in greenhouse project with supplemental lighting.
For most crops you can define what is the ideal total light sum per day they can efficiently use.
The total light sum is the sum of the light perceived from the sun + the sum of the artificial light per day.
Of course your climate computer doesn’t tell you much about mols per day.
Therefore you have to calculate back from the solarimeter values in J/cm² to mols to make a total sum per day insight.
Keep in mind the solarimeter is on the roof, so you have to deduct the transmittance of the greenhouse glass from this value.
From solarimeter to DLI from the sun - J/cm² to mol/dm² conversion:
DLI from the sun = ((measured J/cm²)/100) x 2.15 x glass transmittance %
What the plants receive extra from the grow lights can also be calculated – therefore you convert your PPFD light level to DLI with the numbers of lighted hours
From PPFD to DLI conversion – μmol/sm² to mol/dm²:
DLI from the grow lights = (hours x PPFD x3600)/1.000.000
We also list some useful indicators per crop under the Typical PPFD and DLI values per crop.
Photon 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.
