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Lumens are for humans

It’s all about photons

Many lamp manufacturers still specify the output of their lamps (illuminance) in lumens, though this just specifies how we humans percieve the intensity of that light. Our eyes are most sensitive to green light of 555 nm, but plants are more sensitive to a much broader spectrum. So what is the right way to specify horticultural lamps and how can you calculate with that? What’s in it for you? Enter the photons.

PAR spectrum

Plants primarily use the light ranging from 400-700 nm bandwidth (violet to far red). The light within this bandwidth is called Photosynthetic Active Radiation (PAR). So the bandwidth of the light that plants are sensitive to is much broader than what we see. Using lumens, which are measured according to what the eye is sensitive to, is therefore not a correct representation of the grow light properties of a lamp.


Scientists proved that there is a relationship between the number of photons and the photosynthesis: It takes about 8 – 10 photons to bind one CO2 molecule. They also discovered that there is little difference in the effectiveness of blue or red light. So there is a direct relationship between the number of photons in the PAR spectrum and the photosynthetic potential of a plant (and ultimately the yield of a plant).

For many years now professional researchers have used photon counts in the PAR spectrum as a standard and the greenhouse industry followed very quickly. Most European horticultural lamp manufacturers specify the output of their lamps in PAR photons per second. Because photons come in large numbers we uses a multiplier, in this case Avogadro’s constant (6,0221415 × 1023) to get an expression in mol. 1 mol photons is 6.0221415 × 1023 photons. Now that’s a lot of photons and to get that to levels that become easier to comprehend they are divided by 1 million, thus creating micro-moles (µmol). So 1 µmol is 6.0221415 × 1017 photons.

To illustrate why µmol work a lot better for us: the PPF of a 600W HPS lamp is about 1100 µmol/second. If you would express that in moles it would be 0,0011 mol/s. Now that’s a bit more difficult to calculate with.

Photosynthetic Photon Flux (PPF)

Photons are counted per second as we count a flow or flux of photons. If you count all the photons that a lamp emits in the PAR spectrum per second you get the Photosynthetic Photon Flux (PPF). The only way you can measure this accurately is in an integrating sphere, the Ulbricht sphere. So the PPF is measured in µmol/s and represents all the photons in the range of 400-700 nm per second. But how much ot that will reach your plant and at what distance?

Photosynthetic Photon Flux Density (PPFD)

Let’s say we mount the lamp in a really good horticultural reflector, which has a total efficiency of 95%. That figure means that of the original 100% light of the lamp, 95% is totally emitted by direct light from the lamp or reflected light from the reflector. You could also say your reflector losses are 5%. Now if you spread your 1100-5% on a surface of 1 square meter, you would irradiate 1045 µmol/m2/s (1045 µmol m-2 s-1). This is called the Photosynthetic Photon Flux Density. If I would move closer to the source and would just light half a square meter the irradiance would be 2090 µmol m-2 s-1. And of course spread over 4 m3 you would get 261 µmol m-2 s-1. Double the surface means half the PPFD. Just divide the PPF by the lit surface in m2 to get close to the calculated PPFD. You will always have some stray light losses (much more with open reflectors!) and you have influence by the reflection of the walls, which causes a loss.

PPFD you can easily measure with a quantum meter and a sensor that is specifically designed for the PAR spectrum. Unfortunately real quantum meters are expensive. The Li-Cor meters are used throughout the industry and are recommended. Most meters under $500 use lumens sensors and an internal table to approximate the PPFD in micromoles. We have found them to be inaccurate because they are still more sensitive to certain colors and do not take other colors within the PAR spectrum into equal account.

And how about spectrum?

PPF and PPFD only qualify the amount of photons, and not the quality of the spectrum. If spectrum was not important you would be able to grow any plant under just a single color red LED for example. Plants need different colors for different processes. The color of the light specifically influences the shape, build and development speed of the plant. In greenhouses the sunlight provides quality light. The HPS lamps are just used for extra photons, for quantity. So yes, spectrum is important, specifically when growing indoors where there is no sunlight. Plants have developed under sunlight for millions of years so you can expect them to be adapted for that spectrum and they use all of it as efficient as possible.

Calculating with micromoles

If you know how much light you require for optimal growth of your plants it is easy to calculate how much lamps you need. There is one complicating factor, and that is walls. Walls reflect only part of the light, as low as 40-50% depending on the reflective material. When using diffuse reflection materials not all of the light reflected will reach your crop. So there are serious losses at the edges of your grow room. The bigger the grow room, the less the wall effects. One way to solve the problem is to keep your final fixtures closer to the wall than half the distance between fixtures in the room to allow for some more direct light and reflection at the sides to even out the overlap. An adjustable reflector that sends the light down at the wall side can save you a lot of light.

When you have a room with many lights you will have a great advantage when you overlap your light. Hanging your lamps higher from the crop will create a bigger spread and a lower PPFD per fixture, but you can add the overlap from the other lights so you will still have the same light on your crop but at a greater distance. This is much easier for climate control and a more uniform light coming from different directions, enabling a better penetration in your crop.

Roughly these are a few examples of recommendations for a high light recipe of around 700 µmol m-2 s-1. Calculations made with 10% reflector / wall losses:

  • 400W a) – 1 x 1 m – 1 m2 at a ppfd of ~ 650 µmol m-2 s-1
  • 600W b) – 1,2 x 1,2 m – 1,44 m2 at a ppfd of ~ 690 µmol m-2 s-1
  • 1000W c) – 1,5 x 1,5 m – 2,25 m2 at a ppfd of ~800 µmol m-2 s-1

In practice levels can be lower with different reflectors (open reflectors will have more stray light), older reflectors and a lot of wall influences. Other lamps may result in different densities.

  • a) – Philips GreenPower 400W 230V – ppf 725 µmol
  • b) – Philips GreenPower 600W 230V – ppf 1100 µmol
  • c) – Philips GreenPower 1000W 400V Electronic – ppf 2000 µmol


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