The Story about PPF, PPFD, Lumens and Lux
Confused about what light units to work with?
When you have come across lights in the past, you may have seen tons of different units of light measurements to judge by: Watts, Lumens, Lux and Colour Temperature (e.g. 5400k such as on fluorescent tubes). If you are lighting your home and living space for purely human eyes, these units serve well, you could choose a high lumen bulb to light your living room, or a low lumen one for your bedside light. However, plants are different to us. So suddenly our 40W, 1000 lumen lightbulb does not tell us if it will grow our African Violet or not, as there is no way of knowing whether the light is photosynthetically active or not. Even the light colour, such as 5400K, red, or blue does not, as many such colours also emit normal white light, merely using a filter to change the emitted colour. So what do we use to judge grow lights by? In this article, that very question will be answered!
Lumens, Lux, Geometry and Candles
Lumens is a useful unit, but purely for human use. High lumens can help you illuminate a room, with a typical 75W incandescent bulb or a 25W compact fluorescent bulb emitting 1,600 lumens. So for your living room, you want a few thousand lumens output for decent brightness. This light is anchored to the human eye spectrum, so it is without deep reds or ultraviolet blues, mostly centred around the 550nm (nanometer wavelength) green light.
What is a lumen? It is a unit of luminous flux that is derived from geometry and the light emittance of a simple candle (one candela, another unit). One lumen is equal the emitted light power of one candle, multiplied by a steridian (a geometric term for a certain area of a sphere). So without going further into mathematics, it is based on candles and a human centric luminosity function, which is great for humans, but not plants.
So what is a lux then? A lux is simply a lumen over a 1m2 area, so that is 1 lumen reaching a 100cm by 100cm patch of floor in your room. This is quite handy for measuring the light in a place, as you can judge the level of light with a meter around an area and add lights in dark spots. To give you an idea of scale, 100,000 lux is direct sunlight, but 300 lux is normal room lighting, you can still see with 1 lux however, the light of a full moon. However, is that lux growing your plant with plant relevant light? We don’t know, so we must not use it for grow light usage.
Correlated Colour Temperature (CCT)
Now, if you have had an aquarium or a strip fluorescent light, you will have bought fluorescent tubes for it with a colour temperature scale, typically 5400K, 7200K or 3400K. What this represents is a colour mixture based on natural daylight impression, ranging from the orange flames of a cantle to the light blue of a clear blue sky.
Photosynthetically Active Radiation (PAR)
What plant needs is photosynthetically active light, which we call PAR. This is a mixture of relevant bands of light frequency (related to colour), with peaks around 420nm and 680nm for red and blue light respectively. Other light frequencies are also used and are important to remember however. So how do we measure this PAR light? PPF stands for photosynthetic photon flux. A photon can be thought of as one single light unit without going into quantum physics, so what PPF means is the amount of PAR light molecules hitting a certain area per second. This is typically measured in micro-moles, µmols-1, (a unit of quantity representing many trillions of light photons moving per second), with a typical 1m2 growth area needing to receive between 300 and 1000µmol per second for strong plant growth, depending on the crop. An interesting fact is that during photosynthesis, around 10 photons are used power the consumption of one CO2 molecule. Therefore, we can see how important PPF is when certain leaf organs only accept a specific light frequency for photosynthesis or plant reactions.
Taking PPF one-step further for grow space usage, we have PPFD, which stands for photosynthetic photon flux density. This takes PPF and measures it per unit area, usually in m2. So if your crop is a 20m2 space, you need do deliver enough PPFD to meet the needs of your plants. When choosing your crop you can find out the PPFD requirements and buy grow lights to suit their PPFD needs. It is worth noting that PPFD varies a lot considering how close the light is to the plant, in an inverse square law. This is because typical grow lights spread light in a sphere, cone or pyramid shape, scattering light in a pattern which wastes light, with more wastage being spread at taller heights. While the PPF as described above is the total output of the lamp (regardless of the design), the PPFD provides the information on how many photons actually reach the plant canopy. A lamp can have a massive PPF, but could fail in getting the photons onto the canopy due to ineffective distribution of light (i.e. hot spots) as well stray losses to the sides.
You may have heard of the plant light spectrum or PAR spectrum now and are wondering what it is. The plant light spectrum ranges from 400nm to 700nm and consists of specific light frequencies used or for other specific uses within. What PPF measures is a light mix purely within this spectrum, exactly what plants use. Within this spectrum the balance of light can be varied to suit plant and grower needs, this is what we are doing at excite to produce weird and wonderful results.
Grow Light Usage
What does this mean for you, the grower? Well, armed with this new information, you can judge grow lights by their emitted PPF and PPFD to understand what grow lights actually deliver. Many grow lights such as HPS or MH emit a core PPF, but lots of other waste light and heat. For every 300µmolm-2s-1 of PPFD a plant receives, the required power rating varies dramatically depending on the type of grow light.
Here is a comparison of the PPF/Power efficiency examining each grow light, and projected annual costs. As we can see, LEDs are the clear winner in this comparison, delivering the most PPFD for the least annual cost. PPFD varies also depending on the light patterns produced by the grow light fixture.
Therefore, what can you take from all this? Here is a summary of what you need to know about measuring light to help choose better grow lights:
- Lumens and Lux should not be used to choose grow lights; those measurements were created for human needs, not plant needs.
- Photosynthetically Active Radiation (PAR) and Photosynthetic Photon Flux (Density) PPF (PPFD) are vital to judge how much usable light a plant receives.
- LEDs are the most cost efficient PPFD emitters, with controllable spectra, making them the best grow light to choose to grow strong plants.