Grow Lights - An In Depth Look

When it comes to grow lights one will find there is a lot to know and so many different types of grow lights out there, so how does one decide on what light to choose or even decide if the light you want is good or why one light is better than another?

To do this we need to have a standard that can be used to compare lights and this is what we refer to as PAR (Photosynthetic Active Radiation), PPF (Photosynthetic Photon Flux) units are umol/s and PPFD (Photosynthetic Photon Flux Density) units are umol/m²/s. But what does this mean? PAR literally refers all the light within the wavelength 400nM to 700nM which is the light most commonly used by plants for photosynthesis and it doesn’t tell you how much or how good it is but only that its there. PPF refers to how much PAR is hitting a particular point and PPFD is the average amount of PAR hitting a set area. I heard a very good analogy to explain this, think of the light as a rain cloud and the rain as PAR and a field as your plant, the PPF would be the amount of rain hitting a single spot so obviously a higher value would be nicer but what about the rest of the field? Its no good having all the rain hitting one point it must spread over the whole field and the average amount of rain hitting the entire field is referred to as the PPFD. So a light that has a high PPF value over a wide area is much better than one with a high PPF value over a small area. Most manufacturers have a chart on their website that shows the PPF values over a set area for their light for example here is one from the Horticulture Lighting Group.

So we can now compare one light to another but what is it we are looking for? When it come to growing certain plants they require different levels of PPFD for their different growth stages, for cuttings or propagating one is looking for 75 – 150 umol/m²/s, for seedlings 100 - 300 umol/m²/s, for the vegetative stage 300 - 600 umol/m²/s and for flowering over 600 umol/m²/s is preferred. Every plant has its own preferred range so this is only a rough indication you will see if you plant is getting too much light for a particular stage in its life by looking for nutrient burn, an excess of light will cause the plant to take up too much nutrients and cause a build up of nutrient leading to nutrient burn which is characterised by the edge of the leaves and tip turning brown.

Knowing this it makes it a lot easier choosing a light, if you want a single light then get one that can handle the flowering stage and have a PPFD of 600 umol/m²/s or higher, you can raise this light up higher to reduce the PPFD so you can use it for the other growth stages as well.

As a bit of general information but not really necessary for choosing a light this is a quick summary of what the different wavelengths are for with regards to plants.

• 439 nm is the blue absorption peak of chlorophyll a.
• 450-460 nm is the royal blue that is absorbed by one of the peaks in beta-carotene. It is a readily available LED wavelength commonly used to excite the remote-phosphor in white LED lamps.
• 469 nm is the blue absorption peak of chlorophyll b.
• 430-470 nm is a range that is important for the absorption of chlorophyll a and b, which is key for vegetative growth.
• 480-485 nm is the second absorption peak of beta-carotene.
• 525 nm (green light) is a phototropic activator that researchers are still trying to find the chromophore of. Green light isn’t important for photosynthesis, but it is apparent that plants are gaining direction and environmental signals from it, and that it affects internodal spacing. This is also the wavelength of GaN or InGaN green LEDs commonly used in RGB and tunable applications.
• 590 nm is key for carotenoid absorption. Carotenoids are starch-storing, structural and nutritional compounds.
• 590 nm is additionally the phycoerythrin absorption wavelength. Phycoerythrin is a red protein-pigment complex from the light-harvesting phycobiliprotein family, present in red algae and cryptophytes, and is an accessory pigment to the main chlorophyll pigments responsible for photosynthesis.
• 625 nm is the phycocyanin absorption peak. Phycocyanin is a pigment-protein complex from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is also an accessory pigment to chlorophyll.
• 642-645 nm is the peak absorption point of chlorophyll b.
• 660 nm is often called the super-red LED wavelength and is important for flowering.
• 666-667 nm is the peak red absorption point for chlorophyll a.
• 730 nm, often referred to as far-red, is important for phytochrome recycling. It is needed for all kinds of morphogenic (shape-forming) processes. A few minutes of 730 nm light treatment after the full light cycle is over will revert the phytochrome chromophore from activated to inactive. This resets the chemistry for another lights-on cycle and may be useful in shortening the classic dark side of the photoperiod. This color is important to plants but is not considered in PAR as it is outside of the 400-700nm PAR range.

Now that is a lot of info and goes to show how important a lot of the different wavelengths are so in that regard it is much better to go for a full spectrum light than a light that only targets certain wavelengths.

With all of this you can now decide for yourself what grow light you want whether it is an LED, HID or fluorescent. Some general notes on these, the fluorescents can work for growing plants and produce little heat and are energy efficient but they have a much lower PPFD value than LEDs or HIDs for equivalent energy consumption, HIDs are energy hungry and produce a lot of heat but have excellent PPFD values, however LEDs are the best route to go as they produce the least heat are energy efficient and can have excellent PPFD values if its good LED light.