7-colour LED's - why they work

Plants, like most land-based life forms, need the correct spectrum of light to allow the process of photosynthesis and uptake of CO2.

Plants, like most land-based life forms, need the correct spectrum of light to allow the process of photosynthesis and uptake of CO2 (one of the basic building blocks for life). They also need good humidity (high humidity opens the stomata, which is the pore in the leaf through which the CO2 enters the plant and Oxygen exits). Humidity is not to be confused with condensation, which is generally bad for the plants and causes mold and other problems. The plant also requires water and nutrients.

LUX is a measure of "brightness" in a given area and Lumens are a measure lux over a 1M2. Spectrum requirements differ by species, but most herbaceous plants require light in the blue and red area's of the spectrum. Lux places different weighting to different parts of the spectrum, as related to the human perception of light. Lux is most heavily biased towards 555nm, which is green in appearance (the area of the spectrum in which humans see the best). Unlike humans, plants do not require green light, which is why most of the green is bounced back at your retina by the plant when you look at it.

Watts are an equally bad way of measuring light as Lux and Lumens are. It's just a simple measurement of expressing the energy consumption of a given lamp or device etc. It does not tell you how much of the energy is being converted to something useful, just that the amount of "load" it will place on a given circuit. The only way to realistically compare one crop with another is by looking at the yield per watt/per meter, but even this method ignores all the other variables that relate to it, like ambient temperature and background CO2 and humidity levels.

For this reason, the argument around the use of 3w LED's or 1w LED's is a bit of a red herring, the wattage will dictate the intensity at the point of generation, not at the leaf surface, where it matters. That said, 3w light arrays are cheaper to manufacture and for this reason, the market is moving towards them.

Unlike LUX and Lumens (which have a direct relationship to the human perception of brightness), PPFD umol/M2/s is an abbreviation of Photosynthetic Photon Flux Density by unit per meter per second. This measure looks at the spectrum from the perspective of the plant and measures the volume of the light provided by the light source, over a given space and instant in time.

PPFD is a measure of light quantum (photon quantity) of photo-synthetically active radiation (light energy of an appropriate spectrum, which reaches the surface of the plant, in a given time and over a given area). Plants speed of photosynthesis depends on the light quantum absorbed by the plants. Photosynthesis depends in part on the ability of the stomata to exchange CO2 for Oxygen. You can think of the stomata as the plants mouth for the consumption of CO2, which is stimulated to open by the light landing on the leaf surface. Whereas the roots are the plants mouth for the consumption of nutrients. PPFD is a measure of the availability of the correct food (light spectrum) in a moment in time and over a fixed area of the plants foliage.

PAR (Photosynthetic Active Radiation) is a measure of the available light to stimulate photosynthesis within a given spectrum. It's measured or expressed using PPFD umol/M2/s.

If the light (of the correct spectrum) was water contained in a watering can, you could pour it out of the spout very quickly into a small area if you do not use a rose on the end. However, if you attach a rose to the spout, you can pour the same amount of fluid out of the can in the same time, but over a greater given area. Light is measured in this way using PPFD, as it looks at the speed and quantity of the delivery of the light, whilst considering the spectrum and the area covered.

When the photon flux (light radiation intensity) reaches the highest point (also called the light saturation point), plants have the quickest carbon dioxide exchange speed (carbon dioxide taken in and oxygen/moisture released).

Carbon Dioxide (CO2) is a material required for photosynthesis as the plant uses it to build cells, be they leaf, flower, root or stem cells. These elements of the plant all require CO2 to make them happen and light drives the rate at which this can take place. For example, the light saturation point for many common herbal plants is around 1512 umol/M2/s). When it reaches this point it's at its maximum carbon dioxide exchange rate. This means the plants grow bigger, faster.

LED's have always been good at getting the right spectrum out, but they were really bad (until now!) at getting the light they produced to the plant. They couldn't penetrate the space in front of them and get to the plants leaf surface to make a real difference. Now the latest generation of LED's contain lamps that will penetrate to 2M and produce a 7-colour spectrum that is perfect for your plants to thrive in. Pink or purple in appearance, these lights do not waste energy producing spectrum your plants can't use or don't really need (like green).

In order to get the best results from the new generation of LED lights they have to be used correctly. The PPFD umol/M2/s of a good quality 234w (78x3w) 7-colour light is around 756 PPFD umol/M2/s each. If you use two of these lights in 1M2 tent, you will be producing enough of the correct light to produce 100% of the plants maximum photosynthesis rate (all things being equal, like temperature and other environmental variables).

See fig 1. for details of PPFD umol/M2/s plant needs by tent size for a guide to light selection and quantity required. Please note the matrix gives alternatives, so a 1.2m x 1.2m tent needs 2177 PPFD and could take either 3 x 234w (2223 PPFD) or 2 x 312w (1976 PPFD). The best option would be 3 x 234w for light spread (getting the light to the top of the plants foliage over the whole surface area), although it would be marginally more expensive at the point of purchase. REMEMBER not all LED's are the same, check the specification of the lights you are considering before you buy them or ask the retailer for advice.

Used correctly you can easily expect to exceed like for like HPS grown crops. The HPS system will also cause significant issues around environmental control as a result of heat and high transpiration rate (sweating) of the plants, leading to nutrient burn and shock.

The PPFD umol/M2/s of the LED's is complemented by the reduced stress the plants feel, the lack of fan noise (from the fans going at full tilt, trying to get the heat of the HPS out the room), the reduced nutrient and water requirements of the plants. More easily controlled CO2 systems and humidity controls. You can also expect 20-80% energy saving (depending on the amount of lamps and what they are compared against (HPS or CFL etc.)).

LED's have had some rightly deserved bad press in the past, but that was because the lamps were not at a stage of development that would deliver on the high expectations the market had. Often they were not used correctly. This should no longer be the case as repeated tests have demonstrated, it's a matter of changing what you do and how you do it, this way the technology delivers, rather than disappoints! The future of the LED is assured, growers will feel and see the benefits and the plants will yield the results we all want.

The truth is out there.