Becoming Enlightened - Deciphering the language of LED

Choosing the right product is rarely easy, at least if you do it properly.

Yeah, anyone can look at a range of similar products, be it shoes, cars, holidays, or baked beans, and quickly decide “That’s the one for me, I love the look of that.” But if you plump for a product just because it tickles your fancy, or it suits the depth of your pocket, you might be in for a disappointment. An attractive appearance or bargain-basement price might be hiding poor quality, bad design, or something that just isn’t fit for your purpose.

It's always advisable to do a bit of research beforehand. In some cases, just ask around your friends, see what they recommend. Call upon their acquired wisdom and experience. It’s better to be told that “so and so bought such and such and it was useless!” than to make the same mistake yourself. And if you hear that half of your mates at the gym already have that phone that you’re lusting after, and they absolutely love it, chances are that it’s a safe buy for you too.

But some items are just so new to you that you need to get educated about them first. Particularly if it’s something technological. If you’re after buying a new TV, a computer, or even a washing machine, there’s a whole new language you need to become fluent in before you can even understand the advertising spiel, let alone decide with confidence.

I guess that you chuckled about the washing machine, didn’t you? Well, I had to replace my old one last year. I wanted one with a fast Economy cycle like the old one I had, so it wouldn’t run for ages and use a lot of electricity. I couldn’t find one, and after a lot of bewildered head-scratching I stumbled across some advice from a manufacturer, on the internet. I learned that washing machines aren’t made that way any longer. “Economy” washing cycles used a lot of water at high temperature to shift the dirt from your clothes rapidly. That’s great, but it’s more economical, and better for the environment, if the machine uses less water. Less water means less electricity needed to heat the water, and heating the water is by far the biggest part of the energy used in a wash cycle. But to clean the clothes properly with less water, it takes a little more time. It proved to be true – my electricity use has reduced since I bought the new machine, despite its longer cycle time. So, if you’re buying a washing machine, be wary of that word “Economy”.

Nowhere is terminology more confusing than the LED lighting market. We growers are used to dealing in Watts and Lumens – the amount of energy that a lighting rig will consume, and the amount of light that it will give out. Simple enough. The more technically minded among us may look into the spectrum that a lamp will provide, or its colour temperature, because we know that certain parts of the visible spectrum are better for photosynthesis, and that different colour temperatures are more suited to propagation, to growth, and to flowering. Armed with the knowledge we have accumulated we can confidently choose the best lighting equipment for our particular growing needs. But LED?

The terminology of LED grow lighting can seem baffling at first. I reckon that it’s best to get behind the sales pitch, the heaps of near-unintelligible data that is thrown at you, and first get down to basics – the plants, and what they need. Excuse me if I seem to be oversimplifying all this. I’m not an expert and I don’t mean to be patronising. I just want to make things clearer.

Plants have evolved to use light, usually from the sun, in a process called photosynthesis. The green leaves of plants contain a pigment called chlorophyll, which the plants use to absorb light energy. This energy is then used to break down water (H₂O - mostly taken into the plant via the roots) and carbon dioxide (CO₂ - also absorbed through the leaves), and to synthesise carbohydrates – starches and sugars – which the plant uses to grow and thrive. As a by-product of photosynthesis, plants’ leaves also release oxygen, which is waste material to the plant, but vital to animal life, including ourselves.

The Sun gives out an enormous amount of electromagnetic energy. At the top of the Earth’s atmosphere this energy, known as the “Solar Constant”, has been measured by satellite. The Solar Constant is around 1360 Watts per square meter (1360 W/m²). The thickness of the atmosphere, and clouds, screen out a lot of this energy so that only just over half of it reaches the Earth’s surface, mostly in the form of heat and light. The weather, the seasons, the time of day, and the latitude (distance from the equator) also cause the amount of sunlight reaching a particular spot to vary. Plants have adapted to this variable power source, and they don’t need full sunlight to do well.

Plants use white light, which is made up of all the colours in the visible spectrum, which is electromagnetic radiation in wavelengths between just under 400 nanometres (nm) and just over 700nm. The wavelengths of light are very small and are measured in nanometers (nm). A nanometer is one billionth of a meter (1×10⁻⁹ m).

The colours that we see each result from different wavelengths of light, blues having shorter wavelengths and reds higher.  Certain wavelengths, or colours, of light are needed for photosynthesis. These wavelengths lie between 400nm and 700nm. That’s roughly the blue, green. yellow, orange, and red part of the spectrum, with the red and blue wavelengths being most important for photosynthesis. This part of the spectrum is known as Photosynthetically Active Radiation (PAR).

PAR is not a quantity of light, it’s the range of wavelengths of electromagnetic radiation (in this case, light) that is used by plants for photosynthesis.

To bind one molecule of CO₂ for photosynthesis, a plant needs about ten photons of PAR light. Plants need to bind many molecules of CO₂ and so they need a huge number of photons. (The world’s cereal crops alone are estimated to bind almost 4 billion tons of CO₂ each year). To keep the figures manageable, instead of counting photons it’s usual to quantify PAR in micromoles (µmol). One micromole of PAR contains 6.022 x 10¹⁷, or 602,200,000,000,000,000,000 photons.

Plants photosynthesise throughout the light part of the day, (the photoperiod) as photons of PAR light hit their leaves. How much they can photosynthesise is in part dependent upon how much PAR reaches them over time. The rate of PAR availability is known as the Photosynthetic Photon Flux (PPF) and is measured in micromoles per second (µmol/s).
Most LED grow lights will specify a PPF figure. This is the total PAR light output rate of the unit, and one of the most important figures to look out for when choosing an LED light, because it allows you to compare the output rate of different lights. But for the comparison to be a fair one, the LED lights should be about the same size. Obviously, a larger light will emit PAR at a greater rate than a smaller one, and its PPF figure will be greater.

To get around the problem of comparing LED grow lights of different size, some manufacturers use a measure known as Photosynthetic Photon Flux Density (PPFD) which is measured in micromoles per square metre per second (µmol/m²/s). The PPFD simply represents the amount of PAR made available over one square metre of area each second. At latitudes midway between the equator and the poles, at noon on a clear day in midsummer, the Sun emits a PPFD of about 2000 µmol/m²/s (measured at ground level). But PPFD can be problematic when comparing LED lights:-

• A large LED fixture and a smaller one might both claim a similar PPFD. But while the larger one might cover (allowing for light spread) a canopy area of up to 1.5m², the smaller one might cover less than a square metre. Thus to cover your crop with the same PPFD, you would need to buy a larger number (possibly several times as many) of the smaller LED lights than of the larger one.
• The PPFD value varies with the distance from the light at which it is measured. If measured close to the light, the PPFD will be greater. It is possible that a manufacturer might choose to measure the PPFD of his light at the minimum recommended distance between light and canopy, but most growers will position the light at greater than the minimum distance to allow for growth and to avoid scorching. PPFD can also be increased by use of highly reflective surroundings.
• PPFD is also dependent upon the direction of the light relative to the plants. It will be greater directly below the centre of the light than it will be at the fringes of the lighted area.

Another bit of data to look for when choosing an LED grow light is the Photosynthetic Photon Efficacy (PPE) which is the ratio of the amount of PAR light that the LED grow light emits each second relative to the amount of power it draws. PPE is stated in micromoles per second per Watt (µmol/s/W) or in micromoles per Joule (µmol/J). One Watt = one Joule per second, so no matter which units are used, the PPE value remains the same; 2 µmol/s/W is the same as 2 µmol/J. It’s a simple ratio which can’t be skewed in any way, and it tells you how much PAR (light that is useful to your plants) you will get in return for each unit of electrical energy you will provide. To put it another way, PPE is an indication of how much bang you got for your buck, when you come to pay your electricity bill.
I learned all the above so that I could decide whether to make the change from HID lighting to LED, and then to select the best LED grow light for me. I chose the LUMii BLACK LED 720W 6 Bar Fixture.

Two major factors attracted me to the LUMii BLACK LED 720W 6 Bar Fixture; its relatively low price, and the fact that it is powered and controlled by a piece of kit well known to growers all over Europe, the LUMii BLACK 600W Electronic Ballast. Much better to deal with a ballast I know than to struggle to get to know a control unit that I never used before. Also, because the ballast isn’t built-in, it is easy and inexpensive to replace in the unlikely event that anything should go wrong. Better still, if I use LUMii Extension / Link Leads I can set up the ballast outside the growroom, so that any heat it may generate isn’t added to the grow environment. I can also dim or boost the LED Fixture from outside the growroom.

Next, I liked its size and coverage. The length of the light-emitting part of the bars is about 1 metre, and the array of 6 bars is roughly 1 metre wide. Allowing for the spread of light from the bars with distance above the plant canopy, it is an ideal size for a grow tent 1.5 metre square. Since I’m investing in the future, I decided to buy a LightHouse White 1.5m² Tent, because it’s white reflective lining is designed to homogenously reflect LED light and enhance canopy penetration.  Weighing in at 7.8 kg, the LED Fixture will hang from the poles of a grow tent without problems, and if I use LUMii Rope Ratchets I can adjust the height of the fixture above the canopy as the plants develop.

But the technological clincher for me was the LUMii BLACK LED 720W 6 Bar Fixture’s impressive performance. Although it doesn’t boast big-name LED chips, the technology and expertise used to manufacture the chips for LUMii BLACK and to build the fixture is the at the same level, and from the same part of the world as top-branded chips. If you want designer-branded goods, you’ll pay more, but you won’t necessarily get better goods. This fixture puts out the full spectrum of PAR light, and is good for propagation, growth and flowering stages – the whole grow cycle.

It's output rate, or PPF, is a massive 1870 µmol/s. Other brands of LED lights with similar output cost far more than the LUMii BLACK LED 720W 6 Bar Fixture. Its PPF is similar to the PPF of a typical 1000W HPS lamp, but with much better light distribution and far less heat output. Because it doesn’t throw out much heat, the LUMii BLACK LED 720W 6 Bar Fixture uses power more efficiently too. It has an efficacy, or PPE, of 2.6 µmol/s/W. So, running at its maximum, the fixture will draw or 720 Watts of electricity.

That brings us to the question which everyone is asking; how can a 720W fixture be powered by a 600W ballast?
The LUMii BLACK 600W Electronic Ballast was originally designed to power 600W HPS lamps. The electronic components within the ballast consume around 30 Watts while providing a full 600 Watts to a lamp. The LUMii BLACK 600W Electronic Ballast also has a 600W Boost setting, at which it will provide 690 Watts to the lamp. While on 600W Boost setting, the total power drawn at the plug will be 690W + 30W, or 720 Watts. It is at this 600W Boost setting that the LUMii BLACK LED 720W 6 Bar Fixture achieves its maximum PPF of 1870 µmol/s.

For me, getting value for money is important. You can buy a LUMii BLACK LED 720W 6 Bar Fixture and a LUMii BLACK 600W Electronic Ballast as a kit for less than most LED fixtures of similar size and performance. And yet the fixture has a working life expectancy of 50,000 hours – that equates to five years and eight months of constant use. An excellent investment.
To sum up all of the above simply: -

• Don’t be dazzled by jargon – learn to understand what it means
• Some brands are impressive because they are high quality and they cost more. Other brands, like LUMii BLACK, are impressive because they are known and reliable and give you more for less cost.
• LED grow lights are far more efficient than HID lamps. The lower running costs will pay back the higher initial outlay.
• LED grow lights have a much greater working life than HID lamps
• PAR – Photosynthetically Active Radiation – the part of the light spectrum which is useful to plants.
• PPF – Photosynthetic Photon Flux – the rate at which PAR is emitted by a grow light, measured in µmol/s.
• PPFD - Photosynthetic Photon Flux Density the amount of PPF per unit area, measured in µmol/m²/s. Always consider the actual coverage of the light along with PPFD.
• PPE - Photosynthetic Photon Efficacy – the ratio of the amount of useful light emitted to the amount of electrical power drawn, measured in µmol/s/W or µmol/J.