There was a time when choosing a grow light was simple – all you needed to know was what brand and wattage you wanted. Things have changed. Drastically. The market has been flooded with grow light options and it can be quite challenging to figure out which light is best for you. Below is some advice on how to know what a truly great LED grow light looks like.
Micromole per Joule
The industry standard for measuring grow light efficiency is micromole per joule (sometimes written as umol/j, μmol/j, umolj-1 or PPF/W). That means that for every joule of electrical energy (joule = watt * second) a certain number of photon micromoles are produced. One micromole = 602,000,000,000,000,000. That’s a dizzyingly large amount of photons!
Highly efficient LED grow light range from 1.5 μmol/j and up (this number is constantly improving). Many of the most efficient LED grow lights are low power and, as a buyer, you may have to weigh out whether you are looking for efficiency or intensity, because at times one can be at the sacrifice of the other. Top brand HPS (high pressure sodium) lights are around 1.7 μmol/j.
Wall Plug Efficiency
Another measurement relevant to DIY LED grow light enthusiasts is wall plug efficiency (WPE). This is a ratio of the amount of energy put in and the amount of light produced. This can be expressed as a percentage, such as 60% “wall plug efficiency,” which means that 60% of the electricity that goes through the light gets converted into light. The rest gets turned into heat that will need to be dealt with in the grow light itself as well as the room that houses the light.
It’s not typical to rate a grow light’s wall plug efficiency, but high-quality diodes made specifically for horticulture occasionally have this listed. For instance, high quality blue LEDs at 450nm can reach wall plug efficiencies of 60%, red LEDs at 660nm with 50% WPE, and green 530nm with 25% WPE. Wall plug efficiency can be calculated using a diode’s radiant flux (not luminous flux, which is a measure of how bright a light appears to the human eye and not how many photons it is producing) divided by the total wattage of electricity the diode uses. Remember to convert between milliwatt and watt, as necessary (1000 milliwatts = 1 watt).
Achieving High Efficiency
Brands increase their efficiency by having high quality diodes, running them at low energy, having excellent heat management, and using a high percent of the most efficient diode wavelengths. While some LED companies market themselves as having many different color LEDs, this can often be at sacrificed efficiency, since each color of LED has a different efficiency at producing light. For instance, 450nm, 660nm, and high kelvin white (white LEDs are 450nm with a phosphor coating) are very efficient, while green LEDs are not very efficient.
While there are no doubt that grow light spectrum is important, some studies suggest that even more important than spectrum is light intensity. There are a number of ways of measuring the intensity of a grow light – some good and others bad.
This is the most common measure of grow light intensity and is a measure of electricity (watt = amp x volt). This measure can be misleading, though. Some manufacturers (the better ones) give the actual wattage the unit uses – the wall plug wattage. Others, typically lesser brands, will give you the max wattage rating of the LED diodes. To use a simple example, a grow light could call itself 90 watt if it has thirty 3 watt LED’s, however, it is common practice to run LED’s at half wattage to reduce heat production (and therefore heatsink cost) and increase efficiency. So, what was called a 90w grow light could really be 45w (or less!).
Bottom line, you want actual power draw, not LED wattage rating. It can be good to be suspicious of overly round numbers – you are likely getting the diode rating and not the actual power draw. Knowing the wattage of the diodes isn’t useless, though. You will get a heck of a lot more light out of a 3 watt diode run at 1 watt than a 1 watt diode run at 1 watt.
PPF (Photosynthetic Photon Flux) measures the total amount of light produced by a grow light in terms of micromoles of photons produced per second (often written as umol/s or μmol/s). This is an important number because unlike PPFD (which will be explained below) it can’t be manipulated and tells you the full amount of light coming from the LED grow light.
PPFD (Photosynthetic Photon Flux Density) measures the amount of micromoles of photons striking a square meter per second (often written as umol/m2/s, μmol/m2/s, or μmolm-2s-1).
Full daylight sun at noon in the summer is around 2000 μmol/m2/s. What your plants actually need, however, is likely to be much less than that. Infact, because the Sun’s intensity is only that bright for a small portion of the day and because the angle of that intensity changes throughout the day, providing that much light for an extended period of time would very likely be damaging to your plant. A ‘light response curve’ shows how effectively a plant utilizes light at differing intensities. Depending on the plant, at levels greater than 800-1000 μmol/m2/s the efficiency that a plant uses the light starts to slow. Meaning, you can provide your plant more light than this, but you might not see a huge change in outcome.
It’s worth noting that some LED companies can increase their PPFD numbers by measuring extremely close to the grow light or using spot-light like reflectors or lenses. An LED company should always report what distance their PPFD numbers were taken at (e.g., 24 in, etc.).
This is not a measurement at all, but instead, a type of light that can be absorbed by plants (and coincidentally seen by humans). It ranges from 400 to 700nm.
This is a measure of the total apparent brightness of a light source and not how many photons are produced. As the cliche goes, “lumens are for humans.” You can think of it as the brightness of a single candle (although the real definition is more complex). By in large, lumens are not a useful measure of light intensity for plants since they overweight colors like green and underweight reds and blues. For instance, 1W of radiant flux at 550nm (green, which the human eye is very sensitive to) is 675 lumens. One watt of radiant flux at 660nm (deep red) is only 45 lumens. The red light will be more easily absorbed by your plant in photosynthesis than the green one, but your eye will see the green light as 15 times brighter than the red light!
These measure how much brightness is striking a unit of area. Contrast this with lumen which is the total amount of brightness coming from the light source in all directions. Lux = one lumen per meter squared. Footcandle = one lumen per foot squared.
When choosing a spectrum of light for growing plants, two main factors should be weighed:
- Photosynthesis – You need to choose a light that caters to the energy production of your plants. The McCree curve and absorption peaks of photopigments are worth researching to learn more about this. Read here for more.
- Photomorphogenesis – ‘Photo’ means ‘light’, ‘morpho’ means ‘shape,’ ‘genesis’ can be translated as ‘creation of.’ So, it’s using light to create a certain plant shape. Light can do a lot more than just change the growth pattern of a plant, though. It can trigger or delay flowering and fruiting, change chemical composition, among other diverse reactions. Read more on this here.
Spectral terms relevant for white LEDs:
CRI (color rendering index) describes how “full spectrum” a light is and is typically only used for white LEDs. This scale maxes out at 100. “High CRI” means that a light is producing an abundance of each color within the visible spectrum and therefore “renders” objects illuminated by the light as the color that they would appear under full daylight (which is 100 CRI). High CRI may sound more appealing, but there are instances where low CRI can be more desirable, since low CRI phosphors can be more efficient at producing light and run cooler (due to Stoke’s shift). What “high CRI” typically translates to is more red and cyan than “low CRI.” Not all light is created equal. Here’s some help on this topic.
CCT (correlated color temperature) is a measure of how “warm” or “cool” a light source appears. The scale represents the appearance of a glowing object at different temperatures, measured in the temperature scale of Kelvin (a scale commonly used in physics and chemistry, abbreviated as ‘K’). For instance, if you were to place an iron rod within a furnace and heat it to 2700K its glow would have the same appearance as a 2700K lamp. Higher Kelvin lights produce more blue and lower Kelvin lights produce more yellow, orange, and red.
One of the many significant advantages of LEDs is that they offer greater control in light concentration and directionality. As growers, we need to care about providing light to as much of our plants as possible to increase whole plant photosynthesis. There are two ways that achieve that and which you choose will depend on your particular situation and crop.
The first way to get light deep into your canopy is to have a very concentrated light beam. This is achieved by tightly packing your diodes together and by using reflectors and lenses to direct the light. The other way, which may seem initially counterintuitive, is to provide multiple sources of very diffuse light. By allowing the light to be cast at many angles on the plant, deep canopy penetration can also be achieved.
These different approaches, or some compromise in between, all affect how much area your grow light can effectively cover – its footprint. Make sure that you have measured your grow space and that the lights you are looking at can properly cover that area.
Buy from a company that stands by their product and will guarantee that it will work for years to come.
The cooler an LED stays the longer it will last and the higher its efficiency will be. Most LED manufacturers recommend operating temperatures below 850C (1850F). To achieve this, there are two broad categories of heat management:
- Active cooling – As the name implies, this involves expending energy to reduce a light’s temperature. Fans blowing over a heat sink is the most common method. Liquid cooling is another method that due to the high heat capacity of water has gained an avid following within the computing and lighting community. It’s also how your car’s engine cools! Learn more about this technique here. Quality manufacturers have safeguards put into place that, in the unlikely event of a malfunction, protect the LEDs from overheating and turn off the light.
- Passive cooling – This method doesn’t require energy to cool but can have higher upfront costs and heavier final product weight. Factors that go into a high quality heatsink include material (e.g., aluminum), shape (e.g., pin fin), anodization, and production process (e.g., extrusion, cold forging, etc.).
Total Cost of Ownership
If you plan on using your light for a while, LEDs are a better deal than HID. High-end LEDs are more efficient than HID and have far more ability to provide a specific, tailored spectrum. Lower energy consumption is not only good for you, but it’s good for the environment. Don’t forget, too, the cost and hassle associated with changing bulbs for HID fixtures every year to six months. A good LED light will last you many years, maintenance free. If that wasn’t enough, you also don’t have to feel guilty about using a light that contains mercury and other toxic heavy metals like those contained in HID and fluorescent.