By using LED technology, we can turn electricity into light tailored specifically for plants with no waste heat. LEDs can dramatically improve the efficiency and quality of horticultural lighting by customizing the output spectrum to where plants can use it most. Because LEDs run cool, we can create productive small grow chambers or reduce heating costs in large ones. And, because we can control a mix of different LED colors, we can even mimic the natural seasons by changing the light output spectrum over time. Imagine creating light “recipes” to match each individual plant’s requirements over the growth cycle.
Given their advantages, it’s no wonder LEDs are making a big impact on the horticultural lighting world. But what exactly is an LED?
LED Basic Facts
A light emitting diode is a crystal that emits light when electric current passes through it. This property was first discovered in 1907, but it took until 1962 for LEDs to become commercially available. The LED’s color depends on the elements used to make the crystal. For example, red LEDs contain aluminum, indium, gallium and phosphorus.
The earliest LEDs were red, followed by amber, yellow and green. Blue LEDs eluded researchers until 1992, and full color (RGB) LED displays followed soon after. White LEDs are simply blue LEDs with a yellow phosphor printed on the crystal surface — the blue light makes the phosphor glow yellow and the two colors mix to produce a bluish-white.
An LED’s light output depends on its “quantum efficiency,” or the crystal’s ability to convert electrons (electricity) into photons (light). An LED will produce more light as more current passes through it, until self-heating at very high currents reduces its quantum efficiency. As the LED heats up, it produces less light with more current. This is why it’s very important to keep LEDs cool; cooler LEDs produce more light. Because they don’t radiate any heat, the way to cool LEDs is by mounting them to a heat sink, like a computer CPU.
LEDs Cross Brightness Barrier
The LED most people are familiar with comes in a bullet-shaped 5- mm package, with a clear epoxy body holding two long leads that support the LED crystal, or “die.” The 5-mm LED package is cheap to produce but not good at removing heat from the die, so the 5-mm LED is limited by self-heating to drive power levels below 20 milliwatts (0.02 watts). They’re good for panel indicators and flashlights, but they don’t produce enough light for general illumination.
LED technology advanced to enable 1-watt LEDs by the late 1990s, and finally crossed the lighting-class brightness barrier in 2001 with Philips Lumileds’ introduction of the Luxeon family of 3-watt LEDs. These were followed by the LuxeonIII family of 5-watt LEDs in 2003. Cree and Osram Opto followed suit with their X-Lamp and Dragon series LEDs in the 3- to 5-watt range. Manufacturers continue to develop new LED materials that improve quantum efficiency and new package designs that remove more heat from the LED die.
The light output from multi-watt LEDs is great enough to match conventional lighting technologies in specific scenarios. High-power LEDs now find use in architectural lighting, flashlights and daytime running lights for cars. Because each LED has a narrow beam like a small spotlight, illuminating a large area requires an array of many LEDs—currently an expensive proposition for general lighting applications, but a justified expense where energy savings, maintenance cost reduction and crop yield take precedence. The savings in electrical power and cooling costs make LED lighting a worthwhile investment for eco-conscious greenhouse and indoor growers.
Photosynthesis Basics
To understand why LEDs are so well-suited to growing plants, let’s review a bit of plant biology. Then we can compare LED lighting solutions to HID lamps to determine which is better for plants.
Plants perform photosynthesis using two types of chlorophyll: Chlorophyll-A, with peak response at 430nm and 680nm, and Chlorophyll-B, with peak response at 450nm and 660nm.
While blue light in the mid-400nm range can activate photosynthesis, plants mostly use red light in the 650 to 700 nm range. But pure red light produces abnormal plants, indicating that blue light is required for proper growth. Blue light also tells the leaves to open their stomata and allow CO2 in.
Notice that green light produces no response in the chlorophyll curves. Plants look green because their leaves reflect the green light. Human vision, however, is most sensitive to green light—an advantageous adaptation for a species that evolved in the forest.
Measurements of an air-cooled HPS lamp show that more than 60 percent of the electrical energy is turned into heat and ultraviolet light, and only 32 percent of the electrical energy used is turned into light energy (MacLennan, 1994). HID lamps produce light that’s useful for humans, meaning that most of their light energy output is in the green part of the spectrum, with less than 10 percent of that output in the red and blue regions where plants use light.
Therefore, we see that the typical HID output spectrum is actually the opposite of what plants really need. HID grow lights work only because they are using so much energy that their 10 percent output in the photosynthesis region is enough to grow plants.
Target the Spectrum Plants Need
LEDs allow us to take a targeted approach to horticultural lighting by converting electricity into only the light energy that plants will use. Instead of wasting energy producing the broad emission spectrum of an HID lamp, the LEDs create very narrow emission spectra: 20nm to 40nm spread compared to several hundred nm for an HID lamp.
Meanwhile, LEDs are very efficient at creating light from electricity. At least 20 percent of the electrical energy put into an LED turns into light, and all this light is usable for photosynthesis. For the HPS noted above, only 10 percent of the 32 percent conversion efficiency is in the photosynthesis region, meaning that the HPS is only 3 percent efficient at creating light usable by plants.
