The Red and the Blue of It: LED Lights for Plant Growing

The technical advantages of light-emitting diodes (LEDs) lights—low power requirement, low cost of operation, low heat output, high efficiency, compact form, precise direction and intensity control, shock resistance and long life—have always been clear, but the high initial purchase cost has prevented wider adoption.

Times are changing, though, and LEDs with higher intensities and lower costs are making it this technology an ever-more attractive choice for grow lights. While the cost still gives potential users pause, consider this—the cost per thousand lumens (kl) for LEDs dropped from $36 to $18 over the course of a single year (2009 to 2010) and it is estimated that this price will decrease to as low as $2 per kl by 2015. That is an impressive price reduction in a short time.

LED basics

LEDs are semiconductor devices, which makes them completely different than any other kind of lighting on the market. LEDs are manufactured in foundries, or fabs, using the same kinds of technology and equipment used to manufacture integrated circuits and microprocessors.

White or seemingly broad spectrum LED lamps are available for household use (reading lamps, task lighting, etc.) and for portable use (flashlights, camping lamps, headlamps and the like), but white is not a natural output color for an LED. In fact, one of the advantages of LED grow lights is that they emit monochromatic light, or light in a very narrow wavelength band.

Since photosynthesis is preferentially driven by specific bands of red and blue light, LEDs offer the opportunity to provide energy of the required wavelengths without wasting any of the output on wavelengths that are not useful for photosynthesis. In theory at least, this should make red, blue or red-blue combination LED lamps less expensive than white LED lights because additional steps have to be taken to get an LED device to emit white light.

Two semiconductor materials are placed in contact with each other in an LED light. One material is an n-type semiconductor, meaning it has a surplus of negative-charge-carrying particles (namely, electrons). The other material is a p-type semiconductor that contains a surplus of positive-charge carriers, which are referred to as “holes.”

When a voltage is applied across the material junction, electrons flow through and they fall into the holes. As this happens, the electrons lose energy, which is then emitted as photons of light of a specific wavelength.

This electrically stimulated emission of photons is called electroluminescence. The wavelength of the emission can be adjusted by controlling the chemical makeup of the semiconductor materials and by mixing the light of different colored LEDs in a single device.

LEDs are made in a few basic shapes and sizes, but ones useful for grow lighting are fairly large (as LEDs go), packaged in a base and covered by a plastic “bulb.” The base supports the mounting wires and the bulb (which is made from a type of plastic resin) provides some options in terms of focusing or dispersing the emitted light.

LEDs are direct current (DC) devices, meaning they require some power conversion and conditioning to operate. You can’t just take an LED and plug it in to a wall socket, but, as a practical matter, manufacturers provide the necessary circuitry to allow them to be operated off of standard household or industrial power sources.

Red LEDs

The red LED holds a special place in LED history. Prior to its invention in 1962, other LEDs under research emitted light that was not visible to the human eye (such as in the infrared range). It was Nick Holonyak, Jr. who came up with a way to synthesize a semiconductor material—gallium arsenide phosphide—that resulted in the first useful visible light LED. Even so, red LEDs were at first mostly a subject of scientific curiosity and research due their high cost (hundreds of dollars each) and LEDs did not achieve an economically feasible price point for nearly another decade.

Plants like red light with wavelengths of about 640 to 675 nanometers (nm). It just happens that LEDs with outputs in that range are readily available, and there are two in particular that are well-suited to grow light applications. These are ultra red (660 nm), which are fabricated from gallium aluminum arsenide, and high-efficiency red (635 nm), which use gallium arsenide phosphide and gallium phosphide.

Arsenic and arsenide—sound rather dangerous, don’t they? Yes, arsenic is a toxic material, but in LEDs, it exists in small amounts of a stable solid form that do not represent an environmental threat even if disposed of in a landfill. Also, you have to try pretty hard to break an LED, so even if by some form of accident or abuse red LEDs are broken open, there is little chance of arsenic contamination.

Blue LEDs

The blue LED proved to be much more difficult to come by than the red. The first blue LEDs appeared in the early 1970s, but had a very low output. Commercially practical blues did not appear until 1989. Then, in the 1990s, these lights were revolutionized in Japan with the introduction of high power blue LEDs.

Plants absorb blue light in the range of about 400 to 450 nm. One type of LED in particular is perfect for meeting these wavelength requirements: the ultra blue (430 nm), which consists of a substrate made from the semiconductor materials silicon carbide and gallium nitride.

Red-Blue combination lighting

So, does it make sense to build a light fixture containing a combination of only red and blue LEDs? You bet. An array of red and blue LEDs are available in both circular and rectangular panels designed to be hung above plants or mounted on walls. Some include separate controls to manually adjust the output ratio of red to blue light. There are also compact bulb-like products that can be screwed into a standard Edison-base light fixture.

Large arrays of blue and red LEDs can take the place of high intensity discharge (HID) lamps and fluorescent fixtures. Although the electroluminescent process itself does not generate much heat, the electricity used to make it happen does heat up the LED bases and when a bunch of them are put together, enough heat can be generated to require a cooling fan. Fortunately, most of the heating occurs at the back of the fixture, so LED lights can still be placed closer to plants than HID lamps. Smaller LED lamps can also be placed under the top of the plant canopy and directly among plants to provide light to the lower leaves.

What might the future bring with respect to red and blue LED grow lights? Well, one intriguing idea is to produce a lamp that varies its red/blue ratio over time under computer control. Such lamps would be similar to ones already in production that combine both colors, except the circuits used to control the red and blue outputs would be automatically controlled with a computer. Why would this be desirable? Well, many plants regulate their flowering and fruiting cycle based on the ratio of red to blue light.

As the summer comes to an end in nature, the relative amount of red light received on Earth tends to increase. Many plants have evolved to use this phenomenon as a signal to shift from vegetative growth mode into reproduction mode. So, a lamp that would automatically mimic this gradual shift over a time could conceivably improve crop performance by exposing plants to a more natural light environment.

With further research, it might even be found that varying the red-blue ratio throughout a daylight period to simulate sunrise and sunset could have a beneficial effect on the performance of some crops.

Moreover, it is possible that the use of wavelengths not directly associated with photosynthesis—such as far red or ultraviolet—might have desirable effects on plant processes. So, even if LEDs do not play a direct role in these aspects of plant physiology in actual agriculture practice, it might be able to play a role in the research.

In any case, if the cost of LEDs does drop to $2/kl over the next few years, we can expect to see a rapid and dramatic shift into LED lighting for many uses.

Lots of LED information, both historical and more recent (up to 2008), including a collection of LED spectra, can be found at ledmuseum.org.