LED-based lights have been on the market for a few years. The first generation of LED grow lights used 5-mm LEDs and weren’t powerful enough to directly compete with HID lamps. Only recently have new, high-power LEDs allowed manufacturers to design LED lights with enough power to replace HPS and MH lamps.
The savvy grower should look for the following features in a high-quality LED lighting solution:
High-Power LEDs
The small “bullet” or 5mm LED does not have enough power for horticultural applications. This type of LED is limited to less than 0.02 watts of power. This means that it would take 250 5-mm LEDs to equal the light created by one 5-watt power LED. Clearly, the 5-mm LED is inadequate for growing plants.
Growers should only consider lights made with high-power LEDs. “High-power” LEDs have power ratings greater than 1 watt. LEDs in the 3-watt class include the Cree XR-Lamp, Luxeon Rebel, and Luxeon K2. Some 5-watt LEDs include the LuxeonIII and the Osram Platinum Dragon series. High-power LEDs are identifiable by a large metal pad on the bottom of the LED package. This metal pad provides a direct path for heat to escape the LED, usually by means of a connection to a heat sink.
Reputable LEDs with Accurate Lifetime Ratings
When comparing lighting solutions, it is important to understand how manufacturers specify lifetime hours for different lamp types. HID bulb lifetime is specified as time-to-failure, usually something less than 20,000 hours. Light output from HID lamps degrades long before the bulb burns out, noticeably decreasing as soon as 5,000 hours. Growers using HPS lamps, for example, will routinely change bulbs every 6 months to maintain light levels in their grow rooms.
By comparison, an LED’s lifetime is specified as the time for its light output to degrade to 70 percent of its initial output. Manufacturers specify 70 percent lifetimes between 50,000 and 100,000 hours. However, to achieve the LED’s predicted lifetime rating, the light must operate the LEDs within the manufacturer’s temperature limits. Operating LEDs at high temperatures reduces their lifetime.
In order to trust a light’s LED lifetime rating, find out which LEDs the light uses. Reputable makers extensively test their LEDs and employ strict process control in order to provide a real guarantee of operating lifetime. Poor-quality LEDs exhibit lifetime problems caused by skimping on process control and testing to lower quality standards.
High-Power Cooling System
Like the CPU in your PC, high-power LEDs must be cooled with a heat sink and fan. Because high-power LEDs do not radiate any heat, the metal pad provides the only path for heat to leave the LED. Heat flows from the LED die, through the metal slug, through the circuit board, into a heat sink and then out to the surrounding air.
Look for LEDs mounted on a metal-core printed circuit board (MCPCB), a space-grade technology used for operating electronics at high temperature. An MCPCB conducts hundreds of times more heat compared to the typical fiberglass circuit board. An MCPCB is required for the high power levels that LED horticultural lights endure.
Finally, make sure the MCPCB is mounted to a large heat sink, preferably one with many fins. More fins provide more surface area to dissipate heat into the surrounding air. The heat sink should be cooled by multiple fans to prevent a single-fan failure from damaging the LEDs. The light’s datasheet should list the fan’s predicted lifetime.
Constant-Current Driver Circuit
The electronic circuit powering the LEDs is an important consideration when evaluating LED lights. The LED “driver” circuit is like an HID ballast; it converts AC input power into DC power at the proper voltage and current level for the LEDs. Its most important job is supplying a constant DC current even as the LED voltage changes over time and temperature.
Many simple driver circuits provide a constant voltage, meaning the output current varies with the LED voltage. A constant-voltage driver can cause early LED failure. As the LED’s temperature increases, its voltage drops, causing a constant-voltage driver to supply more current in response to the decreased LED voltage. This feedback loop results in a runaway current that destroys the LED.
Proper LED driver circuits supply a constant DC current, holding steady as the LED voltage changes with temperature. Look for the words “constant-current” in the horticultural light’s LED driver specifications.
Adjustable Output Spectrum
Because LEDs are inherently dimmable, an LED solution should let the grower tailor the light output spectrum. Look for an LED light that provides individual brightness controls for each color of LEDs. By varying the output power of individual colors, the grower can simulate seasonal light changes over a multi-week growing cycle.
For example, more blue light mimics the summer sun (vegetative phase), and more red light simulates the sunlight in the fall (flowering phase). This type of spectrum change is similar to the effect achieved by starting plants under MH lamps for vegetation and then changing to HPS for flowering. Growers can even tailor the light spectrum to suit individual plant type.
Rigorous Testing by Recognized Experts
The LED lighting marketplace can be confusing. How does one separate fact from fiction when reading manufacturer claims about electrical savings and plant growth? Look to see with whom the manufacturer has partnered to conduct product efficacy testing. The LumiGrow light, for example, has performed successfully in extensive testing by independent researchers at the University of California, Davis and Duke University among others. Whether greenhouse growing is your livelihood or your hobby, don’t risk the health of your crops or your budget on inadequately tested LED lights.
