Shedding light on LED for industry

LED primed to replace metal halide, mercury vapor

By Steve Henry

October 12, 2011

Thanks to advancements in semiconductors, optics, and materials, LED lighting has excellent applications in industrial environments, and it is especially suitable for hazardous locations.

Advancements in semiconductors, optics, materials, and manufacturing efficiencies have progressed LED technology in terms of light output, energy efficiency, cost-effectiveness, and color options. As a result, LED applications have grown exponentially, including industrial lighting.

Light-emitting diodes are not a particularly new technology. LEDs first were used 40 years ago as a replacement for incandescent lamps in indicator lamps on electronic equipment. They soon found applications in alphanumeric displays for calculators, clocks, watches, and appliances. As brighter LEDs became available in more color options, they quickly became ubiquitous in traffic lights, animated signage, automotive brake and signal lights, decorative lighting, and flashlights.

All of these applications benefit from LEDs' energy efficiency; minimal environmental footprint; and long, reliable operation, even under extreme conditions.

But for industrial settings and public areas such as parking garages, LED lighting commercialization has always been something of a golden city—rumored to be just over the horizon but never actually within reach. LED technology has not been available as a realistic option for industrial area and task lighting—until now.

At the Gate to the City

Things have changed in the last two years. Thanks to ongoing advancements in semiconductors, optics, materials, and manufacturing, LED technology has steadily progressed in terms of light output, energy efficiency, cost-effectiveness, and color options. As a result, LED applications have grown exponentially, and LED fixtures are now available for industrial lighting applications.

And none too soon. Manufacturers that use mercury vapor ballasts and lamps and metal halide luminaires because they have a broader spectrum than high-pressure sodium and longer life than incandescent must now look at other options. Recent U.S. legislation requires the phase-out of mercury vapor ballasts and lamps and 150- to 500-watt metal halide luminaires.

LED enters the industrial space just in time to fill that void. It meets and exceeds government-mandated efficiency standards. LED is poised to become an increasingly dominant technology for all kinds of industrial and general-purpose lighting.

Dispelling Misconception

Whenever a new technology emerges, it takes time for standards to coalesce and for new concepts to become clear. That is true not only for users but also for the producers that are just getting started with technologies that differ radically from what they've used.

Consider the first few months after the introduction of compact fluorescent lamps. Lighting manufacturers had to play a guessing game regarding which wattages and sizes would become standard, and what fixtures they would need to design to accommodate these standards. Over time standards emerged, products based on incorrect guesses and bad ideas were weeded out, and customers gained access to proven products with reliable performance and compatibility for years to come.

The same is true for LED. Although practical LED industrial lighting has emerged only recently, the field has stabilized. Poorly designed products have been purged from the market, misleading claims have been retracted, and the LED lighting made by the major manufacturers performs as advertised.

Misconceptions, uncertainties, and fears continue to persist, however. As light is shed on the most widely misunderstood issues, users can make more informed decisions today and have a better idea of what to expect from LED lighting in the years ahead.

Why LED Suits Factories

An overview of LED features illuminates why it is suitable for nearly any industrial lighting application. Well-designed LED luminaires are:

• Energy-efficient. The main driver for LED adoption is energy efficiency (see Defining Efficiency sidebar). Achieving the lighting levels required for a particular application at the lowest possible energy input becomes critical as energy costs rise and as government regulations clamp down on wasteful energy sources.
• Long-living. Correctly designed, LED fixtures offer up to 60,000 hours of illumination, with no "droop," or color shifts, and no penalty for frequent on/off cycles.
• Low-maintenance. Because it is rugged and long-running, LED lighting requires very little maintenance. When LED lighting is used in a fixture with an intelligent modular design, even end-of-life replacement of components becomes quick and simple.
• Resistant to Shock, Vibration, Corrosion. LEDs can be used in environments where other technologies fail—prematurely or catastrophically.
• Cold Start-Capable. LEDs provide instant on and instant restrike capabilities to –40 degrees C, with no warm-up time required to achieve full brightness.
• Nondamaging. LEDs do not produce ultraviolet or infrared radiation often emitted by other lighting sources. This lowers cooling costs, simplifies maintenance, prolongs product life, protects eyes and sensitive equipment, and provides a margin of safety in hazardous environments.
• Safe for Hazardous Locations. Available LED luminaires are rated for use in areas where flammable gases and vapors are present under conditions defined by NEC Class I, Division 2 and IEC Zone 2.
• Easily Positionable. LED lighting often is the best choice for areas with low clearance, severe weather conditions, excessive moisture or dust, corrosive atmospheres, and high ambient temperatures.
• Nontoxic. LED lighting is also the only nonincandescent lighting source that contains no mercury. This eliminates any chance for mercury to escape into the environment either in operation or after disposal. Therefore, no special handling is required. Lights with lead-free solder ensure no toxins leach out when components finally reach end-of-life. Too, LEDs' longer lifespan than most other lighting sources' means less material is disposed over time.
• White and Bright. Unlike the dim, bluish-green LED flashlight kept in yesterday's glove compartments, today's industrial LED luminaires provide high-quality light.
• Highly Directional. LED luminaires can be configured to produce nearly any horizontal and vertical distribution of light—from illuminating a tall, narrow fence line for security purposes to area lighting that allows production crews to work efficiently and safely.

Managing LED-generated Heat

Although LEDs generate no or little heat in most applications, AC-powered industrial LED fixtures do produce a significant amount of heat outside the beam. It's important to understand why and how to manage the heat properly.

LEDs operate naturally on direct current (DC). Lighting an LED on an AC circuit without destroying it requires a driver that converts AC to DC and steps the voltage down from 120 VAC (or more) to 24 VAC. Unlike a flashlight battery, the output current is at a very high amperage—much higher than the milliamps required to light the LED. This current is fed into the T junction at the rear of the LED (see Figure 1).

Figure 1 While the surface and beam of an LED produce little heat, the T junction can become hot.

The T junction can be compared to a tiny nozzle mounted on the end of a large fire hose. In stepping the input current down to meet the requirements of the LED, the T junction absorbs a substantial amount of energy, similar to the friction a large volume of water under high pressure creates when it is constricted by a nozzle. This energy is released as heat. Although the beam of an LED fixture may be cool, the back side of the LED array can become quite hot. The T junction is the hottest spot on the fixture.

Accurately determining its maximum temperature is crucial when rating products for use in the potentially flammable atmospheres of oil refineries, paper mills, and other manufacturing environments.

The other main heat-producing component is the driver inside the fixture unit, which is analogous to the ballast compartment in conventional lighting systems. The driver is a solid-state device, and as such it needs to operate within a specified case temperature rating.

Properly managing the heat generated within the LED fixture is important for three primary reasons:

1. Excess heat buildup at the T junction can degrade the phosphor and reduce lamp life.
2. Excess heat at the driver unit can shorten product life.
3. Inadequate heat management can limit the range of ambient temperatures for which the product can be specified.

Luminaires rated for maximum ambient temperatures below 55 degrees C cannot be used in many areas of the world, such as the Middle East, and in many specific applications (for example, smelting and casting).

There are several ways to control the heat.

Large, External Heat Sink. A properly designed LED lighting fixture has a large external heat sink, often visible as a series of bare or powder-coated metallic fins surrounding the LED array itself. This heat sink is designed to pull heat away from the T junctions on each LED and from the driver housing. After several minutes of operation, the heat sink will become noticeably warm to the touch, even while the beam itself remains cool.

Prominent heat sinks may be unfamiliar to most people who are used to seeing LEDs in lower-current applications such as signal lights, or outdoor luminaires that benefit from free-flowing air and nighttime temperatures.

For industrial lighting applications, in which circuitry and lamps are housed in enclosed and gasketed or explosionproof fixtures, these heat sinks are critical. They ensure that LEDs reach their full 60,000-hour lifespan with no degradation in light quality. The heat sink allows luminaires to operate reliably in temperatures as high as 55 degrees C.

Overdriven LEDs. Even with adequate heat sinking, good thermal management requires that luminaires be designed with the optimum number of LEDs to achieve the desired lighting levels. While it is possible to achieve a dramatically whiter, more intense light by adding more LEDs than optimal, inevitably this strategy will overdrive the system. Doing so will reduce lamp life, damage the phosphors, and cause a noticeable droop.

When too many LEDs are incorporated into the design, what began as an impressive display of white light may shift to an unacceptable color within weeks and expire within a few thousand hours of operation.

Locating and Measuring Hot Spots. Luminaires designed for use in hazardous atmospheres must be rated according to stringent requirements to ensure that a spark or hot spot doesn't ignite.

Construction and testing standards for these luminaires are controlled by the International Electrotechnical Commission (IEC), National Electrical Code® (NEC), and other standards and testing bodies. For the most part, the standards are well-understood and consistent, but LEDs require a new approach to temperature rating.

Conventional wisdom based on more well-established lighting technologies suggests that the hot spot is likely to occur on the surface of the lamp, but this is not true with LEDs. The hot spot is at the T junction, which is sealed inside the LED assembly. The T junction is impossible to reach with a thermocouple in order to take a temperature reading directly.

Currently lighting manufacturers and testing bodies use different methods to place the thermocouple as close as possible to the T junction, as well as to analyze the results and estimate the true hot-spot temperature.

It is likely that a single, widely accepted standard will emerge for temperature rating of LED luminaires. In the meantime an investigation of the products, how their temperature ratings are devised, and how much margin of error is built into the results is advised. Because LED luminaires tend to have a lower temperature rating than their alternatives, a suitable lighting system rated at a significantly lower temperature than the safety threshold should not be difficult to find for most applications.

LED Is the Future

LED industrial lighting is here today, and it's here to stay. The benefits it provides simply cannot be ignored. Even local and national governments are taking notice of LEDs as they focus on the problems of energy consumption, greenhouse gases, pollutants, and toxic waste.

LED Is Green. The most important green feature of LED is its energy efficiency. LED has the smallest environmental footprint of any manufactured source. Because a large percentage of electricity in the U.S. is produced by burning fossil fuels, switching to LED lighting also is likely to cut emissions of greenhouse gases and toxic pollutants. In addition, LEDs are mercury-free.

In the years ahead, as standards become firmer, product lines more established, and customers more familiar with the technology, LED lighting is likely to become a relatively common option for manufacturers' plants.

The fundamentals are in place. Some very good LED products are on the market. The knowledge of how LEDs work and the best ways to harness the technology is available. As with anything new, big, and potentially lucrative, a lot of misinformation has proliferated; however, armed with knowledge, manufacturers should be better prepared to evaluate LEDs for their own lighting projects.

While LED technology is not the only option—and in some cases, it's not even the best option—it is likely to become the leading choice to replace many of today's energy-hungry industrial lighting systems.

Defining Efficiency

Comparing the efficiency of dissimilar systems can lead to confusion. Consider the following specifications for lighting over a 100- by 15-ft. walkway at a 13-ft. height (see Figure 2).

• A 175-W pulse-start metal halide lamp requires 208 W of input power to the fixture (the excess power is lost in the fixture's ballast). The light output for each lamp is 17,500 lumens, and the average lighting with four fixtures installed over the walkway is 5.67 foot-candles. Total input power for the installation is 832 W.
• A 165-W QL induction lamp requires 165 W to power the fixture. The light output for each lamp is 12,000 lumens, and the average lighting with six fixtures installed over the walkway is 6.34 foot-candles. Total input power for the installation is 990 W.
• A 48- by 1.7-W LED array requires 98-W input to the fixture (the excess power is lost in the driver). The light output for the fixture is 5,400 lumens. The average lighting over the walkway with four of these fixtures installed is 9.55 foot-candles. Total input power for the installation is 392 W.

It may be instinctive to look at the input power to the entire system versus the power actually consumed by the lamps, and to conclude that LED is the least efficient of the three. That would be a mistake. The critical comparison is input power versus actual illumination at the point where it is needed. In this scenario, LEDs are more than twice as efficient as the metal halide and the induction lamp, providing brighter illumination at less than half the power consumption of either QL induction or pulse-start metal halide.

Comparing Lumen Output. It may also seem logical to conclude that LEDs are less efficient than the alternatives based on the lumen figures given in the previous bullet points—17,500 for the pulse-start metal halide lamp, 12,000 for the induction lamp, and 5,400 for the LED fixture. These figures cannot be directly compared. A lumen rating calculated by totaling the light output of all LEDs in the fixture simply is not comparable to a lumen rating for a lamp based on the measurement of light output in all directions.

Figure 3 LEDs are highly directional, enabling higher efficiency compared to a conventional lamp mounted in a fixture.

What matters is how much light reaches the intended surface, and at what energy cost. In these terms, LED is the most efficient.

Figure 2 gives a lumen value of -1 for the LED fixture. This value is used by photometric software to distinguish LED from other types of lighting in its calculations to produce results that are valid across these very different technologies.

An LED fixture incorporates an array of point sources that direct light precisely where it is needed, with very little scattering or loss. Light distribution is controlled by the placement of the LEDs, as well as by efficient use of optics that take advantage of the focal point presented by each individual LED.

By contrast, conventional lamps cast light in every direction. The fixtures must incorporate hoods, reflectors, and lenses to direct light to where it is needed and block areas where it is not. Due to scattering and absorption, only 40 percent of the available light reaches its intended destination, versus up to 80 percent for an LED fixture (see Figure 3).

ENERGY STAR®. ENERGY STAR-certified LED lighting has been tested and approved to ensure it meets stringent environmental and operational requirements. This is useful to the manufacturers that make the lighting, as well as to the manufacturers that use the lighting.

ENERGY STAR certification also means that a product has been tested to ensure the highest quality. Simply put, this government-backed certification body will not approve fixtures that do not meet customer expectations, despite their energy efficiency. Among many other requirements, ENERGY STAR certification ensures that LED lighting fixtures:

• Use at least 75 percent less energy than equivalent incandescent lighting, and are as or more efficient than fluorescent lighting.
• Be as bright or brighter than other technologies, with good distribution over the lighted area.
• Provide constant light output that decreases only near the end of the product's rated lifetime.
• Provide excellent color quality, with a shade of white light that remains clear and constant over time.
• Turn on instantly, and use no power when turned off other than a maximum of 0.5 W in the control gear.