Choosing LEDs for indoor use

Here are practical evaluation criteria for selecting LED lighting for indoor, ambient applications.

11/11/2010


Everyone is interested in using the latest technology—in this case, digital LEDs. But is LED lighting ready for “prime time,” particularly for general ambient applications such as commercial spaces, offices, and classrooms? It is important to understand how to separate fact from fiction with all the LED lighting choices that continue to flood the market.

The answers depend first on understanding three fundamental performance criteria: thermal management, light output, and color consistency. Additionally, specifiers should consider an often-overlooked but highly practical criterion: How well will an LED lighting system adapt to future technology advances or a reconfiguration of the original space?

Ultimately, the applicability of LEDs for mainstream commercial applications is based on performance and economic viability. The economics of specifying LED lighting typically focus on energy savings, extended life expectancy, reduced maintenance, and improved sustainability. But increasingly, the combination of LED lighting with digital lighting controls is making a more profound impact on the application and selection of LED systems.

As a general rule, LEDs are more “control-friendly” than conventional light sources commonly used today. Intelligent use of digital lighting controls can substantially leverage LED performance—lengthening LED life expectancy and improving system efficacy—making the economic equation more attractive.

Performance criteria

Solid-state lighting systems using white-light LEDs are quickly becoming a viable solution for interior lighting as they begin to rival the performance of linear and compact fluorescent lighting. A well-designed LED system can easily last more than a decade before requiring lamp replacement, greatly reducing maintenance costs. Plus, LEDs inherently are more environmentally friendly than conventional light sources as they do not use mercury or other hazardous materials. And, because LEDs last longer, there are fewer fluorescent lamps going to the landfill, making LEDs potentially a good choice to meet sustainability goals.

In many ways, specifying an LED fixture for an interior application is not that different from specifying any other lighting fixture. The same principles of determining lighting quality, efficiency, and aesthetics still apply. However, there are three primary performance criteria to consider when specifying LED lighting products.

Thermal management--In the current state of LED technology, thermal management of the lighting system is a fundamental performance criterion that potentially impacts all others. LEDs produce more conducted heat than other common lighting sources, including incandescent (more radiant heat), fluorescent, and metal halide. If not managed properly, extreme heat at the pc board interface (as measured by junction temperature, Tj), can significantly impact system performance attributes, including light output, color stability, and life expectancy. This negative property also hinders retrofitting LEDs into existing lighting fixtures.

To sustain superior performance of solid-state components over a long period of time, LED luminaires must be designed to actively extract heat away from the LED itself and then dissipate the heat, preferably outside the occupied environment. For recessed ambient applications, this means heat removal into the ceiling plenum. Depending on the quantity and concentration of luminous flux, luminaires may require very aggressive heat sinks to achieve this, especially for smaller, more intense light sources like downlights. Larger, more uniform area sources can use reflector and housing surfaces to conduct heat away from the LED.

When selecting an LED luminaire, it is important to understand how the product is designed and constructed, and specifically how heat is managed in the overall system. If the method of thermal management is not intuitively obvious, then either performance claims are suspect, or there are probably not enough lumens generated to meet your lighting criteria (without using increased number of luminaires). Many new LED luminaires on the market today are in reality existing fixtures simply retrofitted with LED components. This type of product is usually very limited in terms of lumen output and may not provide adequate optical control to diffuse and shield the intense brightness of discrete LEDs.

A well-designed LED luminaire is fully optimized for superior optical control and thermal management to maximize all critical performance factors such as lumen output, system efficiency, brightness control, and life expectancy.

Delivered lumens and system efficacy--The second important performance criterion is determining the delivered lumens and system efficacy or lumens per Watt (LPW) of the LED system. This will enable you to determine if an LED solution can meet your lighting objectives effectively and efficiently. Manufacturers’ published data is a good source for this information, and it is important to seek data that is tested and reported in accordance with established industry standards. Specifically, ask the luminaire manufacturer for performance reports that have been derived in accordance with IESNA LM-79-08. 

IESNA LM-79-08 (Electrical and Photometric Measurement of Solid-State Lighting Products) is a testing method to measure the performance of LED-based luminaires. This method is termed absolute photometry because the results are actual or “absolute,” not “relative” or scaled to a source that was not actually tested. The limitation of absolute photometry is that it is applicable only for the exact luminaire/light source combination tested.

For first-level comparisons, the LM-79-08 report format summarizes delivered lumens (aka luminaire lumens) as a basis for comparison of LED luminaires rather than the traditional measure of fixture efficiency. In addition, the metric of delivered (luminaire) lumens is common to both absolute and relative photometry and therefore allows for a direct comparison of LED versus non-LED luminaires—at least with regard to delivered lumens and light distribution. Also, the LM-79-08 format provides system input watts based on the actual (or absolute) set of LED components. This allows for more accurate assessment of system efficacy or LPW.

Currently, well-designed LED lighting fixtures for general ambient lighting applications should deliver a range of 3,200 to 3,300 lumens based on 45 to 50 input Watts for a delivered LPW in the 70 to 75 range. This allows for general illuminance levels that are comparable to those of conventional fluorescent systems using typical layouts that do not require additional luminaires. There are viable LED alternatives for other nonfluorescent systems as well.

Color quality--The third key performance criterion to consider is color consistency because it is one of the most visual and experiential aspects of LED lighting applications and therefore a common concern. Questions about color consistency generally fall into three areas:

  • Does the LED color temperature remain consistent from lamp to lamp and fixture to fixture?
  • Does the color temperature (color whiteness) of the LED lamps remain stable over time?
  • Do LEDs render color as effectively as conventional light sources?

The process of manufacturing white LEDs can be inherently inconsistent relative to color quality and requires rigorous production controls to ensure that high quality and consistency are maintained. There is a wide range of quality available today for white LEDs, and color is a significant driver of cost because the best LEDs are typically selected through a testing and sorting process.

There is not an abundance of definitive industry standards in the area of evaluating color quality. Color whiteness—expressed as correlated color temperature, or CCT—is difficult to measure outside of a testing lab or manufacturing environment. In addition, color appearance tends to be perceptual leading to subjective evaluation. Nonetheless, the LED luminaire manufacturer should be expected to provide information relative to the quality and performance of the LEDs they are using as well as validation testing of the LEDs color calibration once installed in the luminaire. It is also important to recognize that color stability over time can be greatly impacted by the thermal management of the LED luminaire. Poor thermal management will accelerate the degradation of LEDs and can create nonuniform shifts in color appearance over time.

A new industry standard called the Color Quality Scale is under development to more effectively measure LED lamps across multiple aspects of color quality, such as color rendering, chromatic discrimination, and observer preferences. Until the Color Quality Scale is developed, the color rendering index (CRI) continues to be the most effective measure of color rendering. An indoor LED fixture for general ambient lighting should have a minimum CRI of 80. Also, ask the lighting supplier about how the color consistency is managed using binning (sorting), color mixing, and/or controls. Effective management of these factors should create consistent color from lamp to lamp and a uniform, quality lighting experience. Because color perception can be subjective, the judgment of a lighting system’s color quality is ultimately best experienced in an actual application when possible. In many cases a mockup can help resolve questions about color quality.

Finally, lighting professionals and their clients should take advantage of the U.S. Dept. of Energy (DOE) Caliper and Lighting Facts programs. The Caliper program anonymously obtains samples of LED lighting products for testing and reports actual performance relative to manufacturers’ published claims. The Lighting Facts program creates a standardized format for manufacturers to report performance of LED products so that specifiers and users can make better comparisons based on consistent metrics.

Investment criteria

Assuming you have successfully identified several high-quality LED lighting products using the three primary criteria, it is now time to evaluate the return on investment of LED technology for your project. Certainly, features such as energy savings, extended life expectancy, and reduced maintenance go a long way to establishing a reasonable payback when measured against the higher initial costs of LEDs.

Two additional considerations, however, should also be factored into your economics equation when determining if LEDs are right for an interior space.

The first is digital lighting controls. LEDs are extremely “control friendly,” and are, in fact, easier to control than any other general lighting source. Unlike conventional sources, LEDs become more efficient as they dim. Also, more extensive dimming and control functions actually extend the service life of LEDs. They are fully compatible with occupancy sensors, daylight harvesting controls, manual dimming, and full-scale energy management systems. LEDs feature a true instant-on capability, and maintain a constant color temperature throughout the entire dimming range. These combined capabilities allow LED systems to deliver substantially greater energy savings when integrated into a digital lighting controls plan.

Beyond capability, the inherent compatibility of LEDs with digital lighting controls may ultimately hold the greatest potential for economic benefit. LEDs are digital light engines that can interface directly to on-board luminaire control logic. The result is an “intelligent” luminaire that has the ability to monitor and respond to its environment, and perform preprogrammed tasks to further conserve input power and reduce luminaire maintenance. One example of such a task is executing a constant lumen output over system life to eliminate the waste of overlighting associated with initial lumens delivered early in a lighting installation. Other potential tasks include: monitor and adapt to variations in ambient temperature, monitor system life, and detect a fault in the system for quicker troubleshooting and maintenance.

Additionally, the LED luminaire is digitally addressable, allowing it to easily network and communicate with other luminaires and like-kind control devices in the same room or throughout a building. Network connections can easily be made using standard CAT5 cabling for true plug-and-play convenience. Again, these facilitate simpler, more cost-effective control solutions that reduce installation and operating costs for better return on investment.

A second economic consideration is adaptability. Once LED luminaires are installed, how easy will it be to replace or upgrade components, and how easy will it be to reconfigure the existing system if the interior space is updated or redesigned?

LED lamp technology continues to change at a rapid pace. Over the next several years performance will continue to improve and costs will come down, just as we now have T5 and T8 fluorescent lamps that outperform T12s. Despite the fact that LEDs will last 10 or more years, there will likely be a time when retrofitting new LED components makes compelling economic sense.

Look for luminaires that are easy to maintain with LED components (boards and drivers) that can be removed easily without deconstructing the luminaire or removing it from the ceiling. In addition, look for LED luminaires that are easy to relocate and are flexible to function in various settings and configurations. It makes sense that if you invest in LED products because of the long-term benefits, the luminaire should be designed to deliver those benefits for a wide variety of applications.

The bottom line: Work with lighting suppliers you trust, and insist on credible performance data using industry-accepted standards to support specification claims. Request application mockups when questions arise about lighting quality and spatial rendering. Consider working with a professional lighting designer. When calculating return on investment, consider the initial cost benefits and energy savings derived through advanced, cost-effective digital lighting control solutions used with high-quality luminaires and components that are designed for the long term. Always be aware of new technologies that allow lighting to enhance design and improve productivity, safety and personal comfort.


- Ranieri is the vice president and general manager of the Lithonia Lighting Fluorescent /LED Business Unit. He has more than 26 years of experience in the lighting industry and has led LED product development in applications of specification downlighting, general ambient lighting, and the integration of digital controls.


Managing digital lumen output

Thanks to digital technology, LED lighting has the capability to easily connect with onboard, intelligent controls. Control logic onboard each luminaire can be used to manage any number of variables and features to improve performance or save energy. Constant lumen output is a good example. Instead of constant power and variable light output over time, which is typical, light output is held constant at a maintained level and power consumption varies over time. This prevents energy waste created by the traditional practice of overlighting with initial lumen output and makes LED system payback even more attractive.

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Video: LED and Lumen Management



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