Finding the right LED

Consulting engineers often are tasked with lighting design as part of their overall electrical design or as a subcontract to the electrical engineering team or contractor. Because of the big push for energy efficiency, many clients are now asking about LEDs. LEDs are have been available for lighting systems for a number of years, but first costs, color rendering, appropriate fixtures for commer...

04/01/2010


 

 

Consulting engineers often are tasked with lighting design as part of their overall electrical design or as a subcontract to the electrical engineering team or contractor. Because of the big push for energy efficiency, many clients are now asking about LEDs. LEDs are have been available for lighting systems for a number of years, but first costs, color rendering, appropriate fixtures for commercial interiors, and controls have not been altogether competitive with conventional fluorescent technologies. Have things changed enough to tip the scales toward LEDs?

 

The answer: Well, it depends.

 

LEDs, or light-emitting diodes, are small semiconductor devices that emit light, usually colored, when an electric current passes through them. This solid-state semiconductor device converts electrical energy directly into light. Many “white” LEDs are really blue LEDs with similar phosphor coatings that fluorescent lamps use.

 

Three big benefits of LEDs: They have a long service life and they consume less energy than most older style T12 fluorescent and incandescent lamps. This makes them good choices for hard-to-reach spots, like gymnasium and high-bay ceilings in warehouses. Their useful life is defined by the Alliance for Solid-State Illumination Systems and Technologies as the length of time it takes until 70% of initial light output is reached, often 50,000 hours, which is about 50 times longer than a fluorescent bulb. In terms of efficiency, a 13-W LED lamp produces 450 to 650 lumens, which is equivalent to a standard 40 W incandescent bulb (See tinyurl.com/LEDoverview ).

 

Payback can be quick—as little as 2 or 3 years, depending on costs and what they are replacing, though a payback of 7 to 8 years is more likely at this time. LEDs aren't necessarily more efficient than T8 fluorescents, however, so engineers may opt for “traditional” lighting to compensate for clients' budget needs. LEDs can direct most or all of the light from a fixture to the target area.

 

They're also good for artistic purposes, and can offer clients several colors. The emitted light can be red, yellow, orange, green, blue, or nonvisible infrared. The quality of this light differs from LED to LED. The color rendering index (CRI; generally the higher the number, the better) and consistency are two key areas to watch. With LEDs, the CRI listing for LEDs actually may not provide a good representation of color rendering. The trick is to get a good, comfortable white light (generally around 3,500 degrees Kelvin), which is important in office, retail, and similar settings. The efficacy of LEDs is generally greater with higher color temperatures (6,000 degrees Kelvin is a high color temperature).

 

LEDs generally dim well. LEDs can be more efficient than other lamp sources when dimmed because LEDs run cooler dimmed. Other lamps such as fluorescents lose efficacy (lumens per Watt) when dimmed. Fluorescent dimming ballasts are also expensive.

 

Like anything, LEDs have drawbacks. LEDs radiate heat, so thermal properties must be taken into account when considering the building's HVAC system—LEDs can be up to 40% of the heat load. The heat from LEDs does not radiate to the target area. This is good for applications that are sensitive to heat (flowers, some museum artifacts). Also, their higher first-cost may price them out of the budget. Finally, white LEDs may turn blue or experience color shift over time.

 

Several manufacturers offer various LED products, and that's where some of the confusion lies. Which product is the best for a given project?

 

Start with the U.S. Dept. of Energy (DOE), which has a wealth of information on applications, measurement, color, and other solid-state lighting information (See tinyurl.com/DOEonLEDs ). Test results, standards information, and research details are all online. Not all manufacturers do this, however, so some LEDs don't have the same proven efficacy as others. This is where much of the confusion sets in for lighting engineers.

 

Tread lightly

Traditional light sources (incandescent, fluorescent, and high-intensity discharge) are rated for luminous flux according to established test procedures. In contrast, there is no standard procedure for rating the luminous flux of LEDs. LED light output estimates (as reported by manufacturers) typically are based on a short (&1 sec) pulse of power applied to the LED chip, with junction temperature held at 25 C. This is because LED chips must be binned for luminous flux and color during the manufacturing process. To run them longer without a heat sink would damage them. LED manufacturers usually list “minimum” and “typical” luminous flux on their product datasheets. There is no standardization of the test conditions, or the meaning of “typical.”

 

Codes and standards

Codes and standards are still tricky, as LED technology is still relatively untested. Many manufacturers work within the LM-79 or LM-80 standard (See tinyurl.com/DOEstandard ). The DOE CALiPER program has revealed many LED products are not meeting manufacturers' specifications. The lighting industry looks to the Illuminating Engineering Society of North America (IES) for measurement test procedures. These test procedures are designated “LM” for lighting measurement, followed by an ordinal number, and the year of adoption or revision. They are developed by the IES Testing Procedures Committee, whose members include representatives of industry, research institutions, and testing laboratories.

 

The document entitled “IES Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products,” designated LM-79-08, was developed by a joint IES-ANSI committee on solid-state lighting and published in 2008.

 

Picking the right light

 

So what does this all mean? It means that you should ask yourself and your client several questions when considering various lighting options.

 

First, where are LEDs going to be used? If you're considering outdoor parking decks, recessed can lighting fixtures, task lighting, or colored decorative lighting, LEDs might be a good choice. Some manufacturers focus on the retrofit market, while others work solely in new construction. Know your manufacturer and where the light is going to be used.

 

Second, what kind of payback is expected? Up-front costs of the fixture, voltage transformation, and the lights may be high. Payback comes in the form of longer re-lamping schedules, utility rebates, and overall energy efficiency. LEDs' lower power usage is advantageous here.

 

Third, what kinds of lamps or fixtures will LEDs be installed with, and will they be independently controlled or integrated with the building's overall control system? Dimmers and building-wide lighting controls (such as timers) are common. BAS integration is where things get difficult. Some lighting manufacturers are opting for proprietary control protocols. Others are working toward compatibility with an open standard system like BACnet. Specify and purchase LED fixtures that are modular that have easily replaceable LEDs and drivers.

 

Fourth, manufacturer quality is a significant issue for engineers to consider. The emergence of LEDs has introduced a bevy of U.S. and international companies making and selling LED products. Do the homework on the lighting company, not just the technology. In any case, for large projects, it would be best to buy and test lighting fixtures before committing to a sizable order. Be wary of sample lighting fixtures with LEDs, as only a few LED chips have very high lumens per Watt along with excellent CRI. Buy one or more lighting fixtures through a third party and compare performance with the sample. Do not specify or purchase an LED product that has not been LM-79 tested with low or no LM-80 information. Lean toward Energy Star-rated LED fixtures and manufacturers with a proven track record.

 

 

 



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