Replacing the Fluorescent Lamp with Linear LED Luminaires

The drive for energy efficiency and new componentry delivers viable LED solutions
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Sponsored by Selux
James Brigagliano, LC, IESNA

Fluorescent Tubes
More energy efficient than CFLs, fluorescent tube lamps—the second most popular type of lamp—are usually identified as T12 or T8 (12/8 or 8/8 of an inch tube diameter, respectively). They are installed in a dedicated fixture with a built-in ballast. The two most common types are 40-watt, 4-foot lamps, and 75-watt, 8-foot lamps. Tubular fluorescent fixtures and lamps are preferred for ambient lighting in large indoor areas because their low brightness creates less direct glare than incandescent lamps.

Fluorescent tube construction. A fluorescent lamp tube is filled with a gas containing low-pressure mercury vapor and argon, xenon, neon, or krypton. The inner surface of the lamp is coated with a fluorescent (and often slightly phosphorescent) coating made of varying blends of metallic and rare-earth phosphor salts. The lamp's electrodes are typically made of coiled tungsten and usually referred to as cathodes because of their prime function of emitting electrons. For this, they are coated with a mixture of barium, strontium and calcium oxides chosen to have a low thermionic emission temperature.

Linear LED luminaires provide soft lighting for Boston University Rich Hall residence in Boston, Massachusetts.

Photo courtesy of Hyexposure Photography

 

LED Technology

Unlike incandescent and fluorescent lamps, LEDs are not inherently white light sources. Instead, LED is a semiconductor light source. When a light-emitting diode is switched on, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is often small in area (less than 1 square millimeter), and integrated optical components may be used to shape its radiation pattern.

LEDs emit nearly monochromatic light, making them highly efficient for colored light applications such as traffic lights and signage. However, to be used as a general light source, white light is needed.

White light can be achieved with LEDs in two main ways: phosphor conversion, in which a phosphor is used on or near the LED to emit white light; and RGB (red, green, and blue) systems, in which light from multiple monochromatic LEDs (red, green, and blue) are mixed, resulting in white light.

Directional Light

One of the defining features of LEDs is that they emit light in a specific direction. Since directional lighting reduces the need for reflectors and diffusers that can trap light, well-designed LED fixtures can deliver light efficiently to the intended location. In contrast, fluorescent and “bulb”-shaped incandescent lamps emit light in all directions where much of the light produced is lost within the fixture, reabsorbed by the lamp, or escapes from the fixture in a direction that is not useful. For many fixture types, including recessed downlights, troffers, and undercabinet fixtures, it is not uncommon for 40 to 50 percent of the total light output of fluorescent and incandescent lamps to be lost before it exits the fixture.

Extended Lifetime

The rated lifetime of LED products is at least comparable to other high-efficacy lighting products, if not better, and for many specific product types, LEDs have the highest rated lifetime. This attribute can be especially important where access is difficult or where maintenance costs are high. In fact, several U.S. Department of Energy GATEWAY demonstrations have revealed that maintenance savings, as opposed to energy savings, are the primary factor in determining the payback period for an LED product.

LED Market

A perfect storm of state and federal government attention to energy-saving LEDs plus industry investment in LED technology is delivering an abundance of products that enter the market about every four to six months. ANSI/ASHRAE 90.1-2010, Title 24 2010 California Building Standards & Codes, LEED® requirements and the DOE are all shaping the LED market. ANSI/ASHRAE 90.1-2010 is pushing increasingly strict energy code models with goals to have market-viable net-zero buildings by 2030.

Lighting programs such as ENERGY STAR, a joint program of the U.S. Environmental Protection Agency and the DOE, and DesignLights™ Consortium, a collaboration of utility companies and regional energy efficiency organizations, are offering rebate programs. Also related is the DOE's LED Lighting Facts® program which showcases LED products performance by reviewing third-party testing and supplying an approved label indicating performance. Commercially available LED Product Evaluation and Reporting (CALiPER) program, provided by the DOE, is a useful resource that shares case studies and extensive testing results.

But the quality and energy efficiency of LED products still varies widely as LED technology continues to evolve and luminaire manufacturers negotiate the learning curve of integrating LEDs into their products. To keep up to date with LED technology developments, design professionals are urged to track the many organizations supporting LED technology.

Successful LED Applications

Some LED products have performed well. These include a wide range of replacement lamps, as well as integrated light fixtures, such as portable desk/task lights, under-cabinet lights, recessed can downlights, track heads, and outdoor fixtures for street and area lighting.

Recessed LED downlights. One of the earliest applications of solid-state lighting for general illumination, LED recessed can downlights are now widely available in a range of sizes and lumen packages, offering a viable alternative to incandescent and compact fluorescent (CFL) products. As the technology has advanced, LED downlights have in many cases become superior to conventional downlights in terms of energy efficiency. This has led to the increasing choice of LED lamps and LED downlight retrofit units as replacements for omnidirectional lamps.

 

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Originally published in Architectural Record
Originally published in November 2012

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