Solid-state lighting: The dawn of a new day?

Forward-thinking cleanroom engineers contend that LEDs are poised to run fluorescents out of Dodge


The automobile replacing the horse and buggy; the electric light bulb replacing the oil lamp; digital replacing analog. Some contamination-control industry watchdogs believe the inevitable transition from fluorescent and incandescent lamps to solid-state lighting (SSL) will be a technological milestone of similar significance for cleanroom owners.

In 2001, the Optoelectronics Industry Development Association (OIDA; Washington, D.C.) declared that rising fuel prices and the threat of blackouts were sparking “a silent revolution in SSL that holds the promise of replacing conventional light sources the way integrated circuits replaced electron tubes fifty years ago.”

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A key point of differentiation in the comparison of LED (below) to fluorescent (above) is to illustrate the greater diffusivity of fluorescent lighting in a cleanroom. Note that the “drop-off point,” or the point on the wall at which shadowing begins, occurs much lower on the wall with LED. This supports the point that LED's inherent directionality continues to be a challenge in cleanroom environments.
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The efficiency and reliability of the Light Emitting Diode (LED) is an innovation driven by materials science. Electrical current is more efficiently converted into photons within the LED, with most light generated by an LED being within the visible region—so, less energy is wasted in the infrared and UV wavelengths.

If SSL were to be actively adopted in the U.S., it would produce enough energy savings to alleviate the need for 133 new power stations of 1000 MW each by 2026, yielding a financial savings of some $115 billion.

As exciting as this prospect is, most observers also agree that the bright expectations for LED have yet to be fully realized, much less applied. This is especially evident to those who have been testing LED lighting in one of the most challenging lighting environments humanity has yet devised—the cleanroom.

The state of LED

The perceived advantages of LEDs appear to make them particularly attractive for cleanroom environments. Conventional wisdom has held that LED technology offers the inherent technology to generate light longer, with less degradation, at cooler temperatures and at lower cost per lumen than traditional fluorescent lamps.

The superior longevity of LED's reputed “100,000-hour life” means less maintenance per lamp, which means less in-situ maintenance required—with no risk of shattered glass in the clean zone.

But there are a few drawbacks. The length of useful life for LED lamps can be inconsistent; LEDs can be vulnerable to temperature issues; LEDs uniformly distribute luminous flux (expressed in lumens) along a linear component. Solutions to all of these concerns are being pursued, and many believe that LEDs will be in position to dominate cleanroom lighting within five years.

The LED's long-term advantage is that, like the semiconductor chip, it's based on solid-state technology. This has allowed improvement of LED technology to follow a progression of steadily increasing performance, mimicking the “Moore's Law” model where semiconductor performance theoretically doubles approximately every 18 months. Following suit, LED lumen output is also doubling approximately every 18 months.

LED testing in real world cleanrooms has leavened the high LED expectations with a dose of reality. LEDs are still in a developmental state that leaves them well short of perfection—at least for a while longer.

LED put to the test

Because LED technology has yet to meet its fullest light-yield potential, LED lighting is not yet competitive with small diameter linear fluorescent lamps.

One of the biggest concerns posed by the LED to date is its light output, with LED and fluorescent technologies differing in the way they deliver lumens to a given target. A typical LED lighting system tailored to a linear application consists of intense miniature downlights; each produces individual focused beams along a tightly spaced string, producing direct illumination. In contrast, linear fluorescent lamps emit a soft, uniform glow across their tubular surface, producing more diffuse illumination.

A recent cleanroom lighting project let us perform a side-by-side comparison of LED and fluorescent lighting systems that were installed in adjacent cleanrooms. Both lighting modes were housed in conventional flush grid lighting systems. Viewing windows allowed simultaneous comparison of lighting conditions in both rooms.

We discovered that perceived brightness appeared greater in the cleanroom with the fluorescent light. Yet measurement of light levels in each room revealed that there was actually 25 percent more light present in the cleanroom with the LED system.

This optical illusion was the result of the LED system creating a “cave effect,” a term known commonly in office environments where low brightness deep-cell parabolic or mini-cube louvers are employed.

While the LED system was actually producing more light at the measured task, it was doing a poor job of inter-reflecting light off the vertical surfaces throughout the space—the cavernous appearance of the room with a shady upper wall and ceiling zone.

The room with fluorescent lighting had quite the opposite effect. More light energy was diffused throughout the space, yielding lower measurable light at the horizontal task plane 30 inches above the raised floor. As any overworked iris knows, it is the reduction of contrast that creates a visually pleasing and ergonomically sound environment. Occupants typically prefer soft uniform lighting rather than high-contrast task illumination.

The challenge of advancing LED technology into the linear lighting market is delivering the high lumen potential in a more ergonomically desirable way. LEDs also require careful mixing and balancing of colors to achieve desired hues in a given cleanroom.

Much activity in microelectronics cleanrooms involves photosensitive processes, with certain light wavelengths being highly disruptive to photosensitive chemicals and processes.

For many years, the solution to cutting out ultraviolet wavelengths in microelectronics cleanrooms has been to use yellow filters over fluorescent lamps—not necessarily an efficient solution. The filters can reduce light output by up to 60 percent, requiring more filtered lamps, thus more energy consumed in the cleanroom to compensate for the shortfall.

This approach is not the most ergonomically desirable either, since it subjects cleanroom workers to long shifts under yellow light conditions. For certain photosensitive processes, LEDs can potentially deliver a uniformly “light safe” environment with white light.

The problem is that the idea of white light with LEDs can only be achieved by carefully mixing the right “recipe” of different colored LEDs within luminaires to render the desired hue of white.

This commingling of different LED colors is called “binning;” however, the trade-off of binning differently colored diodes is reduced overall brightness achieved by the luminaire.

What's in store?

Some promising new technologies on the horizon are expected to ultimately resolve LED shortcomings. A few years ago, the microelectronics industry was abuzz with talk of the inevitable arrival of the new generation of manufacturing technology using 300-mm semiconductor wafers. But it was several years before even the largest manufacturers in the industry did anything with 300 mm, other than talk about its inevitability. When 300-mm processing technology finally hit, it did so with a relative suddenness and force that truly revolutionized the industry.

On a smaller scale, LED lighting is similarly poised to prompt its own mini-revolution in the cleanrooms of the world. The technological advances to date suggest that LED is a more efficient lighting system. Longer life and reduced material composition give LEDs ecological advantages over some conventional lighting systems. Like many emerging technologies, it simply needs to have a few bugs worked out of the system.

STEPHAN GILGES is an architectural and lighting designer with IDC Architects in Portland, Ore. He has served as a design lead and manager on international and domestic projects, providing specialized lighting systems to a variety of clients in science and technology industries. STEPHEN PADDON is a technologist with IDC, with specialization in contamination control technologies. He has experience in a wide range of product yield issues in cleanrooms, including EMI, airborne particulate and molecular contamination, and acoustic and background vibration. AL VIADO is a lead technical specialist in lighting for IDC. He is lighting certified (LC) through the National Council of Qualifications for the Lighting Professions and a former president of the Oregon section of the Illumination Engineering Society of North America. He has been a recipient of four distinguished IIDA awards, with areas of experience in lead design of lighting and power systems for many of the world's most advanced cleanrooms.

Fixtures Update

  • Materials—Fixture bodies and doors are available in steel, stainless steel and aluminum. Aluminum housings provide the coolest environment for the electronic ballast.
  • Optics—Semi-specular white provides a reflectivity of over 90 percent to increase fixture efficiency. In 12- to 14-inch ceilings, a narrower distribution is required to punch the light down. Sealed louvers are available if glare or computer screen visibility is an issue.
  • Special construction—3D CAD systems can model unusual grid configurations to ensure that the fixture fits properly.
  • Sealing—Requirements depend on the application and classification of the installation. Housings may be sealed with silicone, or welded. Door frames are available hinged, inset or overlapped with 4-, 6-, or 12-point gasket compression. The lens is mechanically held and gasketed or silicone-sealed inside the doorframe. On walk-able ceilings, top access fixtures with permanently sealed bottom lenses are used.
  • Non-laminar flow—Fixtures are recessed flush to the grid or solid ceiling. The fixture body must be taller than the grid web to allow for lamp installation.
  • Laminar flow—Fixtures have a teardrop design. Those sized for the smaller T8 lamp minimize disruption to the airflow. Screw slot grid in the ceiling matched with a pre-drilled housing can minimize installation time. Integrated systems have been developed that put the lamps and ballasts inside the grid with either a flush or teardrop lens. They are most often used in rooms with high filtration and are installed by a specialized cleanroom contractor.

Step into the right light

Lamp, ballast and fixture design advances can help significantly lower a facility's initial, operational and maintenance cost

An imperative that is all too often taken for granted in cleanroom design and maintenance is facility lighting. But in these days of tight cost-cutting and smart, more efficient design, cleanroom lighting is becoming a critical specification—and will only gain more importance as the specter of LED looms in the distance.

We've surveyed the latest advances in lamps, ballast and fixture design that will significantly lower a facility's initial, operational and maintenance cost. In turn, this information will help cleanroom owners and operators minimize fixture and lamp requirements on current and future projects.

Fluorescent technology

Linear fluorescent: T8 lamps have become the standard for new construction, and even smaller T5 lamps are entering the scene in indirect fixtures. New phosphor coatings provide 17 to 28 percent higher lamp output and almost perfect natural color. Better lamp design has allowed life ratings up to 30,000 hours, and new low mercury lamps don't require special disposal.

Fluorescent electronic ballasts: Electric ballasts optimize lamp performance and minimize energy. Their high frequency (20,000 hertz) eliminates lamp flicker and extends lamp life. Newer designs are smaller, intelligent and universal. Many operate different wattage lamps and accept 120 or 277 volts. Some can operate four lamps, while “Instant Start” versions operate lamps in parallel. If one fails, the others still operate. It's key to remember that electronic ballasts are sensitive to heat—the fine print states that they need to operate at 70∞C for full life.

For new construction, the trend is toward increasingly smaller linear fluorescent lamps.
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T8 ballast factor opportunity: Ballast factor indicates the percent watts provided to the lamp versus its rated wattage. The low-cost “Normal Output” ballast has a “ballast factor” of .87. The “Low Watt” ballast factor is .75 and the “High Output” is 1.20. They drive the lamp to provide 87, 75 or 120 percent of its rated lumens. Ballasts for T5 and compact fluorescent lamps have a ballast factor of 1.

Ballast controls: Dimming systems and software have the potential to reduce energy through daylight harvesting, sensors and optimizing illumination to task requirements. An open digital addressable lighting interface (DALI) is slowly becoming the global lighting communications standard. Each ballast can be individually controlled and belong to any or all of 16 different groups. The open standard allows interchangeability from different manufacturers, and ballasts and control systems are just entering the marketplace.

Ultraviolet and special color lamps: Special spectrum systems normally use tinted lens, lamp sleeves or coated lamps. I would recommend sleeves or tinted lamps. They are readily available from many sources. Lens extruders require minimum runs of tinted plastic. Replacements may be impossible to obtain, so always stock spares from the initial run.

This metal halide HID lamp combines mercury and metal halide atoms under high pressure. In the arc stream, these atoms generate both ultraviolet radiation and visible light. A specially formulated glass bulb filters the ultraviolet radiation without affecting the visible light.
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Break-proof lamps: These are available from coating companies and some manufacturers. They generally use clear Teflon to contain glass breakage from fluorescent, incandescent or HID lamps, and are commonly used in food process areas.

Induction fluorescent lamp: This is a new system from two manufacturers that uses microwave energy to excite the lamp. Life is rated for 100,000 hours and is available in sizes up to 165 watts. This system is expensive but may be suitable for high-cost maintenance areas.

Compact fluorescent lamps: Interest in this new lamp has exploded in the last two years. They offer high lumens and 83 percent color accuracy, and most are dimmable. Unlike linear lamps, the output is almost unaffected by ambient temperature. They are the new standard in down lights and offer alternatives to metal halide, linear fluorescent and incandescent lamps.

Improved arc tube and a remote starter in pulse-start metal halide lamps offer increased lumens, better color, and longer life when compared to probe-start lamps and ballasts.
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High-intensity discharge technology

Pulse-start metal halide lamps: These are replacing probe-start lamps and ballasts. The improved arc tube and remote starter provide increased lumens, better color, longer life and reduced re-strike time (see Figures 2 and 3). Thirteen sizes are available, from 39 to 1,000 watts. The lamps have a crisp white color, are unaffected by temperature, provide high lumens in a small package and are an economical choice above 14 feet.

Metal halide ballast improvement: Thirty-percent more light from the 320-, 350- and 400-watt pulse-start lamps is now available with the new universal electronic ballast. This improvement provides dimming to 50 percent power, end-of-life lamp protection and silent operation. Lamp life is expected to increase beyond the present 20,000 hours, and fixture reduction eliminates an increase in first costs.

Advances in technology when properly applied can provide a low-cost and energy-efficient installation. Today's technology can provide 100-foot candles at less than 1.5 watts per square foot.

ROBERT CATONE is general manager of Guth Lighting (St. Louis, Mo.) and vice president of JJI Lighting Group. He is a Certified Lighting Professional by the National Council of Qualifications for the Lighting Professions (NCQLP) and a member of the Illuminating Engineering Society Progress Committee. He can be contacted at: [email protected]


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