Both T5 high-output (HO) linear fluorescent and pulse-start metal halide HID lamps are excellent lighting options for high-bay applications in a variety of environments. In some installations, either lamp would be an excellent choice. In others, one lamp may provide greater benefits than the other. Each light source has several attractive attributes that might tip the scales toward one choice over another. Plant engineers involved in lamp and fixture selection can benefit from understanding the similarities and the differences between the two.

Historically, high-ceiling applications have been dominated by HID due to its high light output, energy efficiency, and extended life (Fig. 1). However, the new T5 HO lamps, with their smaller diameter, have greatly improved light output and compete well against HID in many high-bay applications.

Today, both technologies have evolved into very efficient lighting systems. T5 HOs are high-output versions of T5s that deliver up to 67% more light than the standard T5s (Fig 2). Additionally, T5 HOs deliver 95% lumen maintenance, a color rendering index (CRI) of 85, and instant starting. Pulse-start metal halide lamps, which use pulse-start ballasts, compare favorably to an earlier popular HID technology — switch-start metal halide lamps. The upgraded benefits include improved energy efficiency and lumen maintenance, faster warmup to full brightness, faster restrike time if power is interrupted when hot, and extended lamp life.

Both T5 HOs and pulse-start metal halide lamps are excellent choices for a lighting retrofit or a new lighting layout at a factory or warehouse. Key factors to consider when evaluating and comparing systems are:

• Plant activity being lit

• Total fixture cost (with lamps and ballasts)

• Number of lamps required

• Energy usage of each system

• Lumen maintenance

• Color rendering index

• Ambient temperature

• Ceiling height

• Restrike time constraints.

• Other considerations should include potential energy savings achievable through use of dimmers and motion sensors, as well as the practice of daylight harvesting (a conservation measure suitable for spaces with ample natural light), and aesthetic concerns.

  Ceiling heights

  A typical high-bay installation of between 25 and 45 ft calls for between 36,000 and 44,000 initial lumens, or 35-50 ft-candles, on the floor. If the lighting source is to be installed very high above the floor, from about 45 ft and up, pulse start is the best choice because of its high light output (see sidebar “Key cost and performance parameters” for a comparison). At this height, T5 HOs are not a feasible lighting solution because of the number of lamps needed to achieve similar foot-candles.

  However, at a lower height range of 20-35 ft, because of the inherent characteristics of T5 HOs (high lumen output and high lumen maintenance of fluorescents), they become a valid alternative.

  To achieve the target of 35-50 ft-candles at ground level, typically a pulse-start metal halide installation uses single high-bay fixtures (each with one lamp and one ballast) spaced throughout the plant. To achieve similar light levels, a T5 HO installation would have to use a fixture that requires four to six lamps and two or three ballasts, spaced throughout. This is because of the lower lumen output.

  Fixture cost and installation

  If the initial purchase and installation cost is a major factor, the advantage goes to pulse start. A pulse-start metal halide fixture, with one lamp and one ballast, is typically less expensive than a comparable T5 HO fixture requiring four to six lamps and multiple ballasts. Additionally, the T5 HO lamp and fixture technology is newer, therefore the systems tend to be more expensive. These costs will decrease with increased industry capacity and competition.

  Lumen maintenance

  Light output from all lamp types degrades, or dims, over time. In environments where the light level is critical, it is important to configure an application based upon the light level that will exist in the middle of the lamp life to ensure maintained light levels. Lumen maintenance (or design lumens) quantifies how well a lamp maintains the light output level at 40% of its rated life. Pulse-start metal halide lamps achieve a respectable 75% lumen maintenance; the T5 HOs achieve an excellent 95% lumen maintenance. In facilities where the falloff in light level could be critical, the T5 HO wins hands down. You get bright light longer, essentially achieving a close-to-constant light level throughout the life of the lamps.

  Color rendering index

  The CRI of lighting systems may sometimes be the deciding factor in a light source selection. For installations where true color under the lights is important, T5 HOs are recommended over pulse start because of their higher CRI (85 vs. 65). Furthermore, because T5s feature minimal color shift between lamps, they will provide uniformity among fixtures, one next to another, and also from one section of the plant to the next. However, in an industrial setting other factors normally take precedence over CRI.

  Relamping maintenance

  In terms of long-term maintenance cost, the two technologies share similar performance characteristics. For example, a 320-W pulse-start metal halide lamp, and 24, 39, and 54-W T5 HO lamps have 20,000 hr rated life.

  Good lamp maintenance practices for metal halide lamps include shutting them off once a week for 15 min. So, in facilities that operate continuously, the clear advantage goes to T5 Hos, since they do not require cycling.

  With respect to maintenance at the end of life, a T5 HO system having multiple lamps will provide light even when one lamp is out, while a one HID lamp system will leave a dark space when the lamp expires. However, if the HID lamp can readily be replaced from the ground with a pole, then the dark space is easily remedied. The T5 HOs require lifts for replacement.

  If a facility practices group relamping, then the scale tips toward the HID one-lamp source, since less labor is required per fixture.

  Energy efficiencies

  Both the pulse-start metal halide and T5 HO technologies have excellent energy efficiency. A pulse-start metal halide lamp delivers approximately 110 lumens/W, compared to 83-93 lumens/W for T5 HOs. However, using dimmers and sensors over a full range of control settings can reduce T5 HO energy consumption.

  Though the older 400-W metal halide lamps were somewhat less efficient than the T5 HOs, the 320-W pulse-start metal halide lamps have just about eliminated any advantage the T5 HOs may have held in that arena. However, the balance in any equation will shift in favor of T5 HOs if the plant can use a full range of dimmers and sensors to reduce energy consumption, because they work with dimmer controls over a full range of control settings. In contrast, metal halide lamps are limited to one or two stepdown settings.

  Restrike time

  Pulse start does not provide the “instant on” of a fluorescent system. Pulse-start metal halide systems typically warm up to 90% brightness within 2 min. and restrike for hot lamps within 4 min. In facilities where motion detectors can control turning the lights on, off, or to a preset dim level, the T5 HO offers greater dimming range than the stepdown dimming of pulse-start lamps, which dim the lamp to 50%.

  While stepdown dimming could be fine in a warehouse, it is not a good choice in an environment with constant or intermittent traffic. In a warehouse that uses motion or infrared sensors to detect people or machines moving down an aisle to pick materials or parts, the instant-start feature of T5 lamps is compatible, while the slower start of the metal halide lamps is not.

  Daylight harvesting

  In a plant that has the ability to use natural light, such as one with windows and/or skylights, a T5 HO system can take advantage of the practice of daylight harvesting to keep energy costs down. Daylight harvesting integrates electric light sources, light sensors, and dimmers with use of daylight through windows and skylights. As the natural light level within a space changes, the sensors perceive the differences and automatically regulate the dimmers to lower or raise electric light levels to maintain a constant light level within the environment. The more daylight can be used, the greater the energy and cost savings. If a plant can make use of natural light, the advantage goes to T5 HOs.

  Temperature

  T5 HO, like other fluorescent technology, is sensitive to both hot and cold temperatures, and has maximum light output at 35 C. Pulse start, however, operates equally well in a wide temperature range, and therefore has the advantage in hot or cold climates.

  Personal preference

  Aesthetics also could be a differentiator. One plant engineer might prefer the linear, more commercial ambiance that T5s provide, while another might favor the more industrial look of pulse-start metal halide.

  — Edited by Jack Smith, Senior Editor, 630-288-8783, jsmith@reedbusiness.com

  More info

  The author is available to answer questions about this article. He can be reached at ted.simpson@philips.com

  Key cost and performance parameters

  	Pulse Start MH	T5 HO
  Fixture Cost	+	–
  Lamps Required	+	–
  Energy*	even	even
  CRI	–	+
  Lumen Maintenance	–	+
  Color Control	–	+
  Re-strike Time	–	+
  Use w/ Dimmers & Sensors	–	+
  Temperature Sensitive	+	–
  *Both Pulse-start metal halide and T5 HO have excellent energy efficiencies, however with the use of dimmers and sensors the T5 HO lamps have an energy advantage.

  Classifications for metal halide lamps

  Because metal halide lamps operate at high internal pressures, manufacturers provide explicit instructions and warnings to ensure proper use. Adherence to these instructions is critical to the safe operation of metal halide systems.

  Lamp manufacturers classify every metal halide lamp according to the recommended manner in which it should be used. The following are the three American National Standards Institute (ANSI) classifications:

  “E” types are to be used only in suitably rated enclosed fixtures in accordance with UL 1572 and CSA C22.2 No. 9.0 (UL 1598 and CSA C22.2 No. 250.0).

  “S” types may be used in open fixtures when operated in the specified near-vertical position. This category of lamps is limited to only certain lamps in the 350 to 1000-W range.

  “O” types comply with ANSI Standard C78.387 for containment testing and may be used in open fixtures.

  For more information on best practices for metal halide lighting systems and FAQs about ruptures in metal halide lighting systems visit the NEMA website (

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