High/Low-Bay Applications: Fluorescent or Metal Halide?

By Craig DiLouie, Lighting Controls Association

Indoor spaces with high ceilings, such as factories, warehouses, big box retail stores, gymnasiums and all-purpose rooms are most often lighted by high-intensity discharge (HID) lighting systems.

Recently, manufacturers have begun offering specialized T8 and T5HO fluorescent fixtures as an alternative for high-ceiling applications. These fixtures provide distinct advantages versus HID fixtures. Traditionally, fluorescent lighting has dominated the <15 ft. ceiling height niche, but new technology has enabled it to be competitive with HID in higher ceiling heights, even over 25 ft.

Figure 1. Fluorescent lighting provides distinct advantages over HID lighting in applications with high fixture mounting heights; new lamp technology enables fluorescent to be competitive with HID at fixture heights as high as 35 ft. Courtesy: Sacramento Municipal Utility District (SMUD).

Manufacturers offer new fluorescent fixtures that can be substituted for HID for relighting/upgrade as well as renovation and new construction projects. The market potential is significant. However, relighting projects typically require installation of new fixtures, which can inflate payback periods and reduce return on investment.

A second alternative to standard HID is pulse-start metal halide, which provides superior performance and, depending on the ballast, dimming capability. The right choice, as with all things in lighting, depends not just on economics, but on what capabilities the owner needs in their lighting system.

In this special report from the Lighting Controls Association, we will examine the advantages and disadvantages of fluorescent and metal halide in applications with high fixture mounting heights.

Ceiling heights
In the lighting industry, we often hear talk of “high bay” (also called hi-bay) and “low bay” (lo-bay) lighting. What does this mean?

In the construction of some types of industrial facilities, a skeletal framework is used, which forms an interior subspace called a “bay,” which in turn marks the space as “high bay” or “low bay.”

An older definition designated high-bay to mean >25 ft. off the floor, medium-bay to mean 15-25 ft., and low-bay to mean <15 ft.

Some manufacturers define high-bay as being over 15 ft. or 20 ft. off the floor.

The Illuminating Engineering Society of North America (IESNA), the authority in lighting, categorizes spaces as either high-bay (>25 ft.) or low bay ( <25 ft.).

The terms “hi-bay” and “lo-bay” also refer to fixtures designed for these applications, although it is not uncommon to see hi-bay fixtures in low-bay applications, and vice versa.

Metal halide: standard choice for many high/low bay applications
Metal halide is the dominant light source in many high-ceiling applications in warehouses, industrial buildings, gymnasiums, all-purpose rooms and big-box retail stores. At higher ceiling heights, 250W and 400W sizes are common. High pressure sodium lamps are also popular; for this article, however, we will focus on metal halide.

In review, the core operating components of a typical metal halide lamp include three electrodes within an arc tube that contains mercury and other metals in iodide form. One electrode is used for starting, and two for operating. The arc tube is constructed to withstand the internal high temperature and stresses of HID lamp operation. The tube is enclosed in a borosilicate glass bulb that is also highly heat resistant. These lamps are called probe-start metal halide lamps, or simply metal halide lamps.

These lamps are compact, rugged, powerful point sources. They produce enough light to cover a large space with few fixtures. A 400W standard metal halide lamp, for example, produces 36,000 initial lumens. Some standard metal halide lamps are rated as high as 41,000 lumens. They’re also efficient and provide good service life. At 10 hours/start, the service life of a 400W metal halide lamp is rated at 20,000 hours.

Compared to fluorescent, metal halide lamps have several distinct advantages. Metal halide is a powerful light source offering high lumen packages, and, depending on the application, can present a lower installed cost due to fewer fixtures. They are able to operate reliably in a wide range of ambient temperatures, including very cold environments, where fluorescent performance can be dramatically impaired (except for induction lighting, which is considered part of the fluorescent family). And numerous models are available that are specially designed for a wide range of demanding environments such as hazardous locations.

Despite their advantages, standard metal halide systems do not include any dimming capability, experience color shift over time, require four minutes to start, and require about 10 minutes for re-strike after shut-off. Most significantly, at 40% of service life, light output and efficacy experience severe degradation. A 400W metal halide lamp, for example, may produce 36,000 lumens but 25,000 at 40% of life, a 30% decline. Therefore, unless the lamps are periodically group-relamped, a large system’s “average” performance over time is much lower than its initial ratings.

Pulse-start metal halide
Before entertaining a challenge to metal halide by fluorescent, we should consider pulse-start metal halide as an alternative to standard metal halide. Pulse-start technology has been used in low-wattage metal halide lamps for decades, and has recently begun being used in higher-wattage lamps up to 1000W.

“Pulse-start metal halide has so many advantages over standard metal halide that the latter should get a fond farewell and gracefully retire,” says Stan Walerczyk, LC, principal of Lighting Wizards, a consulting firm. “It is a shame how many new standard metal halide hi-bays are still being installed.”

(He adds, “Some lamp manufacturers salespeople try to push the 360W energy-saving standard metal halide lamps to replace 400W standard metal halide lamps on existing ballasts, because it can be a quick sale, and a contractor is not necessary. Except for short-term leases, I usually find better solutions.”)

In a pulse-start metal halide lamp, the number of electrodes is decreased from three (probe start) to two, while a high-voltage ignitor is added that provides 3-4 kV pulses to ionize the gas and produce the glow discharge. Pulse-start metal halide lamps produce higher light output, enjoy higher lumen maintenance, are more energy-efficient (15% more efficient), produce a whiter light, and re-strike faster after an outage.

However, pulse-start lamps cannot be used with standard magnetic ballasts, instead requiring the installation of a compatible pulse-start ballast.

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