Purchasing Advisor

High-intensity discharge (HID) light sources, such as metal halide and high-pressure sodium lamps, have long dominated the market for lighting indoor spaces with high ceilings. These "high-bay" spaces are typically found in warehouses, factories, large retail stores, and athletic facilities. In recent years, however, improvements in fluorescent lamps and the emergence of new high-intensity fluorescent fixtures have made fluorescent lighting the most cost-effective choice for lighting high indoor spaces. These high-intensity fluorescent systems are more energy-efficient than HID solutions and feature lower lumen depreciation rates, better dimming options, virtually instant start-up and restrike, better color rendition, and reduced glare.

What Are the Options?

HID lamps. HID lamps produce intense light in such a small area that they are considered "point sources." As a result, they are often installed in fixtures that direct their light using parabolic reflectors. Compared with other installations (of the older T12 fluorescents, for example), an HID installation requires fewer individual fixtures, which sometimes allows for lower capital and installation costs. HID lamps are popular in applications that feature large expanses lit by distant fixtures, such as indoor and outdoor sports facilities, factories and warehouses with high ceilings, and streetlighting.

Although there are several different kinds of HID lamps, the most popular types for indoor applications are metal halide and high-pressure sodium lamps. A key difference between these two is the type of vaporized metal that constitutes the gas within the inner glass vessel of the lamp, through which the electric arc is struck. Of these two types, metal halides—with their high-quality light, high efficacy, and wide range of sizes—are more versatile.

Fluorescent lamps. Fluorescent lamps emit diffuse light from long glass tubes. This characteristic of diffusivity has enabled fluorescent fixtures to dominate the market for lighting commercial, institutional, and industrial spaces with ceilings less than 15 feet high. In recent years, however, the emergence of more intense and efficient fluorescent lamps coupled with specially designed reflecting fixtures has enabled fluorescent systems to break through the ceiling-height barrier and compete directly with HID lamps in indoor applications.

Electrodeless induction lamps. Inductive fluorescent lamps also have a role in illuminating high-bay areas. These relatively new lamps use radio frequency energy rather than an electric arc to excite phosphors and produce light. Inductive fluorescents have an extremely long life (up to 100,000 hours) and excellent cold-start properties (minus 40 degrees Fahrenheit). They also have the ability to instantly restrike. However, they offer lower efficacy than metal halide and conventional fluorescent lamps, and suffer from high lumen depreciation (about 40 percent). There are also concerns about how their radio frequency energy might affect adjacent electrical equipment.

High-intensity fluorescent fixture designs. To go along with the latest in high-intensity fluorescent lamps, new fixture designs have been introduced. Design variations address aesthetics and some specific customer needs. One manufacturer sells fixtures with conical reflectors that closely imitate the look of a typical metal halide fixture (Figure 1). However, the compact size of these fixtures limits them to the use of shorter compact fluorescent lamps, which are less efficient and have a shorter life than long twin-tube and linear T5 lamps. This manufacturer also sells lay-in and canopy fixtures that use T5 lamps. Another manufacturer offers an entire family of fixtures based on a star design, in which smaller arms radiate out from a central core that encloses the ballasts. The firm offers fixtures with various numbers of arms (which house the lamps) so end users can achieve the desired light level and distribution. Also, the arms can be individually switched to provide a wider range of light level and control options.

Figure 1: Fluorescent or HID?

This fixture uses eight 42-watt compact fluorescent lamps and is designed to resemble a typical HID fixture.

Source: Sportlite

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How to Make the Best Choice

The new fluorescent fixtures designed for high-bay applications have seven advantages over similar HID fixtures: lower energy consumption, lower lumen depreciation rates, better dimming options, faster start-up and restrike, better color rendition (see Table 1), more pupil lumens (see Table 2), and reduced glare. Not only do these advantages make fluorescent fixtures more cost-effective in many applications, they also enable them to provide superior lighting to the spaces they illuminate.

Table 1: The color rendering index of fluorescent and HID lamps
Measured on a scale of 0 to 100, the color rendering index (CRI) describes the capability of a light source to accurately render a sample of eight standard colors relative to a standard source. Light sources that exhibit higher CRIs render color better than sources with low CRIs.
Lamp type	CRI
T8 fluorescent	75-98
T5 fluorescent	75-98
High-color-rendering metal halide	80-93
White high-pressure sodium	60-85
Standard metal halide	60-70
Pulse-start metal halide	65-70
High-pressure sodium	27
Low-pressure sodium	5
Source: GE, Osram, and Philips

Table 2: Conversion factors for lumens to pupil lumens
Correction factors applied to conventional values of lumens per watt yield a value for pupil lumens per watt, which is a measure of how effectively the eye sees the light that is emitted. The pupil is more receptive to light at the blue end of the spectrum.
Light source	Conventional lumens per watt	Correction factor	Pupil lumens per watt
Low-pressure sodium	165	0.38	63
5,000-K T5 fluorescent	104	1.83	190
4,100-K T8 fluorescent	90	1.62	145
Clear metal halide	85	1.49	126
5,000-K pure triphosphor fluorescent	70	1.58	111
3,500-K triphosphor fluorescent	69	1.24	85
50-watt high-pressure sodium	65	0.76	49
2,900-K warm white fluorescent	65	0.98	64
Daylight fluorescent	55	1.72	95
35-watt high-pressure sodium	55	0.57	31
5,000-I 90 CRI fluorescent	46	1.7	78
Vitalite fluorescent	46	1.71	79
Deluxe mercury vapor	40	0.86	34
Standard incandescent	15	1.26	19
Tungsten halogen	22	1.32	29
Note: K = Kelvin; CRI = color rendering index.
Source:Platts

Perform a cost-effectiveness calculation. The cost-effectiveness of fluorescent lighting compared with HID depends on several factors including lamp life, lumen depreciation, hours of operation, and cost per kilowatt-hour. Use this online calculator as a screening tool to compare the costs of HID lighting with fluorescent alternatives. Make sure to compare alternatives that provide approximately equal amounts of light. The calculations include a correction factor for pupil lumens—a factor not universally accepted. To perform the calculations without considering pupil lumens, simply input values of 1.0 for the conversion factor.

Online Lighting Calculator

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What's on the Horizon?

The HID industry is working to improve lamps, ballasts, and luminaires. With lamps, the greatest improvement has been the introduction of the pulse-start metal halide lamp. Pulse-start lamps are superior in nearly all respects to the standard metal halide lamps. They have higher initial lumens, less lumen degradation, improved color uniformity, and faster warm-up and restrike times.

Pulse-start lamps cannot be used with standard magnetic ballasts, so a new pulse-start ballast must be included in any retrofit. One company has introduced a proprietary electronic ballast for use with pulse-start lamps. It increases system efficiency by 15 percent and reduces lumen depreciation to only 20 percent. It appears that much more can be done to improve the ballasts for HID lamps, and we should expect some significant developments in the next few years—particularly in electronic HID ballast technology.

The third area for HID improvement is luminaire design and efficiency. Although the typical "hooded" HID fixture has an efficiency of about 70 percent, efficiencies as high as 90 percent are possible and, in fact, are available from a few manufacturers. However, these high-efficiency luminaires are considerably more expensive than the typical fixture, so they don't yet have a significant share of the HID fixture market.

Although improvements in lamps, ballasts, and luminaires may eventually make HID lighting systems as energy-efficient as the new fluorescent systems, it is unlikely that lighting manufacturers will ever be able to eliminate the warm-up and restrike delay associated with HID lights. This inability to instant-start severely limits the use of occupancy sensors and other switching methods that can save energy.

Lastly, it appears that HID lighting has a ways to go before it can match the very low lumen depreciation of T5 lamps. Even 20 percent lumen loss is a problem when compared with the 5 to 10 percent loss of T5 fluorescent systems. Until these drawbacks can be eliminated, the market share for HID lighting will probably continue to erode.

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Who are the Manufacturers?