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In general, metal halide lamps produce a stark white light, but over time a color shift can occur toward blue, pink, green, or purple, with no consistency from lamp to lamp.

All electric light sources get dimmer over time, a phenomenon known as lamp lumen depreciation or lumen maintenance. A light source that loses as much as 15 to 20 percent of light over its life is considered acceptable, but standard metal halide lamps can lose up to 50 percent of light before they finally fail.

How well a light source reveals colors accurately is charted on the color rendering index (CRI), a scale from 0 to 100. The higher the number, the better the light source reveals true colors. Standard metal halide has a rather mediocre CRI of about 65, rendering flat skin complexions and drab-looking interiors. Better color discrimination aids vision, which is important in a high-activity area like a gymnasium.

While a point source increases optical control, it also renders shadow very precisely. This can lead to harsh shadows, distracting patterns, and areas of bright and dark. Also, a point source is by definition very bright, and in a poorly designed or applied fixture, this can mean intense glare for the users of the space.

Pulse-start metal halide and ceramic metal halide have alleviated some of these problems. Pulse-start lamps provide incremental improvements in strike and restrike time, lumen maintenance, reduced color shift, and efficacy. Ceramic metal halide lamps have greatly improved CRI (in the 80s) and minimal color shift. However, problems persist.

Alternatives to Metal Halide

Given these unfavorable properties, why is metal halide still the standard-practice light source used in athletic facilities today? I think there is a lack of awareness of the new fluorescent products and their benefits.

There are many advantages of fluorescent systems. High lumens per watt (in the 90 range) mean more light for less energy, resulting in energy savings. T8 and T5 lamps have excellent lumen maintenance, losing only about 10 percent of their light over their life.

Common fluorescent lamps can have a CRI in the 80 range. This gives objects and colors a clarity and definition that aids in visibility. Studies suggest that people require less high-CRI light to achieve the same visual acuity as low-CRI light.

Fluorescent lamps also have instant strike and no restrike. This means the lights will come on immediately and if they are switched off, they can be turned back on right away if necessary. This makes them compatible with control systems based on occupancy or daylight conditions.

Energy savings and longer maintenance intervals will result if the lights are turned off when not needed. It should be noted that frequent on/off switching of fluorescent lamps results in shortened lamp life. This can be minimized by using program start ballasts and a longer occupancy control interval.

Because fluorescent tubes are large radiant surfaces, they tend to deliver diffuse, even illumination, making for softer shadows, lower contrast, and increased uniformity of light. A possible downside of using fluorescent systems is that they require more lamps than metal halide systems, and the fluorescent tubes have reduced light output in chilly spaces.

Comparative Analysis

To demonstrate the differences and advantages of fluorescent lighting, I have modeled a typical gymnasium space with a variety of lighting strategies. The space is a 75- by 100-foot (23- by 30-meter) single-court gym area with a fixture height of 26 feet (8 meters). The reflectances are 70 percent for the ceiling, 60 percent for the walls, and 30 percent for the floor.

The analysis examines not only power consumption and light level but also uniformity, expressed as a ratio of maximum to minimum. For this type of space, having evenly distributed light is important for both the field of play and the surrounding visual field.

The fixture types are:

A) Baseline: 15 standard 400-watt metal halide high-bay with glass reflector
B) 63 2-lamp 32-watt T8, 4-foot (122-centimeter) industrial with specular reflector and 10 percent uplight (in 3 rows)
C) 15 4-lamp 54-watt T5 high-output 2- by 4-foot (61- by 122-centimeter) high-bay with specular reflector
D) 15 8-lamp 42-watt compact fluorescent high-bay with acrylic reflector
E) 14 8 lamp 40-watt biaxial wrap style with prismatic lens
F) 15 6-lamp 32-watt T8 high bay with specular reflector

Fixture Type	Avg FC	Max	Max/Min	Watts/SF
A) 400W MH X 15	28.8	37.9	4.2	.9
B) 32W 2 lamp T8 X 63	28.5	40.5	4.2	.5
C) 54W 4 lamp T5 X 15	28.5	35.8	3.1	.5
D) 42W 8 lamp CFL X 15	27.1	34.2	3.0	.6
E) 40W 8 lamp Biax X 14	27.7	38.9	4.5	.6
F) 32W 6 lamp T8 X 15	25.0	33.6	4.2	.5

Minimum recommended lighting levels for gyms is around 30 footcandles (FC) (320 lux) but levels can go as high as 150 FC (1600 lux) depending on the sport, the class of play, and the size of audience.

Note that fixture type C has the best blend of light output, uniformity (Max/Min), and low power (Watts per area). While no one system is perfect, I think it is clear from these examples that the fluorescent systems have lower power and good uniformity, while maintaining more than adequate light levels.

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Eric Strandberg is a lighting specialist at Seattle's Lighting Design Lab, where he promotes good design and energy efficiency on commercial projects. In 1997, he became one of the first to take the National Council on Qualifications for the Lighting Profession exam to become Lighting Certified (LC).

This article first appeared in the Fall 2004 edition of the Lighting Design Lab News and is reprinted here with permission.

An illumination simulation displaying the effect of fixture type B, fluorescent T8 in three continuous rows. Image: Lighting Design Lab Fixture type C, fluorescent T5 high-output high-bay. Image: Lighting Design Lab Fixture type D, compact fluorescent high-bay. Image: Lighting Design Lab Fixture type E, fluorescent high-bay with wraparound prismatic lens. Image: Lighting Design Lab Fixture type F, fluorescent high-bay with specular reflector. Image: Lighting Design Lab   Click on thumbnail images to view full-size pictures.