Using an electric arc to create intense light, high-pressure sodium (HPS) and metal halide lamps fall under the high intensity discharge (HID) category. Metal halide lamps generate white light and, consequently, have a better color rendering index (CRI) rating than HPS lamps, which generate yellow light.
New metal halide (CDM) lamps use ceramic discharge tubes. Because ceramic is a more robust material than the traditionally-used quartz, it can better contain the salts used to produce visible light. Thus, these lamps improve lumen maintenance, color rendering and lamp color stability. They also allow facilities currently using HPS lamps to convert to white light without modifying the existing fixture or ballast.
The case in favor of HPS lamps
Many industrial facilities require lighting that can run continuously. Factories and warehouses have used HPS lamps because they have desirable operating characteristics, such as high lumen maintenance and energy efficiency, and because they don’t require a weekly shut-off cycle. However, they give off yellow light, which affects their CRI rating.
Other choices for factory lighting have their own limitations. Until recently, metal halide lamps were limited to quartz discharge tube technology. While quartz lamps offer white light, they are plagued by lumen maintenance issues, weekly shut-off requirements and poor color stability.
As a result, facility managers have either shied away from metal halide altogether, or have retrofitted them into spaces that don’t require uninterrupted lighting.
Partial retrofits don’t always work to management’s benefit. In plants with partial retrofits to metal halide, facility managers often hear grumblings from employees still working under HPS lighting. They complain that workers on the other side of the factory have better lighting—and that they want it, too.
White light’s advantages
White light is generally perceived as brighter and closer to natural sunlight than yellow light, so it renders color well. At 250 and 400 W, CDM lamps offer a CRI as high as 90. Most HPS lamps have a CRI as low as 22, while incandescent lamps have CRI ratings close to 100.
In a factory or a distribution center, workers with white light overhead have an easier time distinguishing objects’ colors than those with yellow light. This can speed the picking process, boosting assembly line productivity. It also can lead to fewer “misses” during inspection.
Employees performing precision tasks can see more clearly under white light, which can reduce mistakes and improve safety.
Metal halide options
The following is a brief survey of metal halide, white light technologies and their relative strengths vis-à-vis typical facility lighting needs.
Quartz switch start
Quartz switch start lamps provide white light, but have poor color stability and tend to show color drift over time. They average about 63 percent lumen maintenance, which is noticeably lower than HPS lamps and therefore a detriment to full-scale conversions. (The higher the lumen maintenance factor, the less a lamp will dim over time.) And because quartz switch lamps require a 15-minute maintenance shutdown weekly, they’re unsuitable for installations calling for continuous ambient lighting.
Quartz pulse start
Quartz pulse start lamps offer somewhat improved lumen maintenance (70 to 75 percent) and better energy efficiency. But they also lack color stability and, because most manufacturers recommend a precautionary 15-minute shut-off per week for proper lamp maintenance, factories that need 24/7 lighting cannot adopt them facility-wide.
A retrofit of either quartz switch start or quartz pulse start metal halide lamps requires additional investments in labor, new fixtures and ballasts.
Screw-in quartz metal halide
A few manufacturers offer a version of metal halide lamp that can run on 250- and 400-W HPS ballasts. These lamps offer an easy, direct-screw-in retrofit. But because they use quartz technology, the lamps face the same shortcomings in lumen maintenance, color shift and weekly shut-off requirements as the switch start and pulse start metal halide systems.
Screw-in ceramic metal halide
Medium wattage (250- to 400-W) CDM lamps that use HPS ballasts provide a twist-in retrofit for areas that require high levels of light without interruption. They provide 85-percent lumen maintenance, a step beyond pulse start and approach the performance expected from HPS lamps.
Thanks to this new innovation in lighting, that old “factory yellow” may be a thing of the past.
Ted Simpson is Marketing Manager for Philips Lighting Company, 200 Franklin Square Drive, PO Box 6800, Somerset, NJ 08875-6800.