High-Intensity Discharge Lamps

High-intensity discharge (HID) lighting offers very high efficiency, providing energy savings of 50 to 90 percent when replacing incandescent sources. Originally intended for outdoor and high-bay applications, the use of HID lamps has spread to some retail applications as the introduction of ceramic metal halide lamps has improved color-rendering characteristics and as smaller sizes have become available.

Mercury vapor lamps, metal halide lamps, and high-pressure sodium lamps all fall in the HID category. Mercury vapor lamps have become almost obsolete because the Energy Policy Act of 2005 (EPAct 2005) banned the manufacture or import of ballasts for the lamps. Other HID sources with better efficacy, color rendering, and lumen maintenance are available at lower cost.

HID lamps are available in sizes ranging from 15 to 2,000 watts (W). Figure 1 shows that HID light sources boast the highest efficacies now available. (Efficacy is a measure of the performance of a lamp, calculated as light output divided by power input and expressed in lumens per watt.) However, as HID lamps age, their output and efficacy degrade more rapidly than those of other light sources do. Within the HID category, the highest-wattage lamps offer the highest efficacies. However, the high-pressure and low-pressure sodium lamps that offer the highest efficacies also provide the lowest color quality. Low-pressure sodium lamps are not technically HID lamps, but they are often discussed in the same context. Despite high efficacy, low-pressure sodium applications are limited because of the nearly monochromatic, yellow light they produce. One accepted application is street lighting near astronomical observatories, which is allowed because it is easy for astronomical equipment to filter out the yellow light.

Figure 1: Efficacy of high-intensity discharge lamps
High-intensity discharge lamps span a wide range of efficacies, offering the highest values now on the market.
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