June 1997 Volume 3 Issue 12
High Intensity Discharge (HID) Lighting
When it comes to illuminating high-ceiling spaces like warehouses and arenas, and large outdoor areas like parking lots and ball parks, high-intensity discharge (HID) lighting is the most appropriate technology. There are three general types of HID lighting:
· mercury vapour (MV),
· metal halide (MH), and
· high-pressure sodium (HPS).
There are also numerous variations within each type. This article explores differences, commonalties, and some general technical issues.
The table below compares new-lamp efficacy (lumens per watt) and Color Rendering Index (CRI) for 250-watt clear and phosphor-coated lamps of the three types. The lumens per watt figures include ballast losses.
Where a range is shown, the lamps with better colour rendering generally have the lower efficacy. And as a general rule, the higher the wattage of an HID lamp, the higher its efficacy. For example, the efficacy of clear HPS lamps ranges from 41 lm/W for a 35-watt lamp, to 73 lm/W for a 100-watt lamp, to 128 lm/W for a 1000 watt lamp. In HID lighting systems the light can usually be provided either by a few high-wattage lamps, or by a larger number of lower-wattage lamps. The first approach minimizes electricity consumption; the second usually gives more uniform illumination.
HID lighting came into being 64 years ago with the development of mercury vapour lamps. MH and HPS lamps are more recent improvements on the basic MV design, but some things have not changed. All HID lamps must be used with current-limiting ballasts, and many HID lamps are position sensitive. Some are designed for base-up use only, some base-down, and some horizontal. All employ an inner
bulb called the arc tube made of a material like quartz or ceramic that is able to withstand the extremely high temperature of the arc. All also have an outer glass bulb. In some models this bulb is left clear, and in others it is internally coated with a luminescent phosphor — hence the terms clear and coated.
All of these lamps require several minutes warm-up time before they produce full light output, and they need restrike time after a momentary power interruption before they will come back on. There are also safety considerations for HID lamps. Rupture of the arc tube is one concern. This can cause the outer bulb to break and hot particles to go flying — risking fire and harm to people. Breakage of the outer bulb while the arc tube continues to operate is a second concern. This results in excessive ultraviolet radiation from the lamp. Appropriate fixtures (and some bulb designs) eliminate these dangers.
MV lamps employ a quartz arc tube and use argon as a starting gas. They have a warm-up time to full light output of five to seven minutes, and a restrike time of about 10 minutes. Because mercury vapour lamps have lower efficacy and CRI than MH and HPS lamps, they are almost never specified these days for new installations. In fact, to cut energy costs, existing MV installations are frequently retrofitted with MH or HPS lamps. The preferred approach is to replace both lamp and ballast. Replacing the old MV ballast with a ballast tailored for use with the new lamp type generally results in higher efficacy. The alternative is to replace just the lamp with an MH or HPS lamp designed especially to operate with mercury vapour ballasts.
MH lamps are similar in design to MV lamps. Like MV lamps they utilize mercury vapour, argon gas, a quartz arc tube, and an outer glass bulb that may or may not have a phosphor coating on the inside. Where they differ is that the arc tube also contains metallic salts — normally salts of scandium and sodium. These salts cause light to be emitted at additional discrete wavelengths — adding many more lines to the mercury vapour energy spectrum. (See the discussion of emission lines in the recent article “Lighting and Colour Rendition,” $mart Energy User Vol. 3, Issue 9.) Because of these additional spectral emission lines, MH lamps produce a subjectively “whiter” light and have a better Colour Rendering Index than MV lamps. MH lamps have a warm-up time of about 5 minutes and a restrike time of about 15 minutes.
Metal halide lamps are now available in some new formats that make them attractive for incandescent replacement. Among these are bulbs in the 50 to 150 watt range which have a CRI rating of 75. Also available are MH reflector lamps for flood and spot applications that have several times the life and efficacy of similar incandescent lamps. Some of these lamps have a CRI of 80.
HIGH PRESSURE SODIUM
HPS lamps have a ceramic arc tube and use xenon as a starting gas. It takes 3 to 4 minutes to fully vaporize the mercury/sodium amalgam and come to full light output. Restrike time is about one minute — very short, as HID restrike times go. HPS lamps are produced in an exceptionally wide range of wattages (35 to 1000), and the diffuse-coated, colour-improved variety, with its CRI of 70, competes favourably with metal halide lamps for colour rendition.
LOW PRESSURE SODIUM
Although not strictly a form of HID lighting, low pressure sodium (LPS) lighting often appears on HID lists and is included here for completeness. It is the most energy-efficient lighting currently available, with efficacy ratings as high as 180 lm/W. Unfortunately, it has no colour rendering capability. Low-pressure sodium light is monochromatic yellow, and all objects appear yellow, or shades of what might be described as yellowish-brown. It is used primarily outdoors, for safety lighting.
For additional information about HID and other forms of energy-efficient lighting, call Ron Estabrooks or Mike Proud at 1-800-236-5193 (toll free).