As with fluorescent systems, High Intensity Discharge lamps also require ballasts to operate. Although there are not nearly as many specification options as with fluorescent ballasts, HID ballasts are available in dim-mable and bi-level light outputs. Instant restrike systems are also available.

Capacitive Switching HID Fixtures

Capacitive switching or "bi-level" HID fixtures are designed to provide either full or partial light output based on inputs from occupancy sensors, manual switches or scheduling systems. Capacitive-switched dimming can be installed as a retrofit to existing fixtures or as a direct fixture replacement. Capacitive switching HID upgrades can be less expensive than installing a panel-level variable voltage control to dim the lights, especially in circuits with relatively few fixtures.

The most common applications of capacitive switching are athletic facilities, occupancy-sensed dimming in parking lots and warehouse aisles. General purpose transmitters can be used with other control devices such as timers and photosensors to control the bi-level fixtures. Upon detecting motion, the occupancy sensor sends a signal to the bi-level HID ballasts. The system will rapidly bring the light levels from a standby reduced level to about 80 percent of full output, followed by the normal warm-up time between 80 and 100 percent of full light output.

Depending of the lamp type and wattage, the standby lumens are roughly 15-40 percent of full output and the standby wattage is 30-60 percent of full wattage. When the space is unoccupied and the system is dimmed, you can achieve energy savings of 40-70 percent. Fixtures (aka Luminaires)

A fixture is a unit consisting of the lamps, ballasts, reflectors, lenses or louvers and housing. The main function is to focus or spread light emanating from the lamp(s). Without fixtures, lighting systems would appear very bright and cause glare.

Fixture Efficiency

Fixtures block or reflect some of the light exiting the lamp. The efficiency of a fixture is the percentage of lamp lumens produced that actually exit the fixture in the intended direction. Efficiency varies greatly among different fixture and lamp configurations. For example, using four T8 lamps in a fixture will be more efficient than using four T12 lamps because the T8 lamps are thinner, allowing more light to "escape" between the lamps and out of the fixture. Understanding fixtures is important because a lighting retrofit may involve changing some components of the fixture to improve the efficiency and deliver more light to the task.

The Coefficient of Utilization (CU) is the percent of lumens produced that actually reach the work plane. The CU incorporates the fixture efficiency, mounting height, and reflectances of walls and ceilings. Therefore, improving the fixture efficiency will improve the CU.


Installing reflectors in most fixtures can improve its efficiency because light leaving the lamp is more likely to "reflect" off interior walls and exit the fixture. Because lamps block some of the light reflecting off the fixture interior, reflectors perform better when there are less lamps (or smaller lamps) in the fixture. Due to this fact, a common fixture upgrade is to install reflectors and remove some of the lamps in a fixture. Although the fixture efficiency is improved, the overall light output from each fixture is likely to be reduced, which will result in reduced light levels. In addition, reflectors will redistribute light (usually more light is reflected down), which may create bright and dark spots in the room. Altered light levels and different distributions may be acceptable, however these changes need to be considered.

To ensure acceptable performance from reflectors, conduct a trial installation and measure "before" and "after" light levels at various locations in the room. Don't compare an existing system, (which is dirty, old and contains old lamps) against a new fixture with half the lamps and a clean reflector. The light levels may appear to be adequate, or even improved. However, as the new system ages and dirt accumulates on the surfaces, the light levels will drop.

A variety of reflector materials are available: highly reflective white paint, silver film laminate, and anodized aluminum. Silver film laminate usually has the highest reflectance, but is considered less durable. Be sure to evaluate the economic benefits of your options to get the most "bang for your buck."

In addition to installing reflectors within fixtures, light levels can be increased by improving the reflectivity of the room's walls, floors and ceilings. For example, by covering a brown wall with white paint, more light will be reflected back into the workspace, and the Coefficient of Utilization is increased.

Lenses and Louvers

Most indoor fixtures use either a lens or louver to prevent occupants from directly seeing the lamps. Light that is emitted in the shielding angle or "glare zone" (angles above 45o from the fixture's vertical axis) can cause glare and visual discomfort, which hinders the occupant's ability to view work surfaces and computer screens. Lenses and louvers are designed to shield the viewer from these uncomfortable, direct beams of light. Lenses and louvers are usually included as part of a fixture when purchased, and they can have a tremendous impact on the VCP of a fixture.

Lenses are sheets of hard plastic (either clear or milky white) that are located on the bottom of a fixture. Clear, prismatic lenses are very efficient because they trap less light within the fixture. Milky-white lenses are called "diffusers" and are the least efficient, trapping a lot of the light within the fixture. Although diffusers have been routinely specified for many office environments, they have one of the lowest VCP ratings.

Louvers provide superior glare control and high VCP when compared to most lenses. As Figure 13.3 shows, a louver is a grid of plastic "shields" which blocks some of the horizontal light exiting the fixture. The most common application of louvers is to reduce the fixture glare in sensitive work environments, such as in rooms with computers. Parabolic louvers usually improve the VCP of a fixture, however efficiency is reduced because more light is blocked by the louver. Generally, the smaller the cell, the greater the VCP and less the efficiency. Deep-cell parabolic louvers offer a better combination of VCP and efficiency, however deep-cell louvers require deep fixtures, which may not fit into the ceiling plenum space.

Table 13.5 shows the efficiency and VCP for various lenses and louvers. VCP is usually inversely related to fixture efficiency. An exception is with the milky-white diffusers, which have low VCP and low efficiency.

Light Distribution/Mounting Height

Fixtures are designed to direct light where it is needed. Various light distributions are possible to best suit any visual environment. With "direct lighting," 90-100% of the light is directed downward for maximum use. With "indirect lighting," 90-100% of the light is directed to the ceilings and upper walls. A "semi-indirect" system

Figure 13.3 Higher shielding angles for improved glare control.
Table 13.5 Luminaire efficiency and VCP.

Shielding Material


Visual Comfort

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