Measure 8.3.1
Install skylights or light pipes.

Control the Electric Lights to Exploit Daylighting

A skylight or light pipe is useless unless it reduces the energy consumed by electric lights. This is not just a matter of turning off the electric lights in daylighted areas. You also have to avoid the tendency to increase the electric lighting levels in adjacent parts of the space that are not daylighted to compete with the brightness of the daylighted areas.

See Measure 9.5.3 for methods of controlling electric lights in response to daylighting. Combine these with the other automatic lighting controls of Subsection 9.5 that are appropriate for the activities that occur in the space.

Compatible Types of Electric Lighting

Fluorescent lighting is generally the best type to use in combination with skylights because its output can be adjusted efficiently to supplement reduced sunlight. Modulating dimmer systems are available for fluorescent lighting, and these work well with daylighting. Also, fluorescent lamps can be turned on and off repeatedly to respond to changes in daylight. However, frequent cycling reduces lamp life and annoys occupants.

It is common to use high-intensity discharge (HID) lighting in combination with skylights. (HID is the class of lamps that includes mercury vapor, high-pressure sodium, and metal halide.) This is because HID lamps have high output, so they can be installed at the height of the skylight in limited numbers, matching the light output of the skylight. Unfortunately, HID is usually a poor choice to combine with skylights. HID lamps take as long as ten minutes to reach full brightness, and they cannot restart for several minutes after being turned off. This makes them inappropriate for use with daylighting, where passage of clouds in front of the sun may require the output of the electric lights to change continually. A newer type of HID lighting is available that turns on instantly. These operate by keeping the lamps hot continuously, which sacrifices efficiency. HID dimming does exists, but it is limited by problems that are described in Reference Note 56, HID and LPS Lighting.

There is a tendency to believe that fluorescent lighting cannot be used with tall ceilings. This is not true. However, individual fluorescent lamps are limited in light output. As a result, lighting a space with fluorescent lighting typically requires many more fixtures than with HID lighting. See Measure 9.3.3 for a comparison of HID and fluorescent lighting. It makes no economic sense to use incandescent lighting as a complement to skylights, even though this is done often. The saving in lighting cost by daylighting is eliminated by the higher cost of incandescent lighting during the periods of time that it operates.

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