Whenever a person leaves a room, it should be convenient to extinguish all the lights. Even small rooms with two doors need a switch at each entrance unless the doors are close to one another. Larger spaces may need multiple low-voltage switches to cover every exit.
A more elaborate use of controls is to switch the fixture lamps separately. A room with several three-lamp fluorescent fixtures can be designed with two ballasts for each fixture, for one lamp and two lamps, respectively. This provides three levels of light: one lamp, two lamps, and three lamps. These multiple light levels can be used with varying activities or needs in the space and also relative to daylighting conditions. For example, a conference room may use a low light level to show slides on an overhead projector. After hours, cleaning staff may use a medium light level while performing their housecleaning tasks. The full lighting may be helpful for staff work sessions in the space.
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Excerpt from Energy Conservation Projects.