High efficiency lamps are available and delamping may be a possiblity if an area is overlit.
All lamps should be switched off when not in use but HID lamps are commonly left on for long periods of time. They warm up slowly to full brightness over a period of 10 or 15 minutes. To address the energy waste in this practice a two-level ballast is available for high pressure sodium lamps. Occupancy sensors control the switching and when an area is vacant the lamps drop to about 30% of the power used at full brightness. From this starting point the lamps can come up to full brightness in less than a minute.
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