The median time it takes for a lamp to burn out. For example, a 60 watt bulb can be expected, on the average, to burn for 1,000 hours. Based upon continuous testing lamps in laboratories, the 1,000 hour rating is the point in time when 50% of the test samples have burned out and 50% are still burning.

See "Lamp Life".

An average rating, in hours, indicating when 50% of a group of lamps have failed when operated at nominal lamp voltage and current. Manufacturers use 3 hours per start for fluorescent lamps and 10 hours per start for HID (High Intensity Discharge) lamps when performing lamp life testing procedures. Every lamp type has a unique mortality curve that depicts its average rated life.