Dell 4320 User Manual page 111

NOTE:
The lamp life rating for any projector is a measure of brightness
degradation only, and is not a specification for the time it takes a lamp to fail
and cease light output. Lamp life is defined as the time it takes for more than
50 percent of a lamp sample population to have brightness decrease by
approximately 50 percent of the rated lumens for the specified lamp. The lamp
life rating is not guaranteed in any way. The actual operating life span of a
projector lamp will vary according to operating conditions and usage patterns.
Usage of a projector in stressful conditions which may include dusty
environments, under high temperatures, many hours per day, and abrupt
powering-off will most likely result in a lamp's shortened operating life span or
possible lamp failure.
Specifications 111