Projector Lamp FAQ at Projector Central
Projector Lamp FAQ at Projector Centralget all the info u need for a OHP here>
"...The projector image seems dimmer than usual - what does this mean?
It may be time to replace your projector lamp. With metal halide (high pressure mercury) lamps, you'll notice a dimming of the image brightness as the lamp loses power. Some projectors allow you to check the number of hours your lamp has been used through the built-in menu system. Check your user guide for information about this feature."