If your on screen image is getting dimmer then you may need to replace your projector lamp/bulb. Some projectors allow you to check the number of hours your lamp has been used for through a built in menu system, check your projectors manual for details. How does a lamp last?

Most lamps/bulbs last about 1500 2000 hours on average, some claim 4000 hours in low lamp mode, actual life hours will vary depending on how much and how often you use your projector and how you look after your lamp

MODEL:PE6800