Hello, I am Pete, and I'll be happy to assist with your question today.
If the bulb is not lighting brightly enough to send the image to the screen, then it is not getting enough power. The bulb gets its power from the ballast board - this board will need to be tested to check that it is outputting the correct voltage to the lamp.
If you have some experience in electronic testing and the appropriate test equipment you can carry out this test yourself; however be warned - there are fatal high voltages present on the ballast board, so do not attempt this if you have not done it before.
Once confirmed that the ballast board has failed, it will then need to be replaced or repaired. A local electronics company will typically charge around $150 to carry out this type of repair.