Hi, your TV is not a Plasma. It is a Liquid Crystal on Silicon rear projector.
You need to replace the lamp assembly (XL-5100). Extremely simple to do, it is extensively covered within the owner's manual.
It is possible to save a few dollars by reusing the old assembly and replacing just the bulb. However, I recommend you purchase a new XL-5100.
Surprised that you are experiencing a dim lamp for the first time. They typically burn out within 6000 to 8000 hours.