The lower half of my iMac display is dim plus there are vertical light -dark stripes. is their an adjustment for this? I have a model A1224, Thanks...

There are lamps behind the screen and it sounds like either one or more have died, or the inverter power source for them has partially failed. The vertical lines sound like the video card is failing. Can you attach another monitor? If that one is fully lit, it's your iMac's monitor. And if the lines are still there, that would still point to your video card. Either way I imagine this is going to be very expensive such that it would make more sense to get a new or refurb.