Used to run Bauer carbon arcs when I was a a little squid, one of my first jobs was as a projectionist, going on forty years ago... We had dual 70mm projectors where I worked, I was pry one of the last projectionists to be feeding arc lights and doing projector change overs, I know for a fact I was the last to use that set. The lenses on those Bauer's were absolutely gorgeous. The machines themselves were industrial works of art of the highest order. The 35mm system ran a Christie platter with Xenon lamp, functional but boring in comparison. As far as saving electricity or carbon by "turning down" the light output, sorry, it doesn't really work that way with carbon or Xenon. Whatever makes your movie dim is poor maintenance or operation, not some sort of economy. Both systems are such that they can't really be "dimmed". These days you might also suspect a change to cheap digital projector. I don't know of any theaters still running carbon arcs. It would warm my heart if there were.