Van Halen Par Cans

Sometimes I really like the internet.  Today I was looking through some of my things and pulled out my old Van Halen "1984" album... the vinyl album (as an aside, 1984 is one of the few records I have in vinyl, cassette, CD and digital formats).  Anyway, I pulled out the record and was reminded of a mystery that has haunted me for over 26 years.  This is a photo from the sleeve: 

And I never understood what they were.  Ashtrays at the factory? The tops of trash cans?  An assembly line of R2-D2s?  Well, today I decided to look it up and found out that they are "par cans," part of the lighting rig for a rock concert.   Here they are in an assembled rig: 

Mystery solved... thanks, internet!