[Laser] Laser newbie conner cube reflection question

[W4DEX Dexter McIntyre]

I now have a trihedral prism, conner cube or retroreflector (are there 
any more names for the device?)  Using a 5 mw green laser pen with the 
reflector set approximately 250 feet away I get an interesting 
reflection. The reflection is about an 8 CM diameter from the 4 CM 
diameter cube with very noticeable concentric circles.  I tried to 
photograph the display but was unable to get the exposure right to see 
the circles.

http://w4dex.com/laser/laser_250ft.jpg

I'm guessing this is created by phase cancellation but so far I haven't 
been able to confirm this with Google. 

Dex