Thorlabs D3MM Optical Power Meter Head.

This item is very unusual. It is a Davidson Optronics Model 104-0145 90-degree Prism Adapter. The adapter fits onto the ends of the standard 58mm diameter collimator objectives tubes. There are two ports on the side of the �cap�, and the collimator beam is split so one half is directed at 90 degrees to the right, the other half 90 degrees to the left. The reflector is a solid prism with silvered surfaces � I am not sure if it is polished metal or silvered glass. The adapter has a knurled cover that may be turned to block the side apertures.