It is a specially coated piece of glass, coated for about 1% reflection. Do not try to inject another light source, or the PSU will oscillate and the plasma tube will die. You need the light feedback to prevent the plasma tube from oscillating. You also need it because there is no true upper current limit on the K7 PSU. Unlike the other HGM psus, the traditional hack of using the upper current limit to run the tube CW with the computer disconnected does not work on the Surgica K7. In other words, if there is no feedback, the PSU will damage the tube eventually.
There is only one guy in the US who normally has that whole HGM assembly in stock, and he's been very, very, grumpy towards hobbyists lately. HGM and American Laser were both part of Laser Corporation and they shared certain parts and tubes. One shared part is the splitter plate. Your best bet is to find the beam splitter plate from a dead ALC 60X, Omni 532, HGM5 or HGM8. You may have to cut the glass to size, but it will be the same coating.
There will most likely be a stack of black glass filters and diffusers between the silicon cell and the beamsplitter pickoff, calibrated for that particular laser. don't lose it or change it.
Steve
Last edited by mixedgas; 05-27-2014 at 05:49.
Qui habet Christos, habet Vitam!
I should have rented the space under my name for advertising.
When I still could have...