Vacuum Printer question

I’m hoping that someone can help me identify what this is in the second image, and how it is used. It attaches below the shutter, and the glass in it can move freely left and right.

I’m not familiar with this particular unit, but from plater-burners I’ve had or used in the past here are a few possibilities:

  1. safety glass - there should be something between bulb and frame in case a bulb breaks or explodes
  2. light balancing - to aid in even light distribution (NuArc units have a pattern printed or etched into the safety glass for this purpose)
  3. diffusion - not likely but who knows!

Thanks Keith for the ideas. Would be really nice to have a manual for this unit.