The team I am a member of is currently in the process of installing a new steering mirrors in our lasers. The mirrors are about 15mm in Diameter and 7mm thick. They are first surface mirrors coated for a 355nm laser beam with 45deg incidence. 355nm is in the UV portion of the spectrum and therefore is invisible, but we can see a spot where the beam reflects from the mirror. The beam diameter along the beam path is about 3.5mm. The reflective surface is indicated by a small arrow on the side of the mirror. Once the mirrors are installed in their holders the mark is no longer visible. Suppose some sleepy graveyard tech installed a mirror back wards. How without removing the mirror could we verify proper orientation? Or must we remove the mirror to do this? BTW: At full power this is a 7W beam, so please work safely, do not block the beam path with your hands, and take care not to reflect the beam into your eyes!