and must be highly transparent it has been found that the necessary scratch resistance for virtually all objects and the desired transparency can be achieved through the use of a diamond like carbon layer therefore the use of this material is preferred the substrate is exposed to a plasma of carbon ions at a temperature in excess of 2000 u00b0 f after exposure time of a few seconds as determined by the selected final thickness of the dlc layer the substrate is allowed to cool gradually which causes the carbon to coalesce into an amorphous layer the amorphous characteristic is desirable rather than the crystalline structure associated with natural diamonds because a crystalline structure would produce reflections that would interfere with the true image the thickness of the resulting layer is preferably less than 150 microns after a mirror of a selected shape is formed the edges are polished until substantially all surface checking and cracking are removed the polishing operation is particularly important for mirrors to be employed in frameless mounts such as the planar wire mounts removal of surface checking and cracking increases the amount of incidental contact which the mirror will withstand before significant chipping at the mirror edge degrades the reflected image the.