From: astroelectronic@t-online.de   
      
   Hi all,   
      
   let's assume I have a flat glass mirror which had an Aluminium coating, which   
   was removed incompletely. Some parts of the surface are now clean glass, and   
   other parts still have the Aluminium coating on it, most probably with a SiO2   
   layer on top.   
   I did measure the surface topography with a phase shifting Fizeau   
   interferometer against a transmission flat which has a Dynaflect coating. The   
   intensity modulation is good enough for all parts of the surface. There are no   
   areas with invalid data. The    
   phase map shows that the Aluminum coated areas are higher (which was expected,   
   of course).   
      
   My question:   
   Does the phase map show real height differences, or do different materials   
   (aluminium and glass) have different phase changes when light is reflected at   
   the surface?   
      
   Thanks,   
   Michael   
      
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