From: rrllff@yahoo.com   
      
   On Wednesday, December 2, 2015 at 3:23:34 AM UTC-5, Michael Koch wrote:   
   > 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   
      
   At normal incidence I believe the phase change at metals is pi/2 and zero for   
   dielectrics.  However, this is complicated by the protective overcoat on the   
   aluminum.  I don't think there is a simple answer.   
      
   However, for most applications the optical path difference is the important   
   value of interest.  it is this that is measured by the interferometer.   
      
   Google gives some good results.  Others can add to this reply   
      
   http://www.richardfisher.com   
      
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