From: rrllff@yahoo.com   
      
   This is a technology that was pioneered by a company called Digital Optics in   
   about 1990.  They were based in the Carolinas.  They offered design and   
   manufacturing services.  However their product is no longer available since   
   they were bought out.   
      
   The process was (probably still is) by photo-lithography using several   
   exposures.  This creates a stepped phase grating which with about 4 exposures   
   approximates a smooth variation in phase.   
      
   The grating thus produced can be used as an aspheric corrector and/or a   
   chromatic corrector.  This gives the lens designs of complex optical systems a   
   degree of freedom unavailable with spherical surfaces.  Excellent aberration   
   correction can be obtained    
   with a lightweight and compact lens system.   
      
   The main problem with this type of design is the diffraction efficiency.  The   
   visual waveband is too wide to allow high efficiency across its whole range.    
   In general, diffraction efficiency cannot be made greater than ~90%, resulting   
   in a lot of stray    
   light.  There are a few tricks that can be played, however I do not expect   
   this type of lens to be practical for high performance lenses.   
      
   http://www.richardfisher.com   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)