Let's take a look at those two wavefronts side by side.  Remember  
    that these calculated wavefronts do not actually exist.  Rather, they 
    have been back-calculated from single point images.  That is, if  
    that single pixel were lit by light going the opposite direction from 
    the way it will actually go, this is the kind of wavefronts that would 
    be projected onto the two sides of the transistor.  We will use these 
    calculations to produce holograms that actually will focus light going 
    in the correct direction onto those points.  This works because the 
    actual calculations used do not include direction parameters. Thus 
    the calculated results will be the same.  The calculation is basically 
    just a 3D pythagoras in wavelength units. 
 
      So let's look at them together.  You will see from the circular  
    nature of the waves how such would result from light projected from a 
    point directly in front of the screen at the calculated focal distance.