

      The Ray Filters allow halos with pre-selected
      characteristics to be displayed.   

      The Ray Tracer reveals the specific ray paths 
      through crystals that produced a selected section 
      of a halo.

      RAY FILTERS

      The Ray Filter Controls are accessed via the Tools
      menu.   Click 'Ray Filters' and they are displayed.

      The ray filters can be set so that only the rays 
      passing through crystals that meet certain criteria 
      are plotted.

      The filtered rays are plotted in a full display 
      or a 'half screen' display.   In the latter, the 
      right hand half is filtered and the left hand half 
      is left unfiltered for comparison purposes.

      The filter criteria are:

            (a) Minimum number of crystal facets that a ray 
                   must interact with by reflection or 
                   refraction.

            (b) Maximum number of facets.

            (c) Ray entry facets - None, one or several may 
                   be specified.

            (d) Internal reflection facets - None, one or 
                   several may be specified.

            (e) Ray exit facets - None, one or several may 
                   be specified.

      The criteria can be used alone or in combination.


      To illustrate the Ray Filter controls, select the 
      stored simulation "Filter demo".  

      1)  The ray filters are turned on and allow all rays
          traversing 1 to 7 faces through.  Run the 
          simulation.   The unfiltered (left) and filtered
         (right) sides look the same.

      2)  Change the maximum and minimum number of faces a
          ray can traverse to 2.

      1)  Run the simulation again. 
          On the right, only rays that have traversed 
          just two crystal faces shown.   

              The parhelic circle is absent from the 
              filtered display because rays traversing two 
              faces cannot contribute to it.  Some other 
              faint arcs are also filtered out.

              As expected, the 22 degree halo, tangent arcs,
              circumzenithal arc, supralateral and 
              infralateral arcs remain.  This shows that 
              they are formed by rays that pass directly 
              through the crystals without any internal
              reflection.

      2)  Which rays form the CIRCUMZENITHAL ARC?

          To answer this, specific entrance faces for the 
          rays must be entered in the 'Entrance faces' box.   

              We use the face numbering conventions of Tape 
              (see the book 'Atmospheric Halos').  Hexagonal 
              prisms have their end faces numbered 1 and 2 
              while their side faces are numbered 3-8 with 
              the uppermost 'Parry' face as 3.  

              Click the button 'View crystal' for a diagram 
              illustrating the numbering scheme for plates
              and Parry oriented crystals.

          Back to the CZA.  Test the assertion that it is 
          formed by rays entering the uppermost face of plate
          crystals.    Enter '1' in the 'Entrance faces' box. 
 
          Run the simulation again.  The filters only pass 
          the CZA and the infralateral arc.

          Do 'CZA rays' exit the side faces?   Check this 
          by entering the side face numbers 3,4,5,6,7,8 in 
          'Exit faces' 


      5.  Now enter 3,4,5,6,7,8 in the 'Entrance faces box 
          also.  Only the parhelia, 22 degree halo and 
          tangent arcs are plotted showing that these halos 
          are all formed by rays entering and leaving side
          faces. 

      
      6.  The PARHELIC CIRCLE is sometimes said to be 
          formed by rays externally reflected from crystal 
          faces.  

          Test this by clearing all the entrance and 
          exit face filters then setting the minimum and 
          maximum number of faces traversed to 1.

          The parhelic circle is much fainter, especially 
          far from the sun.   Major parts of it must 
          therefore arise from more complicated ray paths.  

       

       HOW then is the PARHELIC CIRCLE formed?

       To answer this, use the Ray Tracer in combination with 
       the Ray Filters.   Leave the controls as they are for 
       now.


       RAY TRACING

       The ray tracer selects a small part of the simulation 
       within a 'the trace Box' and displays the numbers of 
       the crystal faces traversed by each ray that is 
       plotted within the box.  
       
       The first number is the entrance face, the last is the
       exit face.  Other numbers are faces where an internal 
       reflection ha occurred.

       Determine the ray paths in the parhelic circle.

       1. If not already in place, load 'filter demo'.  The
          minimum number of faces should be 1 and the 
          maximum 6. 
          
          Clear all the entrance, internal and exit face 
          filters. 

       2. Run the simulation.

       3. When the simulation is complete, click the button
          "Activate trace".  The 'trace box' displays at the
          simulation centre.

       4. Drag the box so that it overlays the parhelic 
          circle at about the 9 o'clock position.

       5. Click 'start'.
          This time the display remains unchanged but the 
          ray paths of rays falling within the trace box are
          displayed.

          Note the tremendous diversity of the ray paths.  
          Very few rays reflect externally off the crystals.
          Most undergo one or more internal reflections 
          after entering side and end faces.

       6. Change the minimum number of faces traversed to 4.

          Note the complicated ray paths.   The parhelic circle
          is deceptive - it is formed by a greater diversity 
          of ray paths than any other bright halo.


       7. 120 DEGREE PARHELIA
          Use the Ray filters to isolate the 120 degree 
          parhelia and then use the ray Trace to identify the 
          specific ray paths.

          Do you get rays entering the top face of plate
          crystals, reflecting internally from two adjacent 
          side faces then leaving bottom face?


       Many controls are 'frozen' while the ray tracer is on -
       this is essential to maintain the correct internal 
       working of HALOSIM.   Click the 'Hide box' button to 
       remove the Trace Box and free the controls.



          

          

 

