POV-Ray : Documentation : Normals and Radiosity
  POV-Ray 3.6 Documentation Online View Radiosity without conventional lighting Performance considerations Normals and Radiosity

When using a normal statement in combination with radiosity lighting, you will see that the shadowed parts of the objects are totally smooth, no matter how strong the normals are made.

The reason is that POV-Ray by default does not take the normal into account when calculating radiosity. You can change this by adding normal on to the radiosity block. This can slow things down quite a lot, but usually leads to more realistic results if normals are used.

When using normals you should also remember that they are only faked irregularities and do not generate real geometric disturbances of the surface. A more realistic approach is using an isosurface with a pigment function, but this usually leads to very slow renders, especially if radiosity is involved.
normal off (default) normal on isosurface

You can see that the isosurface version does not have a natural smooth circumference and a more realistic shadowline.

More about "normal statement"

More about "isosurface" Radiosity without conventional lighting Performance considerations

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