Hi again, I've tried nesting three concentric spheres thus:


  sphere {
    <0,0,0>,1
    pigment {rgbt 1}
    }
  sphere {
    <0,0,0>,0.9
    pigment {rgbt 1}
    }
  sphere {
    <0,0,0>,0.8
    pigment {rgbt 1}
    }

and the interior sphere ends up black even though it should stay invisible.
Is this an issue with transparency in POV-Ray?

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> and the interior sphere ends up black even though it should stay 
> invisible.
> Is this an issue with transparency in POV-Ray?

Yes.  When tracing the color of a pixel the default setting
calculates 5 surface reflections, or partially transparent surfaces.
With three nested spheres a ray from the camera passes through
at least six surfaces, one for the front and back of each sphere.
When a ray is stopped after some depth the pixel is black.
This can be seen as dark spots in multiply reflected areas,
or as black instead of clear objects, also there is a warning
if you read the message output. The fix is to set the maximum
trace level higher in global settings.

global_settings {max_trace_level 10}

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