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A combination of reflection and a highly pertubated bumpmap may cause some reflected rays to go inwards toward the object. In this case you will see the inside of the object in some places, which may be disturbing in certain scenes.
A possible solution:
Add transparency attribute to this object with
intensity: 0
tran color: color(100,100,100)
This will cause the color absorption inside the object to be very high. and the resulting color contribution of this ray to be almost black.
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