I think it's a kind of material which has rough surface. I think of the opposite of these kind of materials. If a material is very smooth, it will reflect all lights to a single direction and so people not in this direction cannot see the object. I remember it's the reason why people in some directions towards the blackboard cannot see the words on blackboard.
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I think it's a kind of material which has rough surface. I think of the opposite of these kind of materials. If a material is very smooth, it will reflect all lights to a single direction and so people not in this direction cannot see the object. I remember it's the reason why people in some directions towards the blackboard cannot see the words on blackboard.