I related an old industry tale in a thread on another site and thought I might do a copy and paste to share it here:
Reminds me of the story of a Hollywood MasterClass in lighting techniques held by the great DOP who had done a lot of cinema noir drama. He showed lighting a mahogany lined den. He explained that it was low key but that the talent could move from pool to pool of light either lit or silhouetted.
A student queried the flare on the front of a bookcase. The Master explained that it was a sheen and not a flare. The student asked the difference between a flare and a sheen to which came the reply: "$1,000.00 per day!"