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I'm thinking of big halls, especially those that use chandeliers for light. The idea's simple: build a parabolic ceiling that uses mirrors, place the chandelier at the focus, and illuminate the entire hall with light. Since the light would be coming from the ceiling, it wouldn't be possible to "stand in someone's light" either, which would be great for e.g. exams. One would also be able to use just one light source for the entire hall. Why hasn't this been done?

I feel like I'm missing some obvious reason but I can't place it. One guess could be that big parabolic mirrors are expensive, but we have such mirrors elsewhere (e.g. for telescopes, science museums). It could be that such a hall would look really bright if one looks upwards (at the ceiling), but that could be mitigated by, e.g., placing a one-way mirror between the ceiling and ground level that allowed light down but not up. What am I missing?

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  • $\begingroup$ Perhaps cost would be a factor. $\endgroup$ – Solar Mike Aug 11 '19 at 5:49
  • $\begingroup$ Perhaps like the one at the Reichstag in Berlin (designed by Norman Foster) $\endgroup$ – atom44 Aug 11 '19 at 10:15
  • $\begingroup$ The bit about "one-way mirror" doesn't make sense. The question would be better if that sentence were removed. $\endgroup$ – Ray Butterworth Aug 11 '19 at 13:11
  • $\begingroup$ I saw one in a modest size ( 1600 sq.ft.) home . I don't remember the light affect but I do remember the sound; the room was "loud" ( the floor was tile). $\endgroup$ – blacksmith37 Aug 11 '19 at 15:43
  • $\begingroup$ If this was done literally as described, the quality of the illumination would be very poor, because the only light source would be vertically downwards and for any activity using a horizontal surface, you would always be working in your own shadow. And as blacksmith37said, the parabolic ceiling would reflect sound as well as light! $\endgroup$ – alephzero Aug 11 '19 at 15:45
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In the case of a mirrored paraboloid with a lamp at the focus, all the light rays would shine directly downward. Any object would have a sharply defined shadow directly below it; people would look sinister with dark eye sockets and nostrils. This is known as hard light.

Contrast that with a light-colored ceiling (of any shape), which reflects light in many directions. This creates soft light, and is more pleasant to be in.

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