euroherbal said:

If you look closely in the 5th photo of ImpFighter's sequence, you'll see the kneeling pad is INSIDE the verge of the black circle which has the med-chamber is in its middle.

So the BTS photo is not correct, as you can see in the hologram room still that the walls are reflecting on a "clear" floor (no trace of a dark circle).

The dark SE colours make it difficult to see it properly, but I reckon the 'reflective' floor seen in the behind-the-scenes pic will indeed be the same one filmed when we see Vader descending his chamber...and during the 'side onwards' shots of the hologram too.

Here's a big version of the one of the 'side onwards' hologram shots for a clearer look - http://img52.imageshack.us/img52/4872/starwars51954.jpg