So the consensus is that the font does not actually exist, but that Franklin Gothic Demi Bold is a very close match and is a good starting point. When you say Franklin Gothic Demi Bold, do you mean Franklin Gothic Demi (on the bold setting), or is it a completely different font? Either way, can it be download from the net, do you think? (or must it be purchased?)
Here's my first stab at cleaning up the logo a bit:
Actually- I just had another thought: I believe that the original version of THX-1138 has the original green logo as well. Does anyone with the "Special Edition" DVD know if that was still retained? Perhaps that transfer would have the highest quality of all.