In addition what dnmilikan said about IR, I have to say thtat IR isn't convergent which makes it useless when the distance between TX and RX units is large.
On the other hand there is one problem with a battery powered stand alone TX unit. We are using visible 650nm class II laser (for obvious reasons like alignement) therefore we decided to switch off the laser whenever the beam is broken.
The user doesn't want to see red spots (caused by the laser transmitter) on his/her photos. In order to do this we decide to power everything from the main controller unit.
Yes, there are going to be lot's of cables, but the good news is that the same cable will be used all over the system.