If you are using an similar setup you have to take care of the backlash of the robot axis, during your linear move through the fork light barrier all axis have to move in a consistent direction to avoid backlash errors during your measure.
If you need XYZABC Measures you will ned some type of Vision Sensors, i heard about something from Wiest or Leonie about a ready to use system but honestly i dont know anything about them.
If you continue to use the fork light barrier system maybe you should try a di-soric sensor we upgraded to this because it was more reliable then the before used one.
High Performance Receiver (OEPQ 12M 5000 G2-TSSL)
High Performance Transmitter (OSPQ 12 MHF-TSSL)
Maybe this information can help you