After reading this post, I was left with the question of why bother with the luminescence--why not just collect the transmitted light with the fibers and transmit it directly to the detector. The linked journal article makes it clear that the transmitted light is not collected by it entering the ends of the fibers. Instead, the transmitted light "scatters" off of the dye (changing wavelengths in the process), regardless of where and at what angle it hits the fiber, and some of it is scattered in the proper angular range to be transmitted along the fiber.