A port assembly for use with a polymeric bioreactor bag includes the following components: i) a hollow port member made from a material which can be fusibly affixed to the wall surface of the bioreactor bag; ii) at least one fluorophore spot positioned on the port member; iii) a conduit for conveying excitation light from an optical source to the fluorophore, which conduit can be a lens, a curved parabolic collimator, a shaped reflector or a wave guide; and iv) a second conduit for conveying fluorescent emission light from the excited fluorophore to a photo-detector, which second conduit although different from the first, can likewise be a lens, a curved parabolic collimator, a shaped reflector or a wave guide.