Dear Raghu,
Hello, light curtain is a very good idea. whay not IR(not as a light curtain), I'm not absolutely certain that I understood the configuration correctly. but as these objects are very hot, they should emit IR, so an IR sensor(with appropriate wavelength sensetivity) above the production line may be used to observe passage(if I'm wrong, please correct me).
one disadvantage of using temp sensor is over time the board that sensor is mounted on, may become hot and causes the sensor to not be able to sense fast temp changes, unless the sensor is mounted on an object with very low heat capacity(if I'm wrong, please correct me).

best regards