The NTE3100 Interrupter Module is a gallium arsenide infrared emitting diode and a NPN silicon photo transistor mounted is a polycarbonate housing. The package is designed to optimize the mechanical resolution, coupling efficiency, ambient light rejection, cost and reliability. Operating on the principle that opaque to infrared will interrupt the transmission of light between an infrared emitting diode and a photo sensor switching the output from an “ON” state to an “OFF” state.
 
Full spec sheet:
https://www.nteinc.com/specs/3100to3199/pdf/nte3100.pdf