It reflects half of the returning light from the retro reflector onto the detector. Since the light traverses the smoke two times, the unit is called a double pass system. The next improvement is the addition of a chopped light source. This means that the light goes on and off. During the off time the detector is responding to ambient light from the stack or other light leaks.
Ambient light acts as a negative bias in terms of opacity.
The signal from the ambient light is subtracted from the total signal giving the actual signal due to the transmissometer source light is calculated. Some systems use frequency modulation while others just measure the light level with the source on and off. The next trick is perhaps the most important. Instead of shutting down the stack, a movable mirror is placed in the system between the beam splitter and the stack wall. When the mirror is inserted, the light no longer goes across the stack but is reflected toward the beam splitter and the detector.