An added benefit was that the angle of projection of the beam of light was reduced as well as the angle of view of the detector. This produced better measurements of light attenuation. This is the basic transmissometer used in smoke generators for visible emissions training today. There still was a major problem with single pass units. In order to calibrate the zero and span and determine the linearity the smoke stack had to be clear.
Some industrial stacks take days to clear.
By adding a fiber optic cable bypassing the stack, a relative zero could be established if the light that normally entered the orifice at the photocell side were blocked. During the sample mode the fiber optic cable would be eliminated. If the instrument is inherently linear, you only have to establish a zero and a slope to have the instrument in calibration. It is the output you get when the light is on and no smoke is in the path of the light. There should be no voltage generated by the photocell.