Thanks to Koji's help, the second photosensor, which was not working, has been fixed. I have re-calibrated the photosensor after fixing a problem with the circuit.  I have determined the new linear region to lie between 7.6 mm and 19.8mm. The slope defining the linear region is -0.26 V/mm (no longer the same as the first photosensor, which is -0.32 V/mm).

Here is the calibration plot.