A photoelectric sensing unit, especially useful for determining the passage of clear bottles or other low contrast targets on a conveyor, contains two sensors and a signal processing unit to determine the signal differential. The signal differential is compared to a selected value and sensor output is enabled or disabled. High noise immunity, resistance to target reflection, and output reliability result. The unit may correct the selected value to account for environmental change and/or unit component performance. The unit can also figure the speed of passage of the bottles.