The optical based toner-level sensor (TLS) is extensively used in color LaserJet printers to monitor the remaining toner in a cartridge, which helps users know when to change and reorder replacement cartridges. Our recent study found that the optical based TLS signals exhibit large variation. In order to have a better understanding of the optical based TLS technology, statistically designed experiments were used to identify the factors that most significantly impact variation of the signal. Six major factors were investigated in this study. It was found that different toner colors with different toner levels have different significant factors. The governing equation of translational motion for particulate materials is used to qualitatively explain the observed phenomena.