Size or position sensing of intermixed sheets in a sheet stacking tray with sheet edge shadow detection

A non-contact optical sheet size or position detection system for a sheet stacking tray in which thin document sheets of different sizes or positions are stacked on top of one another, said sheet detection system detecting the position of at least one sheet edge of an outside (top or bottom) sheet of said stack, comprising:

• an illumination source for illuminating for light reflection therefrom an area of the surface of said outside sheet of said stack,
• said illuminated area including at least said one sheet edge,
• said illumination from said illumination source being at a sufficiently shallow illumination angle to the surface of said outside sheet to generate a detectable edge shadow directly adjacent to said sheet edge, which edge shadow is relatively darker than reflected said illumination of said top sheet adjacent thereto,
• said illumination angle being substantially greater than a grazing angle to not form shadows from said surface of said sheet,
• and an optical detection system positioned to remotely detect said detectable edge shadow without contacting said outside sheet or said stack,
• said optical detection system providing location information for the location of said edge shadow so as to provide location information for said sheet edge of said outside sheet of said stack,
• wherein said sheet stacking tray is part of an output stacking system with a stacking tray in which thin printed sheets are sequentially stacked on top of other such sheets, and having an offsetting system for offsetting alternate stocks of sheets in said stacking tray, wherein said offsetting system is operatively controlled by said optical detection system sheet edge location information from a detected edge shadow of the top sheet in said stacking tray for controlling said offsetting of alternately printed stacks of sheets.