Touch screen signal processing

A coordinate detection system can comprise a display screen, a touch surface corresponding the top of the display screen or a material positioned above the screen and defining a touch area, at least one camera outside the touch area and configured to capture an image of space above the touch surface, an illumination system comprising a light source, the illumination system configured to project light from the light source through the touch surface, and a processor executing program code to identify whether an object interferes with the light from the light source projected through the touch surface based on the image captured by the at least one camera. Light can be directed upward by sources positioned behind the screen, by sources positioned behind the screen that direct light into a backlight assembly that directs the light upward, and/or by a forward optical assembly in front of the screen that directs the light upward.