A digital image capturing and processing system for illuminating objects using automatic object detection and spectral-mixing illumination technique. The system comprises an area-type illumination and imaging module for projecting a coextensive area-type illumination and imaging field (i.e. zone) into a 3D imaging volume during object illumination and imaging operations. The area-type illumination and imaging module includes a spectral-mixing based illumination subsystem for producing a first field of visible illumination from an array of visible LEDs, and producing a second field of invisible illumination from an array of infrared (IR) LEDs, wherein the first and second fields of illumination spatially overlap and intermix with each other and produce a composite illumination field that is at least substantially coextensive with the FOV of the image sensing array. An automatic object detection subsystem automatically detects an object moving through the 3D imaging volume, while an illumination control subsystem controls the relative power ratio (VIS/IR) of visible illumination and invisible illumination during system operations so as to achieve sufficient image contrast in captured digital images to ensure satisfactory image processing.