A model of illumination permits classifying and analyzing illumination methods. Light sources are modeled as surfaces in space relative to an imaged object, which define source apertures. Sources may be repositioned for scanning and to overcome placement constraints. This aperture model describes collimated and diffuse illumination, brightfield and darkfield methods. With the addition of a field surface it models structured lighting. The models provide a basis for comparing methods. Literature on illumination for machine vision is reviewed, with emphasis on automatic visual inspection.