An illumination system for illuminating a scan region on an object and for providing a white level reference for a detector may comprise a light source for producing light rays and an elongate lens having a first end and a second end positioned between the light source and the scan region. The lens collects some of the light rays from the light source and directs them onto the scan region. A first spot lens is positioned adjacent the first end of the elongate lens and a second spot lens is positioned adjacent the second end of the elongate lens. The first and second spot lenses direct some of the light rays from the light source toward first and second ends of the scan line. A white level reference mark is positioned adjacent the lens and the light source so that some of the light rays from the light source are reflected by the white level reference mark to the detector.