Scanners disclosed in the present invention uses a dual-illumination system comprising a front illumination source and a back illumination source. The front illumination source is controlled to illuminate a scanning object when the scanning object is opaque. The back illumination illuminates a scanning object when the scanning object is transparent. An optical system collects lights reflected from the opaque scanning object illuminate by the front illumination source or transmitted through the transparent scanning object illuminated by the back illumination source and focuses the lights upon an image sensor, thereby an image of either the opaque scanning object or the transparent scanning object can be obtained.