In addition to scanning common reflection documents (i.e., opaque documents), a scanner can also scan transparent documents. A transparent document is a slide (also called positive) or a negative. When a scanner is used to scan a transparent document, a backlight module is required to provide backlight illumination so that the scanner can acquire image data when scanning the transparent document.
Further, because the transparent document is made of transparent material, if a user wants to view the image or text content of the transparent document before scanning, an extra illumination light is required. For instance, he can put the transparent document under a light to view its content. However, this way of viewing the content of the transparent document is inconvenient because it is necessary to find another illumination light.
Accordingly, the present invention aims to propose a backlight module used as a light source for preview of a transparent document to solve the problems in the prior art.