Contact image sensor (CIS) is a kind of linear image sensor and used to scan planar document (such as graph or text document) into electronic image file for further storing, displaying and transmitting. The major applications of CIS include scanner, facsimile machine, multi-function printer and so on.
The working principle of CIS module includes directing light from a light source on the document to be scanned and using a lens set to focus the light reflected from the document to a light sensor such as charge-coupled device (CCD) sensor or complementary metal oxide semiconductor (CMOS) sensor. The light sensor converts the photonic signal into corresponding electric signal and analog (or digital) pixel data can be thus generated.
During scanning, the light sensor can generate analog (or digital) electric signal based on the light reflected from each area of the document. More particularly, darker area on the document has weaker reflected light and brighter area on the document has stronger reflected light such that the light sensor can generate electric signal with magnitudes corresponding to the reflected light intensity. A following-up software processing, such as OCR software, is conducted to process the pixel data (corresponding to the electric signal) into computer image file.
However, the related art CIS device generally uses a lens module with one or more rod lens array 2a arranged in a holder and the assembled holder is mounted in a housing 1a as shown in FIG. 1.
The related art CIS module generally uses rod lens with gradient focusing to focus the light emitted from light source to the CCD sensor and the CCD sensor then converts the photonic signals to analog (or digital) electric signals. The elements of above mentioned CIS module are arranged in the manner shown in FIG. 1. When the object 7a to be scanned is placed on a glass plate 2a atop the housing 1a, the light emitted from the light source 3a illuminates the object 7a and the reflected light corresponding to document image is focused to the light sensor 6a on the circuit board 5a through the focusing action of the rod lens array 4a. 
However, the related art CIS device has following drawbacks:
1. The CIS module generally uses rod lens array 4a and the focusing length thereof is limited by rod material and rod diameter. The CIS module is difficult to scale down.
2. The rod lens array 4a is generally made by injection molding such that the image quality is not satisfactory and the cost is high.
3. The object distance is demanded to be equal to the focal distance to have optimal imaging.
4. The rod lens has shorter depth of field (DOF) such that the housing has more stringent size demand. The yield and cost thereof are influenced.
5. The imaging quality of the image sensor is influenced by particles.