In an image processing apparatus, such as a scanner or a copier, for example, a contact image sensor (CIS) module can be used. A CIS module is an image sensor module configured to optically read image information of an original document and to convert the optically read image information into an electrical signal.
The CIS module can include a light source having elements configured to emit light with wavelengths associated with three different colors: red (R), green (G), and blue (B). The R, G, and B colors are often referred to collectively as RGB. The CIS module can include a light guide configured to guide light from the light source to be incident an original document across its width direction and a sensor array configured to detect the light reflected from the document. The sensor array can include multiple sensor integrated circuits (ICs) to perform a photoelectric conversion of the reflected light. The sensor ICs in the sensor array can be linearly arranged to have a length that generally correspond to the width of the document being read, that is, the sensor array can correspond to a line of the image of the original document.
The light source can be arranged to face the light incident surface of the light guide so that the light radiated by the light source can be incident on the surface of the light guide. The light can be guided to form a line along the main scanning direction of the original document such that the original document, which can be placed on a support such as a glass support, can be substantially uniformly illuminated in a linear shape along the main scanning direction. Red, green and blue light emitting diodes (LEDs) can be used as the light emitting elements of the light source. The light emitting elements can be sequentially driven in a time division manner. The light radiated by the light source and reflected from the original document can be focused by a lens array to form an image on the sensor array. A color image signal, which includes R, G and B color information, can be produced by the sensor array, and can be externally output through an electrical connector, for example.
When the RGB LEDs are sequentially turned on during one period, the reading of the RGB color image associated with one line can be completed.
To obtain two-dimensional information of the original document, the document and the CIS module can be moved relative to each other in the sub scanning direction at a predetermined speed. Because the radiation locations of the RGB light on the document in the sub scanning direction can be different, the timing associated with the reading of the RGB information can be different. Accordingly, when the RGB outputs are synthesized to form color image information, a color shifts can occurs in the sub scanning direction. That is, when the original document is read using the CIS module that used a single channel line sensor having a time division multi-color light source, in order to implement the colors, each of the RGB LEDs may be sequentially turned on during ⅓ of an exposure time of one line to scan the colors. Thus, because a registration error of a ⅓ pixel can occur between R-G and G-B, a color shift can occur. For example, an upper portion of an image can appear reddish, a lower portion of the image can appear bluish, and/or fringes can result in the upper and/or lower portions of the image.
Conventional attempts heretofore to reduce the color shift of the image includes the use of a CIS module employing a three-channel line sensor, and/or the provision of color filters corresponding to the various colors.