Image reading unit with black level control

#The present invention relates to an image reading unit for use in an image scanner for reading images, and more particularly to an image reading unit which outputs an image reading signal in the form of a digital signal.

#In general, in an image scanner for optically scanning an original document to convert the image of the original into an electrical signal, a main scanning operation in one direction is carried out with an image reading unit having light-detecting elements arranged in a line, while the original or the image reading unit is moved in a direction perpendicular to the main scanning direction to carry out auxiliary scanning, whereby electrical signals are outputted in correspondence to the quantities of light applied to the light-detecting elements to read the image.

#In the image reading unit of the image scanner, a light source, such as a fluorescent lamp, for illuminating the surface of the original, an optical lens group acting as a reducing optical system for forming the image of the original irradiated by the light source on the light-detecting element surface, and light-detecting elements for converting the optical image into electrical signals are arranged in that order.

#FIG. 1 is a block diagram showing an example of an image reading unit according to the invention;

#FIG. 2 is a block diagram showing a part of FIG. 1 in more detail;

#FIG. 3 is a timing chart showing waveforms of signals at various points in the image reading unit in FIGS. 1 and 2; and

#A preferred embodiment of the invention will be described with reference to the accompanying drawings.

#FIG. 1 is a block diagram showing the general arrangement of an image reading unit in an image scanner constructed according to the present invention. FIG. 4 is a perspective view showing the construction of an image reading unit in FIG. 1.

#The image reading unit according to the invention includes a light source, namely, a fluorescent lamp 2; a light-detecting element, namely, a CCD sensor 4; a reducing optical system, namely, an optical lens group 3 for forming the image of an original irradiated by the fluorescent lamp 2 on the light receiving surface of the CCD sensor 4, the components 2, 3 and 4 being essential components of a conventional image reading unit; and a built-in image processing circuit which includes: temporary holding means, namely, a sample-and-hold circuit 5; arithmetic means, namely, an arithmetic circuit 6; level setting means, namely, a level setting circuit 7; signal converting means, namely, an A/D (analog-to-digital) converter 8; and timing adjusting means, namely, a delay circuit 9.