| 摘要 |
An image reader installed in an image forming apparatus for reading an image printed on a document and producing a digital video signal representative of the read image. While a CCD image sensor produces an analog signal associated with a document, a background level detector produces a background level signal representative of the density of the background of the document. The background level signal is fed to a reference level input terminal of an analog-to-digital converter. A signal level adjusting circuit is provided for adjusting the ratio between the density of the background of the document and the level of the background level signal. An operation for erasing the background is effected automatically in response to the background level signal. This operation begins after the start of image scanning and, before this operation occurs, the background is erased by using a predetermined fixed level. The time constant of the background level detecting circuit is switchable and is switched over after the start of image scanning. By determining the level of the analog signal from the CCD image sensor, the time constant for background level detection is switched over on the basis of the determined signal level. The image reader erases the background of a document while insuring the production of an image which is printed on the document.
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