This invention relates to an image forming apparatus with document size detection and, more particularly, to an image forming apparatus most suitable for, e.g., an electronic copying machine which optically scans a document to obtain image data to be formed.
As is well known, in electronic copying machines, a detection means for detecting a document size placed on a document table (transparent glass) has been developed. The detecting means has a plurality of light emitting elements and light receiving elements installed at the back surface side of the document table along the longitudinal direction thereof. A document cover is closed and light is radiated from the light emitting elements onto the document table. Light reflected from a document placed on the document table and the document cover is detected by the light receiving elements. A document size is determined by detecting level changes in output signals of the light receiving elements.
In the conventional arrangement, the light emitting and receiving elements corresponding to a document size must be installed at the back surface side of the document table. Therefore, a large number of light emitting and receiving elements are required. In addition, space for installing these elements must be guaranteed.
Furthermore, a surface of the document cover facing the document table is normally white. When the above detection operation is performed while the document cover is closed, there is no significant difference between amounts of light reflected from a document and the document cover. Therefore, it is difficult to reliably detect the document size.
An electronic copying machine normally has a function for copying a document image on a copying sheet with or without a reduction or enlargement mode. If a document image has an unnecessary portion for image formation, a conventional copying machine cannot copy an image without the unnecessary portion.