1. Field of the Invention
The present invention relates to image forming apparatus such as printing apparatus.
2. Description of the Related Art
Several different image forming methods or printing methods are used by commercially available image forming apparatus. Ordinarily, the user selects and uses an apparatus employing a printing method that suits the use to which the apparatus will be put, as noted in Japanese Unexamined Patent Application Publication No. 9-169474.
Electrophotographic image forming apparatus such as laser printers and light-emitting-diode (LED) printers, referred to below as electrophotographic printers, forms a latent image by illuminating the surface of a photosensitive drum, develops the latent image by applying toner, and transfers the toner image from the photosensitive drum to paper or other printing media. Since electrophotographic printers can print at high speed with high quality, they are widely used in business work.
Serial impact dot matrix printers have an impact mechanism with needle pins that strike an inked ribbon, thereby transferring ink from the ribbon to the printing media and forming an image made up of dots. The advantage of a serial impact dot matrix printer is that it can make copies, as the force of the impact is also transmitted to the lower layers of multiple-ply printing media. Disadvantages of serial impact dot matrix printers include slow speed, inferior printing quality, and noisy operation. Serial impact dot matrix printers are now used mainly for office work that requires the making of copies.
Ink jet printers have a printing head with nozzles from which drops of ink are ejected, each ejected drop forming a dot on the printing medium. Ink jet printers can be classified as piezoelectric or thermal, depending on the method by which the drops are ejected. As both types are comparatively quiet and inexpensive, ink jet printers have become very popular for personal use.
Thermal printers have a thermal head with heating elements that heat an inked ribbon, thereby transferring ink from the ribbon to the printing media. A ribbon of the film type is normally employed. An advantage of thermal printers is their very high resolution, resulting in sharp printed images.
A problem with conventional image forming apparatus is that each apparatus employs only a single printing method. Accordingly, there is no single apparatus that combines, say, a multiple-ply copy-making capability with the speed and quality of electrophotographic printing. When copies of an electrophotographically printed document are required, it is necessary either to print the document again or to use a separate copying machine.
Printing the document again takes time, however, and since it is impossible to distinguish between the original document and the printed copy, various problems tend to arise, especially in an office in which the document and copy are handled by more than one person. Typical problems include treating both documents as copies, so that the original document fails to be delivered to its intended destination; treating both documents as originals, so that the same document is delivered twice to the same destination; failing to make a necessary copy; making unnecessary copies; and losing track of copies, which may then fall into unintended hands. The same problems arise if a copying machine is used, because copying machines are also electrophotographic, making it difficult to tell a copy from the original document. Similar problems arise when a document printed by an ink jet printer is copied.