1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer or the like, and particularly relates to an image forming apparatus which is capable of forming an invisible image together with a visible color/monochrome image.
2. Description of the Related Art
An image forming apparatus is known which charges an image carrier such as a photosensitive material or the like to prepare an electrostatic latent image, forms a toner image on the image carrier by developing with toner, transfers the toner image directly or through an intermediate transfer element onto a recording medium, and fixes the toner image. Among such image forming apparatuses, a tandem type color image forming apparatus is widely known, which includes separate image formation units for forming toner images of four colors, i.e., Y (yellow), M (magenta), C (cyan), and K (black). The apparatus separately forms toner images of the YMCK four colors on image carriers in the respective image formation units, and carries out multiple transfer to form a color image, thus having enhanced productivity of color images.
However, even with a color image forming apparatus, documents of only K color (black and white images) are often printed out. When such a document is to be printed out (in other words, when a black and white image is to be formed by using only K color toner), in general, only the image formation unit for K color carries out image formation operation, while the image formation operations by the image formation units for the colors other than K color are not performed, from the viewpoint of prevention of scratch damage and wear of the image carrier, and degradation of the developer, and the like. In addition, a transfer feed apparatus for an image forming apparatus is known, in which a support member is movably provided for supporting a transfer material feed belt from its rear surface side. In a black and white image formation mode, the support member can be moved to separate the transfer material feed belt from chromatic color recording photosensitive materials (for example, referring to Japanese Patent Laid-Open (JP-A) 2001-005305).
In recent years, the trend of utilizing additional information embedding technology has become striking in which additional information is superposingly embedded into an image in order to protect copyright and prevent illegal copying of a digital literary work such as a static image and the like. Examples of the known additional information embedding art include an image forming apparatus and an image forming method for forming a visible image using ordinary toners of YMCK colors and an invisible image using invisible toner having infrared light absorbency on an image output medium such as a recording paper or the like. Security information or the like is embedded into the invisible image, which aims at improving security functions such as counterfeit prevention, counterfeit deterrence, originality certification and the like. Further, coordinate information or the like is embedded into the invisible image, which aims at increasing the added value of the output medium (see, for example, JP-A 2004-012880, JP-A 2003-186238, and JP-A 2002-240387).
An image formation unit for forming an invisible image using the above-mentioned invisible toner can be added to the above-mentioned conventional tandem-type color image forming apparatus. Thus five image formation units are used to form an invisible image together with a color visible image, and additional information can be embedded into the output image.
However, the conventional color image forming apparatus has not taken into account an application where a black and white image is to be formed together with an invisible image, and provides no configuration suited for prevention of degradation of an image carrier, a developer and the like, or prevention of loss of an invisible image.