1. Field of the Invention
The present invention relates to a method of forming monochromatic or dichromatic copy images by the use of a three-component developer.
2. Description of the Prior Art
To meet the need for diverse modes of processing information in recent years, copying machines have been developed for making copies of composite images by successively or simultaneously forming on the surface of an electrostatic latent image bearing member a first latent image and a second latent image having an opposite polarity to the first image, or having the same polarity as the first image but different therefrom in potential, developing the first and second latent images thus formed as a composite image to obtain a toner image, and transferring the toner image onto the surface of a copy material.
As one of the copying machines of the type described, Published Unexamined Japanese Patent Application No. SHO 57-8553, for example, discloses an electrophotographic copying machine wherein the surface of a uniformly charged electrophotographic photosensitive member is exposed to a positive image to form a first latent image and subsequently exposed to a negative image to form a second latent image, whereby a composite image is formed. With this copying machine, the first and second latent images are developed with toners of the same color or different colors.
Furthermore, various processes have also been proposed for forming dichromatic copy images from dichromatic originals. For example, published Unexamined Japanese Patent Application No. SHO 55-117155 discloses a process which employs a photosensitive member comprising a first photoconductive layer laminated to an electrically conductive substrate and photosensitive to a first color (e.g. red), an intermediate layer over the first layer, and a second photoconductive layer formed over the intermediate layer and photosesitive to a second color (e.g. black). Electrostatic latent images corresponding to copy images of a first color and a second color are formed in opposite relation in polarity by specified steps and then developed to visible images with two kinds of toners charged to a polarity oppositie to each other and having different colors.
Another process for forming dichromatic copy images has also been proposed which employs a photosensitive member comprising a photoconductive layer laminated to an electrically conductive substrate and having photosensitivity to both polarities as well as to two colors and in which electrostatic latent images corresponding to copy image areas of first color and second color are formed in opposite relation in polarity by specified steps and then developed with two kinds of toners of different colors.
However, because the composite image forming process and the dichromatic copy image forming processes described require at least two kinds of toners and especially because the latter processes require two toners of different colors, these processes have the drawback of generally necessitating two developing units, which inevitably render the copying machine large-sized. Although a dichromatic developing method has been proposed which uses a single developing unit, the method fails to give sharp dichromatic copies since fog or mingling of colors is liable to occur in copy images.