Oblon, Spivak, McClelland, Maier & Neustadt
Nondestructive multicolor image forming apparatus
 A color copier capable of producing, without disturbing a toner image formed in the first color, an image in the second color and preventing the first color from being mixed with the second color. In the event of development in the second color, a bias voltage is applied to a developing roller at a repetition frequency shorter than 5 kHz. One period of the bias voltage consists of a pulse voltage VPULS having a duration TA (shorter than 100 .mu.sec), and a DC bias voltage VDC having a duration TB. The pulse voltage VPULS is higher than the potential VD of a non-image area in order to enhance a developing potential. The DC voltage VDC is lower than the potential VD and a potential VT of a toner layer of first color and higher than the potential VL of an image area in order to set up a sufficient developing potential. The DC voltage VDC and the potential VT has a potential difference smaller than 500 V. A gap for development ranges from 100 .mu.m to 300 .mu.m. The pulse voltage VPULS may include an overshoot portion at the negative-going edge thereof or may be replaced with a voltage having a triangular waveform. The adhesion of the toner of first color to an image carrier is not susceptible to electric fields formed for development in the second color. For development in the second color, a toner exhibiting a particular moving characteristic in the gap is used. 
Ricoh Company, Ltd.
G
G03G
inventional
Kazuhiro Naoki Koji Akira
Nishido Iwata Suzuki Sawada