Abstract: The operating latitude of the tri-level xerographic process is improved by replacing the standard DC bias that is applied to one or both of the developer housings in conventional tri-level imaging with a chopped DC (CDC) developer bias. Chopped DC biasing is the alternate application of two discrete bias voltages to a developer stucture in a periodic fashion at a given frequency, with the period of each cycle divided up between the two bias levels at a duty cycle of from 5%-10% or 90%-95% depending upon which of the two developer structures is being biased. In the case of the DAD developer structure the duty cycle of higher of the two biases is 5%-10% and in case of a CAD developer structure the duty cycle of higher of the two biases is 90%-95%.

Abstract: Highlight color imaging method and apparatus including structure for forming a single polarity charge pattern having at least three different voltage levels on a charge retentive surface wherein two of the voltage levels correspond to two image areas and the third voltage level corresponds to a background area. Interaction between developer materials contained in a developer housing and an already developed image in one of the two image areas is minimized by the use of a scorotron to neutralize the charge on the already developed image.

Abstract: Printing apparatus for forming toner images in black and at least one highlighting color in a single pass of a change retentive imaging surface through the processing areas, including a development station, of the printing apparatus. The development station includes a pair of developer housings each of which has supported therein a pair of magnetic brush development rolls which are electrically biased to provide electrostatic development and cleaning fields between the charge retentive surface and the developer rolls. The rolls are biased such that the development fields between the first rolls in each housing and the charge retentive surface are greater than those between the charge retentive surface and the second rolls and such that the cleaning fields between the second rolls in each housing and the charge retentive surface are greater than those between the charge retentive surface and the first rolls.

Abstract: An apparatus which develops image areas recorded on a surface by transporting a developer material into contact therewith. The apparatus includes a conductive member and an insulating member spaced therefrom. Developer material transported by the conductive member develops the solid regions of the image areas. The insulating member removes developer material from the non-image areas and develops the lines in the image areas with the developer material.