The white vector--the voltage difference between white areas of a latent image on a photoconductive unit and a developer roller--may be independently adjusted at each photoconductive unit, allowing multiple image forming units to be driven from a shared power supply. The photoconductive unit is charged to a high voltage level relative to the developer roller, and selectively optically discharged to the desired white vector. The voltage of the discharged area may be measured, or may be calculated by increasing the developer roller voltage a predetermined amount, discharging the photoconductive unit until toner is sensed in white image areas, and then reducing the developer roller voltage. The white areas may be discharged using a lower optical power from the writing light source or a different light source, such as a laser, LED or electroluminescent source. A second laser may be of a different wavelength than a writing laser.