In order to save the consumption of toner in color image duplicating devices such as printers, copiers, facsimile machines and any combinations thereof, the prior art color image duplicating devices utilize less than a predetermined amount of toner and generates color images at a low printing density in a toner save mode. In general, there are two kinds of approaches to save toner in image duplicating devices. According to one prior art toner saving method, image data is simplified or reduced. In another prior art toner saving method, a bias voltage for transferring toner onto a static image on a photoreceptor drum is reduced. The first prior art toner saving method loses resolution in a generated image, and a desired image is hardly obtained.
With respect to the second approach, color balance cannot be properly maintained at a bias voltage value that is lower than a predetermined optimal value for generating a desirable color image. Color toner generally includes a predetermined number of color components such as cyan (C), magenta (M) and yellow (Y), and these color components are layered to yield desirable colors. By reducing a bias voltage value for transferring, the original color balance is distorted since these color components of the toner are not equally or proportionately affected. Subsequently, an image generated at a low printing density is also distorted for human color perception. For example, a color toner application ratio among C, M and Y is 50:50:50 at a predetermined optimal bias voltage level. When the bias voltage is reduced, the color toner application ratio is altered to be 60:40:40. The change in the color toner application ratio is also affected by certain characteristics of toner and or a developing device. In the above example, since the C application percentage has increased prior to the reduction of the bias voltage, an image appears to be bluish in color, and no desired color is obtained in the image.
The above described second toner saving method needs to maintain color balance in an image generated at a reduced bias voltage value. Furthermore, it is also desirable to practice the toner saving mode based upon actual usage of toner in the image duplicating devices so that a tone cartridge lasts longer.