HP 8550 Service Manual - Page 215

Background cancellation (background level adjustment) When reading the light reflected by an original, the CCD also reads the color data of the surface (background) of the paper. To correct the image signal, the background cancellation block sets aside background data or correction of the image signal, preventing fogging and improving the level of reproduction. The pixels are identified as representing the background of an original if the level of each frequently appearing R, G, and B signal is 200 or higher (figure 103). Figure 103. Background pixel measurement The collection of extended modes (accessible via the administrative functions menu and image adjustment; see the copy module user guide) provides two modes, each with a different method of correction: l In "background omission mode," the level of RGB signal of the background pixels is corrected to 255 to remove the specified background color. l In "anti-see-through mode" or "second side removal mode," the RGB signal of the background pixels are removed from the image data for the entire image area to remove the image on the other side of the original. EN Copy module 213