Patent ID: 6839078

Claim:
An apparatus for forming a duplex image on a recording sheet, said duplex image including an obverse-side image formed on an obverse surface of said recording sheet and a reverse-side image formed on a reverse surface of said recording sheet, said apparatus comprising: an image-forming section to form said obverse-side image and said reverse-side image on a photoreceptor element by scanning a light beam onto said photoreceptor element, wherein the light beam is modulated with image signals based on pixel-clock signals and reflected from a polygon mirror rotating at an operating velocity based on polygon-clock signals; a transferring section to transfer said obverse-side image and said reverse-side image formed on said photoreceptor element onto said obverse surface and said reverse surface of said recording sheet, respectively; a fixing section to fix said obverse-side image and said reverse-side image onto said obverse surface and said reverse surface of said recording sheet, respectively; and a clock-frequency changing section to automatically change (i) a pixel-clock frequency which is a frequency of said pixel-clock signals, and (ii) a polygon-clock frequency which is a frequency of said polygon-clock signals, in accordance with a degree of shrinkage of said recording sheet caused by a fixing operation performed in said fixing section, wherein the clock-frequency changing section changes the pixel-clock frequency and the polygon-clock frequency at a transition time which occurs at least one of: (i) after said image-forming section completes an image-forming operation for said obverse-side image and before said image-forming section commences an image-forming operation for said reverse-side image, and (ii) after said image-forming section completes an image-forming operation for said reverse side image and before said image forming section commences an image-forming operation for said obverse-side image.