The present invention relates to an illumination scanning system for an electrophotographic printing machine and, more particularly, to a novel illumination detector utilized in a feedback control system.
Generally, in electrophotographic printing systems, originals placed on a platen are incrementally scanned by an illumination source. The source is either maintained stationary and the platen (and document) moved past the source, or else the illuminator and associated mirror, or mirrors, are moved in a plane parallel to, and beneath, the platen to incrementally scan the entire document. The image reflected from the document may be folded along an optical path by one or more mirrors and projected upon a charged photosensitive surface, such as a drum photoreceptor. A latent image of the original document, formed on the photosensitive surface, can then be developed and fixed to produce a final copy.
When the exposure subsystem is initially set up, the illumination system, comprising some type of linear lamp (fluorescent, tungsten etc.) is supplied sufficient power to produce an optimum exposure level of the image at the photosensitive surface. However, with extended operation of the reproduction device, the optical system, the mirrors which fold the optical image, and the lens itself accummulate dirt and other contaminants. The presence of these contaminants results in reduced levels of exposure at the imaging plane since the light rays constituting the reflected image are partially absorbed and/or deflected by these contaminants. This reduced exposure level, if left uncorrected, would result in progressively greater underdevelopment of the latent images being formed on the photoreceptor.
The present invention is therefore directed to an improved optical scanning system wherein a feedback system is incorporated for the purpose of measuring the exposure level at an image plane and providing compensation for decreased image exposure. The feedback system includes a photosensor located within a light guide device. The photosensor positioned near the imaging plane, senses an integrated illumination level and generates a signal to increase the output of the illumination source when a falloff in the exposure level is detected. More particularly, the invention is directed to a document scanning system comprising:
an illumination source adapted to provide incremental line illumination of a document in an object plane; PA1 a projection lens for projecting a light image of said original toward a photosensitive image plane; PA1 means for providing relative motion between said document and said illumination source; PA1 at least one mirror positioned on the image side of said projection lens and along the optical path, said mirror adapted to reflect the projected incrementally scanned image lines onto said image plane, said mirror further adapted to incorporate an optically transparent area extending along the height of said mirror; PA1 an illumination detector comprising a light guide member for coupling light entering said transparent area and adapted to direct said coupled light onto a photosensor contained within said member, said photosensor generating an output signal representative of the level of the illumination impinging thereon; and PA1 control means for adjusting the output of said illumination source in respose to said photosensor output signal.