Exposure control system

• US 5,081,492 A
• Filed: 12/12/1990
• Issued: 01/14/1992
• Est. Priority Date: 12/12/1990
• Status: Expired due to Fees

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Hammond, Thomas J.
• Primary Examiner(s): Grimley, A. T.
• Assistant Examiner(s): LEE, SUSAN SHUK YIN
• Application Number:
  US07/626,596
• Time in Patent Office: 398 Days
• Field of Search: 355/233, 355/228, 355/239, 355/208, 355/200, 355/71, 355/35, 355/75, 355/210, 355/214, 355/246
• US Class Current: 399/52
• CPC Class Codes: G03B 27/526 in which the projection optics move
• CPC Class Codes: G03G 15/0435 by introducing an optical element in the optical path, e.g. a filter
• CPC Class Codes: G03G 15/5037 the characteristics being an electrical parameter, e.g. voltage

Abstract

An exposure control system for a document copier is enabled by positioning a segmented variable reflectance mirror along the optical imaging path. The mirror segments incorporate a material whose transmission changes in response to applied voltage signals, the preferred material being an electrochromic material. In a first embodiment a white reference strip along the platen surface is exposed and the charge levels along discrete areas of the photoreceptor are measured by an electrometer with a plurality of probe segments. Outputs from each probe segment are processed in a control circuit and a signal sent to an appropriate mirror segment to increase or decrease reflectance to enable a corresponding decrease or increase, respectively, of the charge level, at the corresponding discrete areas of the photoreceptor illuminated by the mirror segments.

Claims

1. A document imaging system wherein a document on a platen is scan/illuminated by a scan mirror and illumination lamp assembly and a document image is projected along an optical path to expose a charged surface of a photoreceptor resulting in a first charge level representing the document information and a second charge level representing exposed background areas;

an exposure control system comprising in combination an electrometer positioned adjacent the surface of said photoreceptor substantially extending along its width, said electrometer comprising a plurality of segmented probes, each probe adapted to sense the background charge level of a discrete area of the photoreceptor surface lying therebeneath and to generate an output signal representative of the charge level of said discrete area,

a variable reflectance mirror fixedly positioned along said optical path, said mirror having a plurality of segments whose reflectance varies in accordance with a voltage applied thereto, and

a control circuit for comparing the output signals from said electrometer segments with a reference signal representative of an optimum or uniform discharge level, and for applying output signals at appropriate voltage levels to corresponding mirror segments whereby the reflectivity of the addressed mirror segment is selectively increased or decreased to cause a decrease or increase, respectively, in said discrete background areas.

2. The imaging system of claim 1, wherein said variable reflectivity segments include an electrochromic material.

3. The imaging system of claim 1, wherein said variable reflecting segments are composed of a liquid crystal.

4. An exposure control system for an electrophotographic reproduction machine including, in combination, a scanning optical system having a periodically enabled prescan mode, said optical system including a scan mirror, an illumination lamp assembly adapted to scan/illuminate a document on a document platen and project a light image onto a moving photoreceptor surface,

an electrometer positioned adjacent the surface of said photoreceptor substantially extending along its width, said electrometer comprising a plurality of segmented probes each probe adapted to sense the charge level of a discrete area of a photoreceptor surface lying therebeneath and to generate an output signal representative of said charge level,

a variable reflectance mirror fixedly positioned along said optical path, said mirror having a plurality of segments containing a material whose reflectivity varies in accordance with voltage applied thereto, and a control circuit for comparing output signals from said electrometer probes with a reference signal representative of an optimum illumination level at the photoreceptor surface, and applying appropriate voltage levels to the mirror segments corresponding to the electrometer probe outputs whereby the reflectivity of the mirror segments is selectively increased or decreased to reduce or increase illumination, respectively, at the corresponding discrete areas of the photoreceptor.

5. An exposure control system for an electrophotographic reproduction machine including in combination, a scanning optical system having a periodically enabled prescan mode, said optical system including a scan mirror, an illumination lamp assembly adapted to scan/illuminate a document on a document platen and project a light image onto a moving photoreceptor surface,

a linear segmented photosensor array positioned adjacent the surface of said photoreceptor substantially extending along its width, said array comprising a plurality of photosensors, each sensor adapted to sense the charge level of a discrete area of a photoreceptor surface lying therebeneath and to generate an output signal representative of said charge level,

a variable reflectance mirror fixedly positioned along said optical path, said mirror having a plurality of segments containing a material whose reflectivity varies in accordance with voltage applied thereto, and a control circuit for comparing output signals from said linear photosensor array with a reference signal representative of an optimum illumination level at the photoreceptor surface, and applying appropriate voltage levels to the mirror segments corresponding to the photosensor array output whereby reflectance of the mirror segments is selectively increased or decreased to reduce or increase illumination, respectively, at the corresponding discrete areas of the photoreceptor.