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High Resolution Optical Scanner Incorporating a Variable Radius Curved Mirror

IP.com Disclosure Number: IPCOM000107834D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 113K

Publishing Venue

IBM

Related People

Budd, RA: AUTHOR [+2]

Abstract

A high resolution laser printhead with a multiple-element scan lens set can be costly to produce. Disclosed is a new, simple printhead design which uses a curved mirror of variable radius in lieu of the conventional scan lens set. A high level of performance is achieved at a lower cost. New Printhead Design

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High Resolution Optical Scanner Incorporating a Variable Radius Curved Mirror

       A high resolution laser printhead with a multiple-element
scan lens set can be costly to produce.  Disclosed is a new, simple
printhead design which uses a curved mirror of variable radius in
lieu of the conventional scan lens  set. A high level of performance
is achieved at a lower cost. New Printhead Design

      The various components of the new design are aligned as shown
in the figure. The most salient departures from prevalent low-end
printhead designs are:
      1.   the variable radius curved mirror,
      2.   the orientation of the pre-scan laser beam, and
      3.   the decoupling of the scan and process direction optics.

      As the figure shows, the incoming beam from the laser diode
assembly hits the rotating polygon on axis in the scan direction.
This orientation effectively allows the polygon to be overfilled in
the scan direction (i.e., the width of the beam at the polygon in the
scan direction is larger than the polygon facet width) as the beam
scan angle is small enough for the power variation across the scan
line to be tolerable.  This also keeps the polygon size, and thus
the cost, within practical limits for very small scan direction spot
sizes.  In order to overfill the polygon, the laser beam is no longer
collimated in the scan direction but instead weakly converging.  The
laser beam is, however, still focused onto the polygon facet in the
process direction.

      One disadvantage of this layout is the laser beam tilt in the
process direction which introduces a small amount of scan line bow.
However, as modelling results show, this bow can easily be kept
within acceptable limits.

      The decoupling of the post-scan optics results from using the
curved mirror to focus only in the scan direction while a plastic
variable radius lens is used to focus the laser beam only in the
process direction.  This curved mirror is largely cylindrical with a
base radius to be determined by the layout of the printhead.  A
departure from a pure cylindrical mirror is necessary to compensate
for the optical path length change introduced by the rotating polygon
as it sweeps the beam across the page.  This change in focal power
along the scan line effectively keeps both the scan defocus spread
and the linearity error to a minimum.  It should be also noted that
the preponderance of reflective optical elements in this design
reduces the spot placement errors which are caused by wavelength
shifts.

      A curved mirror of the type described can be manufactured by
the 'Sag Glass' proce...