Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1998-04-20
2001-02-13
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S238000, C347S239000, C348S770000, C348S771000
Reexamination Certificate
active
06188427
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an image display system such as used in a xerographic printer, and more particularity, to an illumination system having a calibration system for providing a uniform distribution of high-intensity light to an image plane.
BACKGROUND OF THE INVENTION
Semiconductor spatial light modulators (SLM's) are one viable solution to realizing high quality, affordable xerographic printers. One promising SLM technology suitable for both printers and displays is the deformable mirror device or digital micromirror device (collectively DMD) manufactured by Texas Instruments Incorporated of Dallas, Tex. The DMD is a monolithic semiconductor device having a linear or area array of bi-stable movable micromirrors fabricated over an array of corresponding addressing memory cells. One embodiment of a xerographic printer implementing a tungsten light source focused via optics on an imaging DMD mirror array is disclosed in U.S. Pat. No. 5,041,851 to Nelson, entitled “Spatial Light Modulator Printer and Method of Operation”, assigned to the same assignee as the present application and the teachings included herein by reference.
In a xerographic printer implementing an imaging DMD spatial light modulator, it is desired to uniformly illuminate the elongated DMD mirror array (typically about 5 inches in length) with a homogeneous light source such that each pixel mirror of the array modulates a uniform intensity portion of light. This is necessary because the DMD mirror array modulates this light to expose a light sensitive rotating printing drum, whereby the intensity and duration of the modulated light directed thereon determines the relative exposure of the charged drum. The exposed portion of the drum comprises a latent image, wherein a quantity of toner will adhere to the drum image, this toner then being transferred to a printing medium such as paper, and fused thereon using heat.
It is also necessary that the energy of the light directed upon the DMD mirror array be of sufficient flux per unit area to fully expose the rotating printing drum to obtain a dark image. If insufficient light energy is modulated and directed to the drum by the DMD mirror array, the printing drum may not be fully exposed, thus degrading the contrast of the image printed on a printing medium.
U.S. Pat. No. 5,159,485 to Nelson, entitled “System and Method for Uniformity of Illumination for Tungsten Light”, assigned to the same assignee of the present invention and the teachings included herein by reference, discloses an anamorphic optical path arranged such that the vertical component of the source light beam is compressed to match the physical shape of the DMD mirror array. The embodiment disclosed dramatically increases the optical efficiency of the system, whereby light energy is compressed to irradiate the DMD mirror array more intensely from a given light source, such as a tungsten lamp.
U.S. Pat. No. 5,105,207 to Nelson, entitled “System and Method for Achieving Gray Scale DMD Operation”, assigned to the same assignee as the present invention and the teachings incorporated herein by reference, discloses a system for enhancing resolution of a xerographic process by submodulation of each individual pixel. The submodulation is achieved by anamorphically reducing the square pixel presentation of light rays to a rectangle having a number of controllable segments within each square pixel scanned line. A conventional tungsten lamp is incorporated in this embodiment.
U.S. Pat. No. 5,151,718 to Nelson, entitled “System and Method for Solid State Illumination for DMD Devices”, also assigned to the same assignee of the present invention and the teachings included herein by reference, discloses an array of LED emitters constructed to efficiently replace the conventional tungsten source lamp. The LED array is geometrically configured, and can be electrically operated by strobing to vary the brightness of light to individual mirror pixels to achieve gray scale imaging, and reduce fuzzy line images. Each of the LED's in the array can be provided with a lens to help collimate the light through optics and onto the DMD mirror array. Using LED's, light is efficiently directed and focused onto the DMD mirror array, with little light being wasted and directed elsewhere. Less optical energy is required of the light source compared to a conventional tungsten lamp to illuminate the DMD mirror array with a particular light intensity. The LED's can be quickly turned on and off, thereby providing the ability to modulate the light energy directed upon the DMD mirror array, and consequently, helps achieve gray scale printing. For instance, during a given line print cycle, the LED can be on for 50% of the cycle time to irradiate the DMD array with half the light energy available for that particular time interval. The alignment of the optics ensures that the energy of each LED is directed upon the DMD mirror array. There is a concern that, the LED array may not produce sufficient and uniform light energy in the cross-process direction should one LED fail or have a reduced output.
As disclosed in commonly assigned patent application Ser. No. 08/735,616, entitled “Illumination System for Hard Copy Apparatus”, there is disclosed an illumination system having an elongated array of light emitting elements in combination with a spreading element which laterally mixes the light from the individual light elements for illuminating an elongated spatial light modulator. A holographic diffuser is implemented to laterally diffuse and mix the light in the cross-process direction, in combination with a cylindrical lens to vertically compress the light. A 10% reduction in light output from one of the light elements will cause less than a 1% localized reduction in light intensity at the spatial light modulator.
It is known that over time the intensity of the light output of the light emitting elements will vary and degrade over time. There is desired an illumination system having a calibration system which has the capability to automatically measure the intensity of illumination across a spatial light modulator to determine the uniformity of illumination across the image plane. Such a system should allow adjustment of the individual light emitting elements light output of an array to insure uniform illumination at the image plane over time.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as an illumination system having an automatic illumination detection system for determining the uniformity of illumination across an image plane, and calibration of individual light source elements to maintain uniformity. This is done by sensing the intensity of light reflected by or transmitted through a spatial light modulator at the object plane as the spatial light modulator is turned on, one zone or block at a time, across the spatial light modulator. Each one of the actuated zones will produce its own light signature which is compared to a set of calibration signatures stored in a look-up table. The look-up table can be originated/derived by computer models or empirical measurements from a golden standard light source implemented in an actual optical system which produces uniform illumination at the image plane. Appropriate specifications on uniformity versus detector readings relative to the standard stored in the look-up table are used for full characterization of the illumination sources. Using the detector readings associated with each activated spatial light modulator zone, variations in light emitting element outputs relative to what they should be are readjusted to previously specified levels. This insures proper illumination across the spatial light modulator by the array of light emitting elements, matching the standard, to ensure uniform illumination at the image plane.
In the preferred embodiment of the present invention, a reflective-type spatial light modulator such as a DMD is utilized, although a transmissive-type spatial light modulator could be used suc
Allen John B.
Anderson Charles H.
Brady III Wade James
Le N.
Pham Hai C.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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