Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-12-20
2004-06-15
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S292000, C355S035000, C358S001900
Reexamination Certificate
active
06751005
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to digital printer apparatus that form a two-dimensional image onto a photosensitive medium and more particularly relates to apparatus and methods of compensating for localized pixel anomalies in a spatial light modulator.
BACKGROUND OF THE INVENTION
Originally developed for forming images in display devices, spatial light modulators are increasingly being used in digital printing applications as well. In printing apparatus, spatial light modulators provide significant advantages in cost and performance over earlier digital imaging technologies, both for line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, and for area printing systems such as the system described in U.S. Pat. No. 5,652,661.
Two-dimensional spatial light modulators, such as those using a digital micromirror device (DMD) from Texas Instruments, Dallas, Tex., or using a liquid crystal device (LCD) can be used to modulate an incoming optical beam for imaging. A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light by transmitting (or by blocking transmission of) incident light from a light source, typically by affecting the polarization state of the light.
There are two basic types of spatial light modulators in current use. The first type developed was the transmissive spatial light modulator, which, as its name implies, operates by selective transmission of an optical beam through individual array elements. The second type, a later development, is a reflective spatial light modulator. As its name implies, the reflective spatial light modulator, operates by selective reflection of an optical beam through individual array elements. A suitable example of an LCD reflective spatial light modulator relevant to this application utilizes an integrated CMOS backplane, allowing a small footprint and improved uniformity characteristics.
Examples of printing apparatus using digital micromirror devices (DMDs), include that disclosed in U.S. Pat. No. 5,461,411. Photographic printers using the more readily available LCD technology are described in U.S. Pat. Nos. 5,652,661; 5,701,185; and 5,745,156, for example.
Conventionally, LCD spatial light modulators have been developed and employed for digital projection systems for image display, such as is disclosed in U.S. Pat. No. 5,325,137 and in miniaturized image display apparatus suitable for mounting within a helmet or supported by eyeglasses, as is disclosed in U.S. Pat. No. 5,808,800. LCD projector and display designs in use typically employ one or more spatial light modulators, such as using one for each of the primary colors, as is disclosed in U.S. Pat. No. 5,743,610.
It is instructive to note that imaging requirements for projector and display use (as is typified in U.S. Pat. Nos. 5,325,137; 5,808,800; and 5,743,610) differ significantly from imaging requirements for printing by photoprocessing apparatus. Projectors are optimized to provide maximum luminous flux to a screen, with secondary emphasis placed on characteristics important in printing, such as contrast and resolution. Optical systems for projector and display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity, since the displayed image is continually refreshed and is viewed from a distance. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as “forgiving” to artifacts and aberrations and to non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. For this reason, there can be considerable complexity in optical systems for providing a uniform exposure energy for printing. Even more significant are differences in resolution requirements. Adapted for the human eye, projection and display systems are optimized for viewing at typical resolutions such as 72 dpi or less, for example. Photographic printing apparatus, on the other hand, must achieve much higher resolution, particularly apparatus designed for micrographics applications, which can be expected to provide 8,000 dpi for some systems. Thus, while LCD spatial light modulators can be used in a range of imaging applications from projection and display to high-resolution printing, the requirements on supporting optics can vary significantly.
Another key difference between display and print applications relates to uniform response of pixel elements. Fabrication methods for spatial light modulators are imperfect; as a result, some pixel sites do not perform properly. Thus, for example, an individual pixel may be “stuck” on or off regardless of the control logic signal it receives. Or, an individual pixel may remain in an unwanted half-way state, neither fully on or fully off regardless of the control logic signal applied to it. In display applications, particularly for moving images, individual pixel anomalies may not be readily visible to a viewing audience. However, in print applications, the same type of pixel condition can be readily apparent and can degrade the value of a print, causing “salt and pepper” effects, for example.
In addition to pixel defects described above, dust, dirt, cleaning residues, and other surface irregularities can contribute to non-uniform pixel-to-pixel response of a spatial light modulator, such as waviness and “water-stain” effects. These other effects can be subtler than those resulting from stuck pixels, however, results can be dissatisfying and the perceived value of print results is lowered when such imaging aberrations recur from print to print.
Dithering methods have been developed for improving the relatively low fill factor and increasing the apparent resolution of spatial light modulators in printing applications. Commonly assigned U.S. patent application Ser. Nos. 09/630,419 and 10/055,781, cited above, disclose various dithering approaches for this purpose. Dithering techniques operate by imaging multiple times with the same spatial light modulator in a pattern, making an initial exposure, then shifting the relative position of the spatial light modulator by a predetermined distance for each subsequent exposure, where the distance shifted in each move is less than or equal to one pixel-to-pixel distance. In this way, dithering allows each pixel from the original exposure to fill in the area surrounding it. The image data provided to the spatial light modulator is preferably changed with each shift operation, to effectively provide increased resolution.
Dithering has also been proposed as a method for compensating for pixel site defects. However, experience has shown that dithering does not provide a satisfactory result with all types of pixel defects. Because pixels neighboring a defective pixel tend to overlap the space between pixels in the original exposure, dithering may help to mediate the effect of a pixel that may be stuck in a partially on state. That is, where pixel defects themselves cause only subtle tone shifts, dithering can be helpful. However, in cases where defective pixels have a fixed on or off state, dithering has been found to make imaging defects more noticeable, providing unsatisfactory results in many cases. Thus, in some cases, dithering can actually enhance the damaging effects of a defective pixel, resulting in an unacceptable print.
A spatial light modulator is relatively expensive to fabricate. As with similar types of devices, perfect performance comes at high cost. Low cost means accepting some number of bad pixel sites in the matrix of spatial light modulator pixels has been an economic compromise. It can be appreciated that it would be advantageous for a builder of imaging apparatus to be able to accept lower spatial light modulator quality and to compensate for modulator site defects a
Barnick William M.
Bernardi Bryan D.
Erwin James C.
Miller William G.
Blish Nelson Adrian
Choi William
Eastman Kodak Company
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