Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
1998-12-28
2001-06-26
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S234000, C348S087000
Reexamination Certificate
active
06252241
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of image scanning, particularly scanning of images by coherent radiation, particularly essentially monochromatic coherent radiation and more particularly optical scanning of images with a mechanical system. The invention relates to such optical/mechanical scanning systems which provide sufficiently high resolution as to enable the scanned image to be used in photolithographic manufacture of printed circuit boards (PCB) and printing plates.
2. Background of the Art
Optical scanning techniques have found important applications in many technical fields. These fields include, for example, laser printers for computers; laser direct writing lithography for production of masks, wafers and optical integrated circuits; high speed photography; IR imaging; image information transmission; thermal and mass transfer of images; graphic arts imaging for newspapers and other printed materials; direct imaging for printed circuit board fabrication; and the like. Many different scanning technologies, software, hardware and qualities are available commercially and in the literature. A summary of scanning methods is presented in the articles by Leo Beiser (
Laser Focus/Electro-Optics,
Feb. 1985) and Henry E. R. Lassiter (
Laser Focus World,
Jan. 1991).
A significant challenge in the generation of scanned data particularly occurs with respect to the provision of large format image recording systems. One aspect of the technical problems faced in producing high resolution image data within the large format field is the technically contradictory objectives to record a large image area, yet use the smallest possible pixel size in the generated data while still providing the data in the shortest time or at least a reasonable time. The issues tend to be contradictory as it takes more data and more time to provide image data when using small pixels. When working with a fixed or standard large area format image, the greater the resolution, longer exposure times are needed to generate the data with high resolution. For example, with a typical plate size in Printed Circuit Board applications of 20″ by 24″ (50.8 cm×61 cm), the total number of binary pixels is about 256×10
6
pixels for 5080 (2000 dots per centimeter) dot per inch resolution. If this image were to be recorded in 1 minute, it would require a sustained data generation rate of about 0.5 Gbit/sec. Such high rates and high resolution in scanned data systems pose a significant challenge to imaging systems that are presently under development or recently available, and these high rates and high resolutions have not yet been met satisfactorily with commercial equipment.
Existing optical scanning systems tend to be limited in data rate acquisition by mechanical speed limitations, light switching speed limitations or both. For example, X-Y stage-based scanning systems have relatively low imaging rates which are limited by the low translation speed of the stages. External or internal drum scanning systems, on the other hand, may achieve medium to high imaging speed rates, but are limited to exposing flexible materials because of the physical construction of the system. Images on non-flexible surfaces must be converted to a flexible surface if the images are to be used on a drum system. This conversion to an intermediate image for use on the drum system is itself likely to reduce the ultimate resolution of the final image by introduction of errors in the formation of the intermediate image. Even when software is used to compensate for variations in image data received on a curved or drum surface, there will still be some additional loss of resolution from the process. Serial scanning systems, i.e., systems in which a single channel is used throughout the data path and light modulating to record an image, are limited by their light switching speed, since they require a light modulation switching speed which is the same as the incoming data rate.
Furthermore, polygon based scanning systems in which the scanning trace is a straight line (or any other type of scanning system having a single channel, a serial scanning system) are limited in format size, since the entire area of the format must be covered by the scanning field of view. When an angular or deflected scanning beam is used to attempt to cover a wider area than would be provided by a non-varying straight line polygon scanning system, additional resolution issues are introduced into the data acquisition system. A common compromise in such systems is to make a trade off between the spot size for field of view and writing energy. U.S. Pat. No. 5,216,247 (Wang et al) is a representative example of such tradeoff. It offers relatively high speed scanning and a potential for relatively higher resolution.
Unfortunately, such an optical scanning system, by its very nature, needs to work with a very low numerical aperture, severely limiting the spot size used and the exposure energy capability of the system. For an image format of 22 inches by 28 inches (55.9 cm×71.1 cm), circles as large as 60 inches (152.4 cm) need to be scanned to avoid scan line distortion. For this, a minimum working distance of 30 inches (76.2 cm) would be required to keep the field of view below 90 degrees. This working distance makes the achievement of a numerical aperture (NA) above 0.2 very complex and expensive. This limitation on the NA also limits the spot size. Additionally, this prior art is still limited by light switching speed. It is a serial scanner by its nature and construction, and it must be switched by a single light switch that would need to possess the switching speed of the system or, in other words, the data rate. As previously mentioned, this is a significant issue in the development of a commercially viable system.
The same type of limitation exists in linear scanning systems, which are built around a flatbed substrate carrier. Although flatbed scanners may feature perfectly orthogonal scanning systems, such systems must also compromise between the numerical aperture (and, hence, spot size and exposure energy) and the speed desired for large format imaging. In one type of implementation, where the scan covers part of the image, the imaging speed is limited by the x-y stage direction change. This forces the use of a low NA, large spot and low exposure energy density.
Another limitation associated with existing rotational scanning systems, such as shown in
FIG. 3
, is the inherent distortion associated with the rotational scanning method. In such systems, an exposure head is rotated at the same time that the working plane is translated linearly to facilitate complete coverage of the image. This mode of scanning can result in incomplete overlap of consecutive swaths of the exposure head, as shown in
FIG. 4
, unless the linear movement is so incremental as to force the scans to overlap. This, however, directly increases the time needed to scan a given area since the number of scans along the line of linear movement must be increased. As shown in
FIG. 4
, the scan pattern transcribes, in a single pass, concentric outer radius R
1
and inner radius R
2
. A subsequent scan transcribes the same pattern, but is linearly offset by the translation of the stage. It is apparent that the two consecutive scans may not perfectly overlap, as they are not concentric. It is impossible to compensate for such lack of overlap unless the incremental distance of movement between each scan is significantly less than the width of the scanning spot, resulting in image distortion.
A review of rotational scanning systems recording image on planar surface is given in U.S. Pat. No. 5,216,247 by Wang et al.
Various scanner systems known in the literature include the following, such as U.S. Pat. No. 3,588,218 which describes a multiple spot scanner with an optical relay system in which at least one optical beam is periodically refocused to define a plurality of scanning light spots along a common scan locus. This sy
Creo Ltd.
Lee John R.
Pyo Kevin
LandOfFree
Rotational scanning image recording system having both a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rotational scanning image recording system having both a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rotational scanning image recording system having both a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2476377