Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1998-10-07
2001-03-20
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S239000
Reexamination Certificate
active
06204875
ABSTRACT:
FIELD OF INVENTION
This invention is related to optical output scanning and in particular to controlling exposure using extended light sources to achieve high resolution output at a high exposure level.
BACKGROUND TO THE INVENTION
Constructing a scanner with an extended light source such as an arc lamp or a high power laser diode or a laser diode array poses a difficult task for a designer. Because of the extended size of the source, it is impossible to collect all the light transmitted by the source into one or more small spots, for example of diameter on the order of 10 micrometers at a sufficiently high exposure level. Laser image recorders typically use lasers with a very well defined optical beam, for example, a low power laser diode, or a gas laser. Such laser diodes provide a high enough beam quality to be focused with high efficiency to a relatively small spot, and thus have been widely used in office laser printers and in the graphic arts industry.
For low sensitivity media printers such as a laser thermal printer, a higher power laser needs to be used to achieve a high throughput. CO
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and Nd-Yag lasers do provide high, clean power with nearly Gaussian beams, and can be focused onto small spots with high efficiency, but these are extremely expensive. In addition, a Nd-Yag laser has a wavelength of 1064 nm, while for thermal imaging a lower wavelength, typically 830 nm, is preferred. The high power is of particular importance when writing on thermal materials such as thermal offset plates. In this type of material, heat is used to cross-link polymer chains. The cross-linked polymer is later used as the ink attracting layer (the printing layer) in an offset press. The necessary energy may not be applied in a very short time period (less than 100 ns) since this would ablate the polymer layer. This is in contradiction with modern fast imaging systems that expose more than 50 million pixels per second, which corresponds to only 20 nanoseconds per pixel.
Another type of material to which the present invention is applicable is for exposing liquid materials. For example, in stereo lithography, a laser scans the surface of a tank filled with a liquid. Upon receiving the laser light, this liquid polymerizes to solid form. The solid layer is then lowered into the liquid until the solid is covered by the liquid. A new laser scan will harden the next layer of material, and so on. The object material is thus built up layer by layer. In the future, it is envisaged that a liquid might be applied to a plate prior to exposure. During exposure, this liquid would cure on the imaged parts. The cured and non-cured parts would then be used for printing. This process is similar to a thermal transfer process wherein a layer from a donor sheet is transferred to a plate.
It is the object of the invention to present an imaging apparatus that delivers the power of an extended line source with high efficiency to a row of small pixels in a fixed array on the recording material. An example of a suitable light source is a high power laser diode, for example one with power of more than about 1 W. Another example is a laser diode array. The imaging system is set up such that the energy delivered to each pixel, and the shape of each pixel in a fixed array on the recording material is essentially the same.
A second object of the invention is to provide a method of applying high-energy exposure doses in fast image scanning systems. The resulting imaging system can provide the energy in an approximately 5 &mgr;s time interval despite the approximately 50 million pixels per second imaging rate.
A third object of the invention is to provide a means for altering the width of an imaged scan line. This enables changing the writing resolution of a scanning system without requiring sophisticated zoom optics. Such a variable resolution system enables higher imaging efficiency because the beam shape provided by the proposed method provides for a more rectangular profile. A rectangular profile is desirable when working with a binary material. With a Gaussian beam, only the center portion (that reaches the material threshold level) of the beam contributes to the image formation process. The portion of energy outside this center is lost.
RELATED METHODS AND SYSTEMS
Since it is physically impossible to focus all the light from a large light source onto one small pixel, one solution is to use a larger source and to consider the source as being made up of many smaller sources. The dimensions of these sources are chosen such that they can be imaged with high optical efficiency onto a small recording spot. For imaging, the recording device needs some means for modulating the intensity of the recording spot. Since the light source is spatially split up in many smaller sources (“sourcelets”), and each of these corresponds to a pixel, or a sub-pixel on the recording surface, a modulator means for each of these sourcelets would typically be necessary.
Several related devices have been described in the prior art. U.S. Pat. No. 4,577,932 to Gelbart (hereinafter: the “first Gelbart” system or method), describes using a multi-beam modulator in the form of an acousto-optic modulator in the scophony mode. The light of a laser diode is imaged on the acousto-optic modulator to form a line shape in the acoustical column. The acoustic wave in the acousto-optic modulator can be imaged onto the recording surface by freezing its motion. This is accomplished by operating the laser device in a pulsed mode at a rate synchronized with the acoustic wave speed. Thus, the laser diode is triggered to deliver a stroboscopic light flash. Because of the short duration of this light flash the modulated acoustic traveling wave can be frozen on the image plane. This first Gelbart method images the acousto-optic modulator such that a number of parallel tracks are written in the focal plane. This method has two main drawbacks. First, the optics that image the acoustic wave need to be of very high quality. The row of cells in the acoustic wave can be considered to be a linear array of pixels, and these have to be imaged onto the recording surface without distortion. If not, the size of the pixels in the focal plane will not be the same resulting in a “banding phenomena” well known in the graphic arts. A second drawback is that each of the pixels corresponds to a different spatial part of the source. Since the illumination of the source is not evenly distributed over its aperture, the intensity of each pixel in the corresponding trace on the recording surface may be different. This has dramatic effects on the image quality. Ideally, each of the spots in this kind of multi-beam scanner needs to have pixels of substantially the same intensity and of substantially the same shape. Also the spot location ideally needs to be within very narrow limits on the recording grid.
Another system described in U.S. Pat. No. 5,049,901, also to Gelbart (hereinafter the “second Gelbart” system or method), overcomes the intensity distribution problem by using a two-dimensional modulator device. The apparatus delivers a number of parallel traces equal to the number of columns in the two-dimensional modulator. During the recording surface scan, the image data is shifted through the columns of the device in synchronism with the recorder surface speed such that the modulating device maintains a stationary position relative to the fast recording movement. The intensity of each trace is calibrated by enabling a selected number of modulating channels in the modulator row, such that the summed intensity of all enabled channels is equal. Although this method overcomes the problem of calibrating channel intensities, it does so at the cost of efficiency. The system has to be set up such that the intensity of all channels equals the intensity of the least efficient one. Also, the system still requires a high quality imaging lens. That is, the location of the parallel tracks still needs to be substantially perfect.
Another system described in U.S. Pat. No. 5,517,359, also to Gelbart (he
De Loor Ronny A.
Rosenfeld Dov
Barco Graphics NV
Feggins K.
Inventek
Le N.
Rosenfeld Dov
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