Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1998-11-05
2001-04-03
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S256000, C347S134000
Reexamination Certificate
active
06211897
ABSTRACT:
FIELD OF THE INVENTION
A printing system and method utilizes a blur filter which includes one or more pieces of double refracting crystal. This arrangement improves the smoothness of printing spots in a laser thermal printer and thereby prevents artifacts caused by hot spots within the printing spots.
BACKGROUND OF THE INVENTION
In laser thermal writing, the optical power of lasers is used to affect media as, for example, by a transfer of dye/ink from donor media to a receiver media. In order to enhance printing speed, high power density is needed at the media. One way to achieve high power is to use a number of independently modulated diode lasers. One such system is described in U.S. Pat. No. 5,109,460 to Baek assigned to Eastman Kodak Company, where each of the lasers is coupled to an optical fiber. The fibers are aligned on a v-grooved silicon substrate to form a fiber head which is then imaged onto the media using a printing lens. Another system is disclosed U.S. Pat. No. 5,453,777 assigned to Presstek, in which a number of pigtailed fibers are used where the fiber ends are arranged with spacings of 1 inch on the printing head, and each of the fibers is imaged by a dedicated printing lens onto the media.
High power laser light spots achieved by the above systems can affect the media either by the process of dye transfer, dye ablation or other light/media interaction mechanisms.
In some applications it is important to accurately control the printing process. For example, in laser thermal proofing systems, which use the process of laser thermal dye transfer, the density of the final print has to be very tightly controlled.
When the printing spots are not smooth in terms of their spatial distribution, occasional localized ablation can occur. The exact onset of the ablation process is not as well controlled as is the dye transfer process.
Variations in print density can then occur since more dye is typically transferred in the ablation process as compared with the dye transfer mode.
Also, when an array of spots produced by the printing head at the media is such that there are significant gaps between the spots, it may be beneficial to spread the light from each of the spots even when each one of them is an ideal uniform spot so as to fill up the gaps and prevent uneven printing.
In the case of fiber based systems, it is desirable to achieve a uniform illumination at the end of the fiber. However, in many cases the laser light has a measure of coherence which can produce laser speckle at the fiber ends. The spots pattern is highly dependent on the propagation through the fiber and can shift and change its shape as the fiber is moved such as when the printing head is translated in one direction during the process of printing.
One way to control and minimize the hot spots at the end of the fibers is to use a highly incoherent laser illumination by using as many fiber modes as possible. However by distributing the laser light into many fiber modes, the numerical aperture at the end of the fiber may increase and with it the depth of focus at the media will decrease. A short depth of focus is hard to maintain, and it commonly requires the use of special auto focus mechanisms.
SUMMARY OF THE INVENTION
An object of this invention is to overcome the drawbacks noted above by providing for a low cost means for smoothing the printing or laser spots in a laser thermal printer, or to fill up gaps in an array of printing spots without increasing the numerical aperture of the printhead, by using a blur filter made of one or more pieces of double refractive crystal material.
The effect of a double refractive crystal material is to convert about half of the light in a printing spot into a slightly displaced copy of the original spot. The doubling of the printing spot has the effect of smoothing the intensity variations at the end of the fiber or filling up the gaps between printing spots to allow for better control over the printing process.
It is noted that the blurring of printing spots as provided by a filter based on double refractors is very different than blurring a printing spot by defocusing the image. A good blur filter functions as a low pass filter where it is effective in high spatial frequencies but does not affect the mid frequencies where most of the image information is. Defocusing in general will affect the high and the mid frequencies while a double refractor based on splitting can be designed to affect only the high frequencies. While we use the term “blur filter”, others have used other terms such as “anti-aliasing filter” or “low pass filter”.
The present invention provides for a thermal printing system which comprises a light source that provides for a first array of laser spots in the form of a line on media located at a media plane; and a double refracting device disposed between the light source and the media plane. The double refracting device splits an image of each of the first array of laser spots so as to create a second array of laser spots.
The present invention further provides for a method of controlling a uniformity of spots of a laser thermal printer. The method comprises the steps of forming a first array of laser spots; imaging the first array of spots onto media at a media plane; and splitting the images of the first array of laser spots so as to create a second array of laser spots.
The above, and other objects, advantages and novel features of the present invention will become more apparent from the accompanying detailed description thereof when considered in conjunction with the following drawings.
REFERENCES:
patent: 4978970 (1990-12-01), Okazaki
patent: 5053861 (1991-10-01), Tsai et al.
patent: 5109460 (1992-04-01), Baek et al.
patent: 5453777 (1995-09-01), Pensavecchia et al.
patent: 5572341 (1996-11-01), Fergason
Kessler David
Tredwell Timothy J.
Blish Nelson Adrian
Eastman Kodak Company
Le N.
Nguyen Lamson D.
Novais David A.
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