Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-04-30
2001-07-24
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S256000
Reexamination Certificate
active
06266080
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to laser recording and in particular to recording of thermal materials, also known as thermographic recording and heat-mode recording.
BACKGROUND OF THE INVENTION
In recording on photoinic materials, such as silver halide films, printing plates, photoresists, etc. it is known that the rate of exposure, or dwell time of the recording spot (typically a laser spot), is of little importance as long as the total exposure is correct. This is the well-known “Law of Reciprocity”. The exposure is defined as the product of the power of the light multiplied by the time. The power is normally measured in Watts and exposure is Joules or Watt-Seconds. When recording on materials known as thermal, or heat-mode materials, the rate at which the exposure is delivered is crucial, since a low exposure rate (low power for a long time) will not cause the desired increase in temperature, as most the heat will dissipate. On the other hand shortening the exposure time and using a very high power can cause the exposed material to break down or ablate, creating debris and not functioning properly (unless the material is designed to operate by ablation). This problem does not exist in photonic materials as they usually require significantly lower power. If any exposure system has to have a specified scanning rate (to achieve a desired throughput or productivity) and the material has a specified sensitivity (usually specified in Joules/cm
2
), these two parameters uniquely set the exposure power, since if during one second X cm
2
need to be exposed, the power has to be “X” times the material sensitivity. For most materials used in thermal imaging the sensitivity is in the range of 0.1-1 joule/cm
2
and writing rates are 10-100 cm
2
/sec. This determines the writing power to be in the range of 1-100 W. If this power level is delivered in a single laser beam for writing high resolution features (1-20 microns) the power density (defined as Watts/cm
2
) is very high and causes ablation. Prior art solutions involve splitting the laser beam into many parallel beams (or using many parallel lasers) in order to reduce the power density per spot. Another solution, shown in
FIG. 1
, is to use spots which are larger than the required addressability, shown as “a” in FIG.
1
. Digital images are made up of pixels and normally addressability is a single pixel. The disadvantage of the latter method is loss of resolution as the spot is larger than a single pixel. If the first method is used it is difficult to change the power density once the power was set (to achieve a desired imaging speed). Another method is to pulse the lasers in order to increase power density, however it lowers the reliability of the lasers. For devices required to image a wide range of thermal materials it is desired to be able to vary the power density without affecting resolution, power or writing speed. It is also sometimes desired to achieve high power densities without change in resolution, power or laser duty cycle, in order to use ablative recording materials. The ideal exposure method will allow the power density to be changed from very high (for ablative materials, typically requiring 1 MV/cm
2
) to low (for chemical reactions, typically requiring under 200 KW/cm
2
)
SUMMARY OF THE INVENTION
The invention allows to vary the power density without affecting other imaging parameters by using an optical spot smaller than the addressability of the imaging process and scanning this spot to generate each individual pixel. One pixel is defined as the smallest element of the image, equal to one unit of addressability. If the optical spot is rectangular, with the long dimension equal to the addressability, the power density can be changed by changing the narrow dimension of the rectangle. As long as the narrow dimension of the rectangle is smaller than the addressability the resolution is practically unaffected. A second benefit of the invention is that the exposure function created by such a square spot has a steep and abrupt transition, which helps maintain the size of the written pixel even if laser power or material sensitivity are changing.
REFERENCES:
patent: 4307408 (1981-12-01), Kiyohara et al.
patent: 4651170 (1987-03-01), Chandler et al.
patent: 5111302 (1992-05-01), Chan et al.
patent: 5479263 (1995-12-01), Jacobs et al.
patent: 5521748 (1996-05-01), Saraf
patent: 5570224 (1996-10-01), Endo et al.
patent: WO95/18984 (1995-07-01), None
Creo SRL
Le N.
Oyen Wiggs Green & Mutala
Pham Hai C.
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