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method for accurate exposure of small dots on a heat sensitive positive working flachdruckplatte
method for accurate exposure of small dots on a heat sensitive positive working flachdruckplatte
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机译:于热敏正性工作平板上的小点精确曝光的方法
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摘要
A method is disclosed for accurate reproduction of high-quality halftone images comprising microdots by means of lithographic plate materials which comprise a heat-sensitive positive-working coating that requires wet processing. Such microdots have a dot size ≤25 µm and may be obtained by stochastic screening or by amplitude-modulated screening at a ruling of not less than 150 lpi. It has been established that the "physical right exposure energy density" (physical REED) lies in the range from CP to 1.5*CP, wherein the physical REED is defined as the energy density at which the physical area on the plate, occupied by a microdot corresponding to a 50% halftone in the image data, coincides with the 50% target value; and wherein CP is the clearing point of the plate which is defined as the minimum energy density that is required to obtain, after processing, a dissolution of 95% of the coating. An accurate reproduction of microdots can therefore be achieved by exposing the material with light having an energy density in the range from CP to 1.5*CP. Loss of microdots by overexposure is thereby avoided.
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