| 摘要 |
In photocopies of confidential computer lists, it is to be recognisable that they have been copied from an original which was not allowed to be copied. For this purpose, there is provided on the original a screen print which consists of two screen patterns of the same scope of tone value which cannot be distinguished from one another by the human eye. One of these screen patterns forms a basic pattern while a coarser screen pattern forms a warning text inserted in the basic pattern, e.g. PIRATE COPY. During photocopying, the screen dots of the basic screen are not recorded so that this basic pattern is copied so as to correspond to a uniformly inked surface. In contrast, the screen dots of the coarser screen pattern are copied, thus resulting in the warning text becoming visible on the copy.
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