Numerous variations exist, including printing the marks using a raster of lines in one direction on a background of lines in another direction, or using fine line patterns that alias into a visible moire pattern when copied. This makes it possible to see a pattern that had been invisible. This means that if a grey region consists of a grid of very small dark dots the filtering will produce a lighter grey, while a region of larger dots will be affected differently ("big-dot-little-dot"). In addition, human vision is sensitive to luminance contrast ratio. Please refer to your PMS book for accurate color representations. Features smaller than the resolution will also not be reproduced. Note: Screen colors and densities may vary depending on monitor and/or printer settings.
It will also not be perfectly aligned with the directions of the document, causing aliasing. A scanner or photocopier will act as a low-pass filter on the original image, blurring edges slightly. Void pantographs work by exploiting the limitations and features of copying equipment. Typically they spell out "void", "copy", "invalid" or some other indicator message. VOID PANTOGRAPH - The word VOID images when copied or scanned and is positioned per Check21. Normally these are invisible to the eye, but become obvious when the document is photocopied. Multi Tone Watermark image combines dark, light and shaded tones. In security printing, void pantograph refers to a method of making copy-evident and tamper-resistant patterns in the background of a document.
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