The watermark is the oldest and most well-known security element for paper documents. Watermarks are recognizable images, patterns, logos or lettering in paper viewed by transmitted light, which are produced by varying the thickness or density of the paper. Watermark elements appears as various shades or darkness and lightness. Genuine watermarks are absolutely unforgeable. The effect of a watermark is visible when the document is held up to the light (daylight or artificial light source).