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測試區

水印是一個可肉眼觀察的具有明暗紋理的圖像或圖案，它需要通過透射光觀察（或者在黑暗的背景上通過反射光觀察）。水印的產生是由於紙張厚度或密度的變化。^[1]主要有兩種方式製作紙張中的水印：水印輥工藝，以及更複雜的筒模工藝。

水印的形式多種多樣，有些可直接觀察，還有一些則需要通過檢測。許多輔助產品也被開發出來，例如驗水印液，它可以潤濕紙張檢測水印而不破壞紙張本身。水印通常作為鈔票、護照、郵票和其他需要防偽的文件的一種防偽手段（參見防偽用紙）。

水印在對紙張的檢查時非常有用，因為它可以被用來用來標註日期、規格、商標和地點，以及紙張的質量。

編入數字音頻、視頻、圖像或者其他文件中一段識別碼，被稱為數字水印。

水印輥工藝

水印是在覆水的金屬印或者水印輥在製作中的紙張上施壓而產生的印記。使用水印的歷史可追溯到約1282(?)年的義大利博洛尼亞，^[2]但是水印輥直到1826年才由約翰·馬歇爾發明。造紙者在他們的產品以及郵票、貨幣、政府文件上使用水印作以識別。在法國，這一工藝在第二次世界大戰期間由維希政權引入，諸如阿道弗·康明斯基等人使用它來偽造證件。水印輥的發明革新了水印的製作工藝，使得公司為紙張製作水印更加簡易。

水印輥（又稱整飾輥、飾面輥）是一個輕型的包覆銅網的空心輥筒，通常是裝設在最初的兩三個真空吸水箱之後的成型網面上。它可以在改善紙張勻度的同時，整飾紙面，並賦予紙所需的水印。紙張上隱約可見的線條由平行於水印輥軸線方向的直線紋產生，粗線由周圍的鏈條鋼絲產生。因為鏈條鋼絲位於直線紋的外側，它們對紙漿會產生更大的影響，因此它們比直線紋更粗。

壓紋轉移到紙張纖維上，壓縮並減少紙張在這一區域的厚度。由於圖案部分的紙張較薄，它可以透過更多的光從而變得更加「透亮」。如果這些線條相互平行並且可以相互分辨，同時/或者存在水印，那麼這種紙稱為直紋紙。如果線條不可分辨或者相互糾纏，同時/或者不存在水印，那麼這種紙稱為網紋紙。這一方法叫線划水印法。

Cylinder mould process

Another type of watermark is called the cylinder mould watermark. A shaded watermark, first used in 1848, incorporates tonal depth and creates a greyscale image. Instead of using a wire covering for the dandy roll, the shaded watermark is created by areas of relief on the roll's own surface. Once dry, the paper may then be rolled again to produce a watermark of even thickness but with varying density. The resulting watermark is generally much clearer and more detailed than those made by the Dandy Roll process, and as such Cylinder Mould Watermark Paper is the preferred type of watermarked paper for banknotes, passports, motor vehicle titles, and other documents where it is an important anti-counterfeiting measure.

Watermarks on postage stamps and stationery

Elephant head watermark used on early stamps of India.

A US postal stationery envelope from 1883 showing a clear watermark on laid paper.

In philately, the watermark is a key feature of a stamp, and often constitutes the difference between a common and a rare stamp. Collectors who encounter two otherwise identical stamps with different watermarks consider each stamp to be a separate identifiable issue.^[3] The "classic" stamp watermark is a small crown or other national symbol, appearing either once on each stamp or a continuous pattern. Watermarks were nearly universal on stamps in the 19th and early 20th centuries, but generally fell out of use and are not commonly used on modern U.S. issues, but some countries continue to use them.^[4]

Some types of embossing, such as that used to make the "cross on oval" design on early stamps of Switzerland, resemble a watermark in that the paper is thinner, but can be distinguished by having sharper edges than is usual for a normal watermark. Stamp paper watermarks also show various designs, letters, numbers and pictorial elements.

The process of bringing out the stamp watermark is fairly simple. Sometimes a watermark in stamp paper can be seen just by looking at the unprinted back side of a stamp. More often, the collector must use a few basic items to get a good look at the watermark. For example, watermark fluid may be applied to the back of a stamp to temporarily reveal the watermark.^[4]

Even using the simple watermarking method described, it can be difficult to distinguish some watermarks. Watermarks on stamps printed in yellow and orange can be particularly difficult to see. A few mechanical devices are also are used by collectors to detect watermarks on stamps such as the Morley-Bright watermark detector and the more expensive Safe Signoscope. Such devices can be very useful for they can be used without the application of watermark fluid and also allow the collector to look at the watermark for a longer period of time to more easily detect the watermark.