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水印是一个可肉眼观察的具有明暗纹理的图像或图案，它需要通过透射光观察（或者在黑暗的背景上通过反射光观察）。水印的产生是由于纸张厚度或密度的变化。^[1]主要有两种方式制作纸张中的水印：水印辊工艺，以及更复杂的筒模工艺。

水印的形式多种多样，有些可直接观察，还有一些则需要通过检测。许多辅助产品也被开发出来，例如验水印液，它可以润湿纸张检测水印而不破坏纸张本身。水印通常作为钞票、护照、邮票和其他需要防伪的文件的一种防伪手段（参见防伪用纸）。

水印在对纸张的检查时非常有用，因为它可以被用来用来标注日期、规格、商标和地点，以及纸张的质量。

编入数字音频、视频、图像或者其他文件中一段识别码，被称为数字水印。

水印辊工艺

水印是在覆水的金属印或者水印辊在制作中的纸张上施压而产生的印记。使用水印的历史可追溯到约1282(?)年的意大利博洛尼亚，^[2]但是水印辊直到1826年才由约翰·马歇尔发明。造纸者在他们的产品以及邮票、货币、政府文件上使用水印作以识别。在法国，这一工艺在第二次世界大战期间由维希政权引入，诸如阿道弗·康明斯基等人使用它来伪造证件。水印辊的发明革新了水印的制作工艺，使得公司为纸张制作水印更加简易。

水印辊（又称整饰辊、饰面辊）是一个轻型的包覆铜网的空心辊筒，通常是装设在最初的两三个真空吸水箱之后的成型网面上。它可以在改善纸张匀度的同时，整饰纸面，并赋予纸所需的水印。纸张上隐约可见的线条由平行于水印辊轴线方向的直线纹产生，粗线由周围的链条钢丝产生。因为链条钢丝位于直线纹的外侧，它们对纸浆会产生更大的影响，因此它们比直线纹更粗。

压纹转移到纸张纤维上，压缩并减少纸张在这一区域的厚度。由于图案部分的纸张较薄，它可以透过更多的光从而变得更加“透亮”。如果这些线条相互平行并且可以相互分辨，同时/或者存在水印，那么这种纸称为直纹纸。如果线条不可分辨或者相互纠缠，同时/或者不存在水印，那么这种纸称为网纹纸。这一方法叫线划水印法。

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.