Imaging watermarks

Infrared imaging reveals watermarks

Watermarks in paper can usually be seen by holding the paper in front of a light. In many cases however, written text, printing or drawings can obscure the watermark and complicate its identification and interpretation.

Currently, X-ray imaging is the preferred method to view a watermark without the interference of text or drawing on the paper surface. Nevertheless, X-ray imaging is a relatively expensive and time-consuming method, which usually cannot be carried out by the conservators themselves.

Infrared imaging method

The method to reveal a watermark in infrared is fairly straightforward. Using a high resolution infrared camera an infrared image of the paper is recorded using transmitted illumination. Another infrared image is taken using only the reflected illumination.

Subsequently, in PhotoShop, both images are subtracted from each other during which the grey levels of the subtracted image are regulated to exactly cancel out the details of the reflected infrared image.

Practical example

In this case, the watermark in this soiled paper was difficult to perceive due to the dense and elaborate writing and drawings. The worn state of the paper did not promise any significant results with X-ray imaging.

As can be seen in the picture beneath, the final image clearly shows the watermark and even the indentations of the sieve mesh used to produce the paper.

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