Bull's Head and Mermaid - The Bernstein Project - Österreichische ...

Ill. 4: Soft-X-ray radiography
is increasing by up to 22 percent for A4 formats while the
intensity of film exposure is decreasing accordingly towards
the edges. For this reason modified soft-X-ray devices have
been used in the last years. For example, the disturbing air
layer was replaced by helium. Helium blocks X-rays less and
thus makes the influence of the varying distance to the Xray
source negligible.
Image subtraction method
The image subtraction method for watermark imaging has
been used approximately since the late 1990s. In this approach
a transmitted light image is subtracted digitally from
a reflected light image, which deletes eventual pictures/marks
on the reflecting side of the paper. Merely the paper
structure (visible in the reflected light image) and possible
pictures on the verso of the paper remain visible (Ill. 5 a, b, c).
Ill. 5a: recto image
1 X-ray source
2 Items with watermarks
3 Film
4 Object slide
The ‘image subtraction’ also known as ‘backlight’-method,
seems very appealing, because it is appropriate for the watermark-
and paper structure imaging of single-sheet items
such as drawings or prints. In contrast to most of the other
imaging technologies it is noticeably cheaper, 1:1 images of
paper structures up to A1-format are possible, the needed
equipment is very easy to handle and to transport for everybody.
Literature: Brown / Mulholland, Using microfocus X-radiography;
Holle / Schreiner, Sichtbarmachung von Wasserzeichen;
Kaiser, Neue Erkenntnisse; La Chapelle / Le Prat, Les
relevés de filigranes; Needham, IDL, ILC, WILC; Nordstrand,
Beta-Radiographie; Schnitger / Ziesche, Elektronenradiographie
von Wasserzeichen; Thienen / Veldhuizen, Watermarks
and Incunabula; Ziesche / Schnitger, Elektronenradiographische
Untersuchungen.
G.D. / M.v.D. (G.D. / L.D.)
Ill. 5b: transmitted image Ill. 5c: result of image subtraction
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