Test Arrangements

The test conditions and environment were kept constant while testing equally comparable materials or methods. In the tests where we used our sample plate we always recorded the same fragments.

The outcome of ordinary photography was most difficult to optimize. Considerable amount of time was spent in trying different illumination conditions and suitable developing procedures before the real test photographs. Hundreds of negatives and paper copies were made. Simultaneously, a test set of negatives from the real Petra Scrolls was being scanned in different commercial litographic firms, with request to produce the best possible pictures.

The amount of manual work and sensitivity to faults makes photography less appealing when comparing to modern digital imaging techniques described next.

The direct scanning method was also hard to test, because a typical sample plate is larger than a typical A4- or U.S.Letter sized flat scanner. Placed upon a scanner, the few millimeter high edges of the scanner casing and the glass plate cause the samples to be too high for the scanner to focus correctly. So, the cover had to be taken off, exposing the CCD cells to dust and voiding the warranty. Opening the cover of a scanner has to be done with special care and in a dustless environment. The first dust particle landing on the CCD will cause a line to appear in all further scans, and several will render the scanner practically useless.

The same illumination problems we had with photography were also present when testing the video grabbers. Fortunately, the result could be seen in real time, and so the lights could be adjusted to optimum quite easily. Also, with a special type of CCD sensor element, we were able to stretch the usable band of spectrum to near-infrared area. The red and infrared filters were applied to these tests, too. The electronics (amplification, white level) of the camera itself were slightly modified to give best results.

Two different thermographic cameras were tested; the glass plate effectively prevented the camera to detect the differences of temperature or emissivicity in the fragments. Slight heating to approximately 45°C with a tungsten lamp did not help

We used Helsinki University Central Hospital's X-ray equipment to search for differences in density. The glass plate captured X-rays far more efficiently than the thin, soft papyri fragments. Several variations in power and exposure times gave, at best, flat grey pictures without a single detail.

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Antti Nurminen, 34044T, andy@cs.hut.fi