Direct Digitizing

Digitizing images of carbonized papyri samples directly with a flat scanner, digital camera or CCD video camera are all very good methods for producing good-quality digitized images. The process is, in principle, very straightforward and quick, and the variables involved are more easily controllable than when producing digitized images through photography.

Five different digitized images are presented along with the histograms of the images: The COHU 4710 camera uses a frame-transfer type CCD matrix which is sensitive to wavelengths from 400nm to 1000nm. Here is presented two images, one in the visible wavelength area (figure 1), and one in the near-infrared area (750-1000nm)(figure 2). The contrast in near-infrared area seems to be slightly better. The number of active picture elements is 699x576, so that only 1x1 inch areas at a time can be digitized with enough spatial resolution. Our resulting image had to be slightly stretched with software to be of comparable size with the other images.

The HP 3c was tested as it was, without modifications. The plate was a few millimeters above the scanner glass, hence causing the image to be very slightly blurred. The bundled software controlling the HP 3c scanner was unable to set the exposure adequately. An attempt to modify the tonal curves (histogram) at the scanning stage caused the grey scale values be stretched, but in a poor manner, so that we were not able to use the enhanced sensitivity of the HP's CCD cells. The scanner has the ability for internal tonal correction, so it is potentially capable of adjusting the black and white levels correctly. Thus, this test does not really make justice to it, but it points out some of the difficulties with flat scanners. Current price for this scanner is less than $1500.

The top casing of Umax 1200S scanner was removed to be able to focus correctly, and the reference black and white stripes embedded in the casing were compensated by a black and dark grey stripe made in a photo laboratory. The new reference stripe caused the automatics of the scanner to be set to nearly optimal exposure values for our papyri samples. A more modern Umax scanner, Umax PowerLook with 10 bits of greyscale depth and optical resolution of 600x1200dpi, with downloadable tonal correction curves and with full version of Adobe PhotoShop, costs approximately $4000.

The Canon EOS DCS 3 digital camera has a 2150x2000 CCD matrix element and sensitivity of 12 bits, thus making it an attractive alternative. The resolution is adequate and the 12 bits provide good control over the darkest grey levels. With very little optimization we managed to take a couple of quite a good pictures. The major drawback of this digital camera is its current high price (July 1995): The camera with one 50mm lense costs 105.000FM ($25.000).

Figure 17: Test image recorded by COHU 4710 CCD camera, enlargened from 462x150 pixels to 640x208.

Figure 18: Test image recorded by COHU 4710 CCD camera in near-IR bandwidth (enlargened).

Figure 19: Test sample scanned with HP 3c flat scanner.

Figure 20. Test sample scanned with Umax1200S flat scanner.

Figure 21. Test sample photographed with Canon EOS DCS 3 digital camera.

Figure 22: Histogram of COHU 4710 CCD camera image (Fig.17)

Figure 23: Histogram of COHU 4710 CCD camera near-IR image (Fig.18).

Figure 24: Histogram of HP 3c flat scanner image (Fig.19).

Figure 25. Histogram of Umax1200S flat scanner image (Fig.20).

Figure 26. Histogram of Canon EOS DCS 3 digital camera image (Fig.21).

A good choice for the measurement of goodness of a digitized picture is the amount of information the image contains, or the information density. Primarily, that depends on the spatial resolution, and for dark objects such as carbonized papyri, the sensitivity of the equipment's CCD elements (or simply the number of grey scale values obtainable). Typical maximum optical resolution of current flat scanners is 600 dots per inch (dpi), and has been accepted as the default resolution for many digital image archives such as the Duke University Papyri Archive. For digital cameras and CCD video cameras one cannot state a certain dpi-value, because one can always use magnifying optics to digitize a very small area with the equipment's maximum CCD matrix size. In practice, one should be able to digitize at least several words, or one whole papyri fragment, at a time. A fragment of approximately 3.5x3.5 inches could be digitized with a digital camera with 2048x2048 CCD matrix to obtain the resolution of 600dpi. For practical purposes, that would be enough.

The amount of different grey scale values as a second basis for information density along with spatial resolution is not simply defined. In principle, all the digital image sources discussed here can provide all 255 different grey scale values. For very dark objects the different equipment produce very different results, as can be seen in figures 22-26.

Setting a flat scanner for optimally recording a dark object depends on the scanner's automatics and driver software. Many scanners adjust the black and white levels automatically, which leaves only little possibilities for optimizing the results. Typically, the automatic adjustment produces poor images. Also, usually the removed top casing contains reference-black and reference-white stripes that the scanner checks before every scan, and they must somehow be reproduced. The scanners can be controlled by bundled software provided by the manufactorer, or using standard TWAIN draivers (if the scanner is TWAIN compliant) with common image processing software. For some scanners, there is also software packages expecially made for chosen image processing programs that take full advantage of all the scanner's abilities together with the features of the image processing program (for example, the Umax PowerLook is shipped with Adobe PhotoShop with "plug-in" software for acquiring images from the scanner).

There are currently very economically priced scanners offering 8, 10 or even 12 bits of depth. Good results might be obtained with a 8 bit scanner, but the more sensitive 3x10-bit CCD's provide better control over shadow details and otherwise dark images.
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Antti Nurminen, 34044T,