Technical: Digital Conversion Methodology

Image Creation

All of the images from the Centennial Exhibition Digital Collection (CEDC) were scanned at the Free Library of Philadelphia (FLP) Digitization Center. The Library purchased two Agfa Duoscan T2000 XL flatbed scanners that allow reflective scanning of images up to 12x18 inches, and transparent media scanning up to 9 1/2 x12 inches. For photographs with significant bowing, or that were too large for the flatbed scanner, a contract photographer created high-quality 4x5 inch color transparencies, which were then scanned. In addition, a selection of approximately fifty lithographs, fifty trade cards and some related materials were scanned.

Resolution and file formats for access files and thumbnails were selected to facilitate speed of transfer via the Internet and to minimize the requirement for high-speed CPUs and a large amount of memory on the end user computers. All master files were saved as uncompressed TIFF files with appropriate header data, adapted from the Library of Congress TIFF header tag requirements.

During the first quarter of the project, the project director and scanning technician prepared a complete shooting script for all the material in the project. Each item in the script was assigned a unique seven-digit number that was incorporated into the file name and used to retrieve the images from the web server. CEDC items have been batched in nineteen different categories. Photographs made by the Centennial Photographic Company are batched by size (7 categories). Additional categories include photographs by other photographers (3 categories), lithographs, engravings, manuscripts (2 categories), sheet music, trade cards, a two-volume scrapbook, maps, and realia.

A prescanning form was developed by the project director for each category in the CEDC for entering basic descriptive and administrative metadata for each item. This metadata was then entered into a MS Access database upon scanning. Technical metadata was embedded in the TIFF header during digitization. Batched forms were transferred to the catalogers after the access and thumbnail jpeg's had been saved to the server.

Quality Control

Prior to the start of scanning the photographs, a rigorous benchmarking process was instituted to ensure that adequate detail was captured in the digital master files saved to CD-ROM in TIFF format. As a result of the benchmarking process, all photographic material with a size less than 8x10 inches (560 items) were scanned at 600 ppi with a bit depth set to 8 per color channel. Photographs 8x10 inches or larger were scanned at 400 ppi with a bit depth set to 8 per color channel (721 items). If the larger photographs were scanned at 600 ppi, the scanning time would have been more than 10 minutes for 17x21 inch photographs generating master files as large as 160 MB. This was deemed unacceptable. This would not have been the case if the originals were being scanned in 8-bit grayscale. However the decision was made early in the project to create digital files that most closely capture the current sepia tonal range of the photographs based on current as well as projected uses of the collection.

The digital files for web viewing were created from the master TIFF in Adobe PhotoShop at 72-dpi 24-bit color and saved as jpeg files using high compression. The reference jpeg files are scaled to 7.5 inches along the longest dimension. The thumbnail files are scaled to 2 inches along the longest dimension. A curve was applied in PhotoShop to slightly darken the images for viewing purposes, adding contrast and increasing the blue channel.

To maintain quality assurance during digitization, each scanner was calibrated twice a month and each monitor, twice a day. Quality control for the first thirty scans consisted of checking each master TIFF displayed on one of the monitors in the digitization lab against the original photograph. After the first thirty scans, every 10^th scan was checked against the original. All scanned files were opened to ensure full functionality. The reference jpegs and thumbnails were spot checked on various workstations and opened in different browsers and evaluated for appearance.

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