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Imaging Decisions

In September of 1998 the Bancroft Library and the Library Photo Service completed a one-year project with an important digital imaging component: the Robert B. Honeyman, Jr. Collection Digital Archive Project. The resulting work is now available as part of the California Heritage Collection.

Now with 20-20 hindsight, we can look back on the image capture decisions, and consider how they influenced the project results.

Project goals:
The main goals of the project were to provide public access to images of each item in the collection for purposes of study and teaching, and to provide improved intellectual control of the collection by compiling an on-line catalog with links to the images.

Project scope:
The Honeyman collection in the Bancroft Library includes about 2300 items related to California and Western US history, including oil paintings, lithographs, sketches, maps, lettersheets, items in scrapbooks, and more.

Project image requirements:
The website offers each image at three resolutions: a small thumbnail version, for browsing; a larger "medium resolution" version, sized to approximately fit on a typical computer screen; and a "high resolution" version for viewing image details, at twice the resolution of the "medium" version.

Additional imaging goals
Several additional purposes for the digital images captured in this project are foreseen. In the future it should be possible to offer higher resolution versions of the images for more detailed study via the web. Also, these images can provide a permanent record of each item in the collection and its condition at the time of capture. Finally, because images from the Honeyman collection are frequently published, the archived digital images may be supplied to publishers instead of photographs made conventionally.

Imaging equipment available
In addition to traditional (film) cameras, the Library Photographic Service has a digital camera: a PhaseOne Powerphase scanning back which fits on a Hasselblad camera. The Powerphase is used on a copy stand and controlled from a PC (personal computer, either Powermac or Wintel); it is capable of capturing an image measuring 7000 pixels square. Also, an Epson 836 flatbed scanner became available during the project; it can scan originals as large as 12x17 inches at 800 ppi (pixels per inch).

Capture Specifications for Digital Masters

File format
Digital masters are captured in 24 bit RGB color and stored in uncompressed TIFF format, as managed by the PhaseOne software. In the case of the Epson scanner, the scanner software runs as a plug-in to Adobe Photoshop, so the file is created by Photoshop 4. This public domain file format is widely readable.

Capture resolution and master file size
Because of the wide range of sizes and types of originals represented in the Honeyman collection, no single value for capture resolution could be set. Instead, a number of discrete resolution values were used, and originals were sorted prior to capture into groups suitable for each resolution, based on the capacity of the Powerphase camera. (The camera's capture resolution is adjusted by moving the camera on the copystand nearer or farther from the subject; the nearer the camera, the higher the capture resolution and the smaller the area of coverage). The highest resolution used was 600 ppi; at this resolution the camera will capture an area about 11.5 inches square, a comfortable fit for an 8-1/2 x 11 inch original. Several arguments were used to establish 600 ppi as the preferred resolution level for capture: 600 ppi is sufficient to capture extremely small text legibly; it is sufficient for high-quality publication at double life-size; and the resulting TIFF file sizes (up to 143 MB) are small enough to be processed and stored on suitably-equipped PC's. This is the sorting table:

Resolution

Coverage

(ppi)

(inches)

600

11.5

450

15.5

300

23

200

35

150

46

(for example, an 11x17 inch original would be copied at 300 ppi because it fits in a 23x23 inch capture area, but can't be completely covered in the 15.5x15.5 inch area covered at 450dpi)

As a result of applying the sorting table, most of the capture files are in the range of 5000 to 6000 pixels in the long dimension, and their file sizes tend to be between 60 and 100 MB.

The file processing workflow was developed and tested to handle file sizes up to about 140 MB.

Included targets
A one-piece target is imaged at the edge of each capture. It combines the graycale target and the color patches from a Kodak Q-13 Color Separation Guide and Grayscale with a centimeter scale, all in a very compact layout created using a hobby knife and two-sided adhesive tape. The information from the target is intended to provide information about the tonality and scale of the image to scholars and technicians. The crucial "A," "M," and "B" steps of the grayscale are marked with small dots to make them easy to identify for making tonal measurements during capture set-up and file processing. Several different-sized versions of the combined target are suited to the range of sizes of the originals.

Tonal metric
The RGB data in the image files is captured in the native colorspace of the capture device (camera or scanner); that is, no color management step such as applying a Colorsynch profile is used on the digital masters prior to saving. Before capture occurs, the camera (or scanner) operator uses the controls in the scanning software to adjust the color balance, brightness, and contrast of the scan so that the grayscale target in the image has the expected RGB values. These values are as follows: for the white "A" patch, R, G, and B values all at or near 239; for the middle-gray "M" patch, RGB = 98; for the near-black "B" patch, RGB = 31. These expected RGB values are appropriate for a 24 bit RGB image with gamma 1.8.

Cropping and background
Originals are depicted completely, including blank margins, against a light gray background paper (Savage "Slate Gray"), so that the digital image documents the physical artifact, as well as reproducing the imagery that the artifact portrays. A narrow gap between the grayscale target and the original allows for cropping the grayscale out of the composition if desired for some new purpose.

Adapting to different formats of originals
Some aspects of image capture were determined or suggested by the formats of the originals:

  • Subcollections suitable for flatbed scanning: The lettersheets, clipper ship cards, and sheetmusic covers are all small enough to fit on the 12x17 inch platen of the Epson scanner, so that these groups were routed to the flatbed. By operating both the scanner and the camera in tandem, the production rate of image capture was increased.
  • Miscellaneous flat art: this group, mostly stored in folders, represents the bulk of the Honeyman Collection, including watercolors, lithographs, maps, etc. Many items are mounted on larger backing sheets, and many are in mounts with window mats, making flatbed scanning difficult, even if size and fragility are not an issue. These items were sorted by capture resolution and routed to the Powerphase.
  • Bound originals: scrap books and sketch books were captured with the digital camera. In cases where more than one item per page is cataloged, separate digital captures were made for each item; otherwise, the entire page is recorded.
  • Framed works: the 136 framed items in the Collection were captured using a film intermediate. They were photographed at the Bancroft Library using 4x5 Ektachrome film, and the film was scanned on the Epson scanner. This was done to minimise the handling of these awkward and vulnerable objects in two ways: the originals would not have to be shuttled between the Bancroft Library and the Photographic Service, at opposite ends and levels of the Library complex, for the project; and the 4x5 film intermediates can be made available to publishers as need arises without further handling of the originals (no system is in place as yet to supply digital files to publishers).

Storing the digital masters
The digital capture files were recorded on external hard drives at the capture stations; the hard drives were than moved to other PC's equipped with CD-Recordable drives, where the digital master files were transferred to CD-R disks.

Specifications for the viewing files
The viewing files for the Honeyman website are designed for convenient transmission over the internet and satisfactory viewing using typical PC hardware and web browser software:

  • A thumbnail GIF, maximum dimension 220 pixels
  • A medium format JPEG-compressed (JFIF) version, maximum dimension 750 pixels, transmitted filesize around 50 KB.
  • A higher-resolution JPEG version, maximum dimension 1500 pixels, transmitted filesize roughly 200 KB.
  • Gamma adjustment: the viewing files are prepared for a viewing environment of gamma 2.2

Making the viewing files
The viewing files were made from the CD-R files in batches, on yet another PC running Adobe Photoshop 4 or Debabelizer 3. The master files are opened, gamma-adjusted, downsampled to size, sharpened with unsharp mask, and saved in GIF and JPEG formats. The derivatives from digital camera files are also color-managed with a Colorsynch profile created using Agfa Fototune Scan.

Dan Johnston, Senior Photographer
Library Photographic Services, The Library
djohnsto@library.berkeley.edu


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