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imaging best practices

Best Practices/Standards/Guidelines

Image scanning at UC Berkeley
Capture specifications for digital masters
CDL image standards
File format
Capture resolution and master file size
Included targets
Tonal metric
Cropping and background
Storing the digital masters
Making the viewing files
Technical metadata

Image scanning at UC Berkeley
Scanning for most UC Berkeley Library projects is done in the Library's Digital Imaging Laboratory (DIL). Digital image capture equipment at DIL includes:

  • PhaseOne Powerphase Digital camera, which fits on a Hasselblad body and captures images up to 7000 x 7000 pixels (LxW). This camera is used to capture originals in bound volumes, and originals too large for flatbed scanning.
  • Epson 836XL and 1640XL flatbed scanners, which can scan originals up to 11x17 inches. These scanners are primarily used with unbound originals, especially document pages and loose photos.
  • Linotype Circon flatbed scanner, which is well-suited for scanning negatives and transparencies up to 11x14 inches at resolutions up to 2400 pixels per inch
  • Nikon LS2000 film scanner, for 35mm film scanning

Capture Specifications for Digital Masters
Our philosophy of digital image capture can be summarized as "scan once, re-use for many purposes." We expect that our master files should be useful for disparate, demanding purposes including scholarly study and high-quality publication, so we follow standards for resolution, image composition, and file format to ensure the ongoing value of the images.

CDL Image Standards
We follow the California Digital Library Standards that pertain to imaging. See the following documents:

File format
Digital masters are captured in 24 bit RGB color and stored in uncompressed TIFF format. Scanner- or camera-specific image capture software is used to manage the technical details of image capture, and then Adobe Photoshop is used to save the TIFF file. This public domain file format is widely readable.

Capture resolution and master file size
DIL's preferred capture resolution for reflective originals is 600 pixels per inch, yielding RGB TIFF master files in the 60 to 100 MB size range for originals close to letter size (8.5 x 11 inches). Larger originals are usually captured at a lower resolution, to keep the file size in the same range, and smaller originals may be captured at higher resolution.

Included targets
For reflective originals, a one-piece target is imaged at the edge of each capture. It combines the grayscale target and the color patches from a Kodak Q-13 Color Separation Guide and Grayscale with a centimeter scale, all in a 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 "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 an ICC color 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 entirely, including blank margins, against a suitable background paper (usually white or 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.

Storing the digital masters
The digital capture files are grouped on a local server for quality review, technical metadata preparation, viewing file preparation, and are then archived on recordable CD media.

Making the viewing files
Viewing files are made from the masters using scripted actions in Adobe Photoshop. The master files are opened, gamma-adjusted, downsampled to size, sharpened with unsharp mask, and saved in GIF and JPEG formats. The viewing files are intended for viewing on a typical computer monitor, as represented by the sRGB colorspace. In some cases, the master files are explicitly translated into the sRGB colorspace, with an ICC color profile created using Colorsynergy InCamera software to characterize the color capture of the camera or scanner.

Technical Metadata
Technical metadata captured for each original:

  • Width in pixels
  • Height in pixels
  • Ppi (aka dpi)
  • Filters used in capture
  • Glassvused to hold item flat
  • Master file name (eg <SObjID>A.tiff)
  • Network location of a master digital file
  • Frame selected to be the master
  • Date captured in digital format
  • Photographers batch ID
  • Scan type (eg scanner, digicam)
  • Scanner make
  • Scanner model
  • Scanner serial number
  • Bit depth (probably 24 bit)
  • Light source used during capture
  • Catchall description of color management
  • Name of color profile file for this setup
  • Grayscale, RGB, CMYK
  • Master file format (TIFF)
  • Intensity correction function for capture
  • Digital compression used during capture
  • Code for one of several standard derivative resolution setups
  • Name of this derivative file (eg <MasterfileName>A.jpeg)
  • Location of this derivative file
  • Host where this derivative file lives
  • Directory where this derivative file lives
  • Creation date for derivative file
  • ID for one of several standard derivation setups
  • Software to produce derivative file from master
  • Derivative file format (eg GIF, JPEG, JTIP)
  • Grayscale, RGB, CMYK
  • Resolution class
  • Conversion Gamma
  • Input color profile
  • Output color profile
  • Digital compression used during creation of a deriviative
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Last updated 03/08/2007. Server manager: contact