Physical Issues

1. Tonality: (pixel depth or bit depth)

pixel: The picture elements that make up an image, similar to grains in a photograph or dots in a half-tone. Each pixel can represent a number of different shades or colors, depending upon how much storage space is allocated for it. See 8-bit image or 24-bit image

a. bi-tonal: (1 bit pixel) either black or white, no shading. Best used for modern printed texts. Probably OK for lithographs.

b. gray scale: (8 bits, 256 shades of gray). The gray scales of scanners and terminals are determined by the number of grays, or steps between black and white, that they can recognize and reproduce. Fine for typescript and handwritten documents if you don't wish to illustrate the quality and feel of the paper.

c. color: (24 bits): Most historical materials will be scanned this way to reflect the age and quality of the item -- even if a black and white photograph.

Ohio Memory Examples http://www.ohiomemory.org/om/imaging.html

2. Resolution: Number of pixels (in both height and width) making up an image. The higher the resolution of an image, the greater its clarity and definition

• DPI: Dots per inch. A measurement of the scanning resolution of an image or the quality of an output device. Expresses the number of dots a printer can print per inch, or monitor can display, both horizontally and vertically. A 600-dpi printer can print 360,000 (600 3 600) dots on one square inch of paper.
• Higher the DPI the sharper and clearer the image. However, the higher the DPI, the greater server space needed.
• Look at the most detailed part of the photograph and scan for clarity of that aspect.
• Recommended DPI's
  1. bi-tonal -- 600 dpi
  2. gray scale -- 300 dpi
  3. color -- 300 dpi, up to 600 dpi

3. File formats and compression:

1. TIFF: Tagged Image/Interchange File Format. A file-storage format implemented on a wide array of computer systems. Considered an industry standard. Suited for black and white text documents. Will lose no information. Can be considered archival.
2. JPEG: Joint Photographic Experts Group. Used to refer to the standard they developed for still-image compression, which is sanctioned by the International Standards Organization (ISO). May lose some clarity in information. Supported by all major computer platforms and Internet Web browsers. Not acceptable as an archival format.
3. GIF: Graphic Image File format. A widely supported image-storage format promoted by CompuServe that gained early widespread use on on-line services and the Internet. Well suited for low resolution screen images and is often used for thumbnails.
4. PDF: Adobe Acrobat Portable Document Format provides a convenient way to view and print images at high resolution, and may also be used to group several files into chapters and books.

Thumbnail: A small image (usually derived from a larger one). Permit a user to view a dozen or more images on a single screen.

Archival: image can be PRESERVED electronically, without any information loss.

4. Special Text Considerations: OCR: Opitical Character Recognintion: with special software, the ability to have every word recognized and then become searchable. The software misses (ie. doesn't recognize) a certain number of words in the scanned document, making this a labor intensive process.

Many definitions derived from:
Getty Institute.
http://www.getty.edu/research/institute/standards/introimages/
Technical Recommendations for Digital Imaging Projects http://www.columbia.edu/acis/dl/imagespec.html.