Scanning

Scanning projects have a number of components, each of which should be thought out before the project begins. An explanation of each of these components, along with issues to consider during decision-making, is provided via the links below.

Image capture resolution is measured in pixels per inch (ppi). A high ppi means a finer image resolution, resulting in a more detailed image.

How much resolution is enough? It depends on the type of material you are scanning, and how small the details are in relation to the overall size of the object. An item with fine details requires a fairly high resolution. By comparison, an item with relatively large details may not need as high a resolution.

The intended use of the item also makes a difference. Will the digital image serve as simple decoration for a web page? Or will it be a source of information about the original object--will researchers want to be able to view small details? If the image is meant for publication, the editor or printer you are working with may have specific requirements for image resolution.

In general, the resolution that clearly captures the smallest significant character or detail is the correct resolution. This table provides some general guidelines. Remember, though, that you will have to refine these specifications based on the particular features of the items you are scanning.

For more information:
http://www.carli.illinois.edu/mem-prod/contentdm/guidlines_for_digitization.pdf

Bit depth is defined as the number of bits per pixel. Each of the following bit depths will result in a successively larger file:

Bitonal
A 1-bit pixel has two possible values, black or white. 1-bit bitonal images are appropriate for some machine-produced documents and books with no illustrations, or with black-and-white line art only. They are not sufficient for documents that include handwriting, photographs, or illustrations with half-tones (greys). Bitonal scanning will produce the smallest possible file.

8-bit Greyscale
An 8-bit pixel has 256 possible values along a spectrum of greys ranging from pure white to pure black. 8-bit greyscale scanning is appropriate for producing images that include grey tones, but no colors—such as black-and-white photographs, and books or documents containing handwriting or half-tone illustrations.

In choosing between bitonal and greyscale, it is a good experiment to scan an item both ways, and compare the results. You may be surprised at what the greyscale scan picks up, and what the bitonal scan leaves out.

8-bit Color
An 8-bit color pixel also has 256 possible color values, meaning a very limited color palette, and a smaller file size than 24-bit color would produce.. 8-bit color works well with images containing flat, sharply demarcated areas of color, such as a bar graph, pie chart, or simple logo.

24-bit Color
A 24-bit color pixel has 16.7 million possible color values, meaning it will produce greater color fidelity to the original object. It works well with images that have subtle transitions between areas of color, as in most photographs and art reproductions. 24-bit color produces large files, so this choice should be carefully considered. Color is important in some academic disciplines, such as the study of art and artifacts, or the natural sciences. Consider whether a lower level of color fidelity will in any way impair a user’s understanding of the original object.

48-bit is also available, and produces even larger files than 24-bit. It is generally used for high-end print production or high-value preservation imaging.

This site does not discuss color models in any depth, but will touch on two of the most common ones, RGB and CMYK, very briefly.

RGB/sRGB is recommended for images intended to be viewed on the Web. It is designed to work with standard computer monitors in rendering colors. When you save a digital image file, if you are not given a choice, it will most likely be automatically saved as RGB or sRGB.

CMYK is geared toward the color system used in commercial printing; it is based on traditional camera color separations. If you are producing images on request for a publisher, they may request CMYK. It is best to ask them specifically about this (as well as the image resolution, file formats, and delivery media).

For comprehensive information on color in digital imaging, see Real World Color Management by Bruce Fraser, Chirs Murphy, and Fred Bunting.

Suggested specifications by material type

File formats

From the beginning of a digital imaging project, saving images should be thought of as a long-term activity, extending well beyond the moment that you save the file to disk.

There are many formats in which you can save an image file; the choice depends on the purpose of the file. It's best to create two copies of each image: a use copy and an archival copy. Use copies are those to be exhibited or shared. Archival copies are backups, permanent copies of images, that may be drawn upon when:

• The use copies are electronically altered for presentation (resized, cropped, color levels adjusted, etc.)
• The use copies are lost or damaged
• The file format chosen for the use copies becomes obsolete

Terms used in this explanation of formats include:

• Compression: The process of altering a file's encoding to reduce its size.
• Lossless: Lossless compression does not remove any data from the image.
• Lossy: Lossy compression removes some data from the image, meaning the file cannot be restored to its original state.
• Open vs. proprietary formats: Open file formats are based on publicly available software standards. Proprietary formats are based on standards copyrighted by private companies. For archival copies of digital images, it is best to choose an open format, since it has the better possibility of being openly available and supported over the long term (though no formats, open or proprietary, are guaranteed to be supported forever!).

TIFF (Tagged Interchange File Format)
File extension: .tif
TIFF is an open standard; images can be saved as either uncompressed, or losslessly compressed, images. In Web terms, it is not very portable; not all browsers support it, so remote users often have to download a plug-in in order to view TIFF images. However, it is favored as a format for archival copies of image files; it is a good idea to keep local copies of files in this format, and convert them to other formats for the Web. For instance, you might save an image as a 300-dpi TIFF, and display it on the Web as a 72-dpi JPEG.

PNG-24 (Portable Network Graphics)
File extension: .png
PNG is an open standard for lossless compressed images. PNG-24, which supports 24-bit color, is intended as an archival format. PNG is more portable than TIFF, being supported by most Web browsers.

JPEG 2000
File extension: .jp2
JPEG2000 is an open, lossless compression standard that promises three improvements over traditional image formats. * It uses wavelet compression, which promises more efficient compression and less distortion of image files. * Metadata can be stored within the file in a variety of formats. * The standard allows for dynamic delivery of the image files over the Web. A single large, high-resolution copy of the image can be stored on a Web server, and when users click to see the image, smaller-resolution JPEGs are automatically generated from it.

This means that there is no need to save archival copies and use copies of the same images. In other words, this standard promises to solve some of the problems of metadata storage and image delivery. However, special software is required for embedding metadata and for realizing the sophisticated delivery options. Open source and commercial software applications have been, and continue to be, developed.

GIF (Graphics Interchange Format)
File extension: .gif
GIF images are created through a proprietary lossless compression scheme. GIF is recommended for objects with large, clearly divided areas of unvarying color, such as brochures, logos, and graphs or charts. It is not recommended for photographs or other items with subtle transitions from one area of color to the next.

JPEG (Joint Photographers Expert Group)
File extension: .jpg
JPEG images are created through an open lossy compression scheme. JPEG is ideal for digitally reproducing photographs and other continuous-tone media. It is not recommended for items with sharp-edged areas of color or graphic type.

PNG-8 (Portable Network Graphics)
File extension: .png
PNG can be used for both use copies and archival copies (see below). PNG-8 is a lossless compressed format. It supports 8-bit color and is similar to GIF. Like GIF, PNG-8 is good for flat images with sharp color transitions.

Questions to ask if you are outsourcing

If you decide to have an outside company produce your digital imaging project, there are several issues you should address before the work begins. In addition to the usual questions about time and price, you should specify:

•  File resolution in pixels per inch ("300ppi") or in pixel dimensions ("1800 x 1200 pixels").

•  File format. Open formats are preferred for projects needing long-term preservation.

•  Storage media. State a preference for media type, whether portable hard drive, CD, or DVD. If CDs or DVDs are being used, specify that a high-quality brand must be used.

You may also want to request resolution targets or color targets, to ensure the quality of the company's work.

Additional Information Sources

Technical Advisory Service for Images
http://www.tasi.ac.uk/advice/advice.html

National Archives and Records Administration Technical Guidelines
http://www.archives.gov/preservation/technical/guidelines.pdf

BCR CDP Digital Imaging Best Practices Version 2.0
http://www.bcr.org/cdp/best/digital-imaging-bp.pdf