Resolution:
Resolution is the ability to distinguish fine spatial detail. The spatial frequency at which a digital image is sampled (the sampling frequency) is often a good indicator of resolution. This is why dots-per-inch (dpi) or pixels-per-inch (ppi) are common and synonymous terms used to express resolution for digital images. Generally, but within limits, increasing the sampling frequency also helps to increase resolution.
Bit Depth: Bit depth is an indication of an image's tonal qualities. Bit depth is the number of bits of color data which are stored for each pixel; the greater the bit depth, the greater the number of gray scale or color tones that can be represented and the larger the file size.
The most common bit depths are:
· Bitonal or binary, 1 bit per pixel; a pixel is either black or white
· 8 bit gray scale; 8 bits per pixel; a pixel can be one of 256 shades of gray
· 8 bit color, 8 bits per pixel ("paletted color"); a pixel is one of 256 colors
· 24 bit color (RGB), 24 bits per pixel; each 8-bit color channel can have 256 levels, for a total of 16 million different color combinations
File Formats:
TIFF - an acronym for Tagged Image File Format. It is one of the most popular and flexible of the current public domain raster file formats. It provides a high level of detail with lossless compression.
TIFF ITU-T.6 - A 24-bit storage format commonly used by Adobe PhotoShop and other bitmap editors. This format is also suited for bitonal text documents; it provides a high level of detail (up to 600 dpi, or 4,800 x 6,600 pixels for a letter-sized page).TIFF ITU-T.6 format should be used for archival files. With lossless compression, the data that results from compressing and then uncompressing the image is exactly the same as the original, uncompressed file. CCITT Group 4 compression is lossless and, therefore, permissible for masters and derivatives.While compression is allowed for archival files, it is discouraged, as it adds complexity to the format migration issues of long-term preservation. When compression is used, it must be lossless and not proprietary.
JPEG - A 24-bit, lossy (some information lost) compression format which is well-suited for screen and print presentation. JPEG is supported by all major computer platforms and by Internet web browsers. With lossy compression, the picture quality of the compressed file is reduced when compared to the original file, and can not be restored, except by going back to the original. The advantage is that the file sizes are much smaller, and image quality is acceptable in most cases. It is not acceptable as an archival file format. JPEG stands for Joint Photographic Experts Group.
JFIF JPEG File Interchage Format - A specific implementation of the JPEG standard, commonly used by bitmap editing programs, viewers, and Web browsers.
GIF - An 8-bit, lossless compression format which is well-suited for low resolution screen display of images. GIF is often used for image thumbnails and screen versions of text documents, and is supported by all major computer platforms and Internet web browsers.
PNG - The Portable Network Graphic format is expected to provide a higher-quality replacement for the GIF, particularly for images delivered to World Wide Web browsers.
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. PDF can provide additional navigational tools such as hyperlinks among pages within a document, and from one PDF document to another. Although this is a proprietary Adobe format, both the file specifications and the viewer software are freely distributed. Plug-ins are available for major web browser to enable them to view PDF files without launching an external viewer.
California Digital Library. Best practices for image capture.
http://www.cdlib.org/news/reports_guidelines.html February 2001 [PDF]