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Federal Agencies Digitization Initiative Still Image Working Group — August 2010 printing images all render high-bit images at 8-bits per pixel, so there is limited practical benefit to saving high-bit images and no way to verify the accuracy and quality of high-bit images. Also, it is best to do a good job during digitization to ensure accurate tone and color reproduction, rather than relying on post-scan correction of high-bit images. Poorly done high-bit imaging has no benefit. Color Mode Grayscale image files consist of a single channel, commonly either 8-bits (256 levels) or 16-bits (65,536 levels) per pixel with the tonal values ranging from black to white. Color images consist of three or more grayscale channels that represent color and brightness information. Common color modes include RGB (red, green, blue), CMYK (cyan, magenta, yellow, black), and LAB (lightness, red-green, blue-yellow). The channels in color files may be either 8- bits (256 levels) or 16-bits (65,536 levels). Display and output devices mathematically combine the numeric values from the multiple channels to form full color pixels, ranging from black to white and to full colors. RGB represents an additive color process: red, green, and blue light are combined to form white light. This is the approach commonly used by computer monitors and televisions, film recorders that image onto photographic film, and digital printers/enlargers that print to photographic paper. RGB files have three color channels: 3 channels x 8- bits = 24-bit color file or 3 channels x 16-bits = 48-bit color. All scanners and digital cameras create RGB files by sampling for each pixel the amount of light passing through red, green and blue filters that is being reflected or transmitted by the item or scene being digitized. Black is represented by combined RGB levels of 0-0-0, and white is represented by combined RGB levels of 255-255-255, This is based on 8-bit imaging and 256 levels from 0 to 255: this convention is used for 16-bit imaging as well, despite the greater number of shades. All neutral colors have equal levels in all three color channels. A pure red color is represented by levels of 255-0-0, pure green by 0-255-0, and pure blue by 0-0-255. CMYK files are an electronic representation of a subtractive process: cyan (C), magenta (M), and yellow (Y) are combined to form black. CMYK mode files are used for prepress work and include a fourth channel representing black ink (K). The subtractive color approach 1s used in printing presses (four color printing), color inkjet and laser printers (four color inks, many photo inkjet printers now have more colors), and almost all traditional color photographic processes (red, green and blue sensitive layers that form cyan, magenta and yellow dyes). LAB color mode is a device independent color space that is matched to human perception: three channels representing lightness (L, equivalent to a grayscale version of the image), red and green information (A), and blue and yellow information (B). One benefit of LAB mode is that it is matched to human perception, and also LAB mode does not require color profiles (see section on color management). Disadvantages of LAB include the potential loss of information in the conversion from the RGB mode files from scanners and digital cameras, the need to have high-bit data, and the fact that few applications and file formats support it. Avoid saving files in CMYK mode; CMYK files have a significantly reduced color gamut (see section on color management) and are not suitable for master image files for digital imaging projects involving holdings/collections in cultural institutions. While theoretically LAB may have benefits, at this time we feel that RGB files produced to the color and tone reproduction described in these guidelines and saved with an Adobe RGB 1998 color profile (or, alternatively, an sRGB color profile), are the most practical option for master files and are relatively device independent. We acknowledge that the workflow described in these guidelines to produce RGB master files may incur some level of loss of data; however, we believe the benefits of using RGB files brought to a common rendering outweigh the minor loss. Digitization Environment Our recommendations and the ISO standards referred to below are based on using CRT monitors; however, the criteria specified below also applies to LCD monitors, as LCDs have now replaced CRTs in most imaging environments... Be aware that inexpensive LCD monitors may have artifacts that make it difficult to distinguish image quality problems in the image files, and the appearance of colors and monitor brightness can shift with the viewing angle of the LCD panel. We recommend using a high-end LCD monitor designed for the graphic arts, photography, or multimedia markets. Technical Guidelines for Digitizing Cultural Heritage Materials: Creation of Raster Image Master Files