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Colour Management for digital imaging is defined to be the communication of the associated data required for unambiguous interpretation of colour content data, and application of colour data conversions as required to produce the intended reproductions. Colour content may consist of text, line art, graphics, and pictorial images, in raster or vector form, all of which may be colour managed. Furthermore, colour management considers the characteristics of input and output devices in determining colour data conversions for these devices.
There are three fundamental issues that colour management has to deal with: interpretation of digital values, colour space transformations, and image state conversions.
When one receives digital files containing colour information, the first thing necessary to view these files is to interpret the digital values in terms of colour appearance. This process makes use of colour interpretation information in the file. In the absence of such information, default assumptions are applied. This process is analogous to the decompression of compressed image data.
Colour space transformations are necessary because different colour encodings are supported by different devices. It may be necessary to take colours encoded in one colour space and convert them to another colour space that is understood by the receiving device. These transformations can be to a device independent colour space based on standardized colour measurement methods, or to a device dependent colour space based on the digital values required to produce colours on an actual device. Some devices can accept data in different colour spaces, so long as they know which colour encoding is being sent.
The third issue is that the best colour image quality is usually achieved by optimizing images for the media on which they will be reproduced. Some level of cross-media consistency must be maintained so that reproductions are reasonably faithful to the original, but this does not mean that the exact appearance of the original is maintained. In many cases, doing so will produce noticeably inferior results. Therefore, in addition to performing colour space transformations, it is often necessary to also perform an image state conversion, where colours that are appropriate for some source medium are optimized for a second reproduction medium.
Colour management is achieved in part through the measurement of equipment and systems that deal in one way or another with colour. Their input and output characteristics may be measured through a set of methods and conditions defined by IEC standards. Then, it is the job of the colour management system to take colour data created using one device and perform conversions to produce images suitable for open exchange, or consumption and reproduction by other devices.
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RGB and CMYK
Humans see colour because light from the external world strikes the retina where there are about 125 million photoreceptor cells, of which there are four kinds: rods and three kinds of cones that absorb light from a portion of the spectrum of visible light. There are cones that absorb long-wavelength light peaking at 565 nm (red), those that absorb middle-wavelength light peaking at 535 nm (green) and those that absorb short-wavelength light peaking at 440 nm (blue).
Apart from the sun itself, the source for this light can be emission by, transmission through, or reflection from, an object. In electrotechnical terms, light that is emitted generally comes from a screen of some sort, such as a cathode ray tube (common examples include a television screens and computer monitors), or others kinds such as plasma screens or liquid crystal displays. In parallel to the way the cones in our eyes operate, this light is usually described as RGB (red, green and blue).
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Light that is transmitted or reflected – and this is the case for the greatest part of the human visual experience – comes from all the physical objects that surround us. These colours are created by the absorption, refraction, or diffraction of the incident light and, in IEC terms, relate to photographs (transparencies and prints, where cyan, magenta, and yellow dyes are used) and printed documents where CMYK (cyan, magenta, yellow and black) inks are used.
RGB monitors can display colours that CMYK printers cannot print. Inversely, some CMYK colours cannot be created using RGB devices.
Transforming colour information from source to destination depends on the colour characteristics of all devices in the colour chain, as well as the viewing conditions. While IEC colour measurement and management standards have a variety of uses, the primary colour management approach embodied in the IEC standards is to facilitate conversions to and from standard RGB colour encodings in the input and output equipment, so the desired colour reproduction is achieved. These standard encodings are then used for exchange of colour data.
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Colour Data Systems
Another important element is colour resolution, measured in dots per inch (dpi), lines per inch (lpi), samples per inch (spi) and pixels per inch, or ppi (see glossary). The metric system may be used here, as in lines per centimetre (lcm). In computers, pixels per inch is a measure of the sharpness, or the density of illuminated points, on a display screen and the term "dots per inch", extended from the print medium, is sometimes incorrectly used instead of pixels per inch.