DigitalView has been appointed as the Certified Service Partner for the Southern African Region
with their colour quality management systems, helping Marks & Spencer to monitor print quality.
Colour management addresses a fundamental problem with the way we use numbers to represent colour digitally. The colour modes we most often use - RGB and CMYK - don't specify colours unambiguously. Rather, they have their roots in the control signals that we send to our colour reproduction devices - displays, desktop printers, or even printing presses - to make them produce something that we then can see and interpret as colour. The problem is that each device will produce a different colour when we feed it the same set of RGB or CMYK numbers. So the scanned image doesn't look like the original when viewed on screen, and your final printed output doesn't look like either the original, or the on-screen rendition.
RGB and CMYK numbers instruct a colour reproduction device such as a display or printer how much of each colourant to use. With displays, RGB numbers tell the display to fire electron beams of a given strength for the red, green, and blue guns in the case of a CRT, or the strength of the red, green, and blue filtration in the case of an LCD. With printers, CMYK numbers tell the printer how much cyan, magenta, yellow, and black ink to lay down.
The actual colour that results depends on a host of physical factors. The colour produced by a display in response to a set of RGB numbers depends on both the displays tonal response and the actual colour of its red, green, and blue phosphors or filters. The colour produced by a printer depends on the colour of the paper, the way the paper absorbs ink, and the actual colours of the cyan, magenta, yellow, and black inks. All these properties vary, sometimes dramatically, from device to device. One monitor's red phosphor may be a tomato red while another's is closer to a fire-engine red. One printer's yellow ink may have a greenish cast while another's tends towards orange. The RGB and CMYK colour modes are often called "device-dependent" or "device-specific" colour in recognition of the fact that the actual colour produced by a set of RGB or CMYK numbers is dependent on the device to which the numbers are sent.
To accomplish this feat, colour management uses data files called profiles, and a software engine usually known as a CMM, or simply as the colour management engine. Device profiles describe the colour appearance generated by RGB or CMYK numbers on a specific device - a display, a printer, a scanner. The colour management engine, when asked to perform a conversion, changes the numbers that get sent to devices by using the description of the devices' behaviour in the profiles, so that the colour appearance is preserved.
What makes colour management possible, however, is a set of mathematical colour models that use numbers to represent the actual colour appearance that humans see, rather than the set of instructions a device needs to produce that colour. Because the colour specified in these models is unambiguous and doesn't depend on the quirks of any particular colour reproduction device, we call such models "device-independent" colour. Since these colour models represent the colour that we actually see, they act as a universal translator between different devices' RGB or CMYK values.
The numerical models used by colour management to represent colour appearance were developed by the Commission Internationale de l'Eclairage (CIE), the international standards organization charged with developing standards for all aspects of lighting. To use colour management effectively, you needn’t concern yourself with the details of these models - they have esoteric names such as ClE XYZ (1931) and CIE LAB (1976). All you need to understand is that these models represent the human experience of colour - the actual appearance - without reference to the particular recipe any given device requires to reproduce that colour.
Profiles are data files that record the relationship between RGB or CMYK numbers and the colour we actually see. Device profiles describe the behavior of a physical device or class of physical devices. For example, "Letitia Sanchez's Epson 2200 Premium Luster.icc" is a profile that describes the actual perceived colors that Letitia Sanchez's Epson 2200 printer produces on Premium Luster paper in response to different RGB values, while “Epson 2200 Premium Luster.icc" - a profile installed by the Epson driver - describes the average perceived colours that Epson 2200 printers produce on Premium Luster paper in response to different RGB values. Profiles built for a specific device are generally referred to as custom profiles, while profiles that describe an entire class of devices are usually called "generic" profiles.
Some profiles don't describe the behavior of a physical device. For example, profiles such as sRGB, or Adobe RGB(1998), simply describe standardized colour appearances for RGB values that are not dependent on the quirks of any particular physical device. They offer idealized environments for defining and editing RGB colour.
As mentioned earlier, colour management does only two things - specify a colour appearance, and attempt to match that colour appearance on different devices. Colour management cannot match a colour appearance until that colour appearance has been specified. Attaching a profile to a document, or an element in a document, simply specifies the colour appearance represented by the RGB or CMYK numbers.
Matching the colour appearance requires a conversion from a source profile (which specifies the colour appearance) and a destination or target profile, which supplies the new RGB or CMYK numbers required to make the destination device produce the specified colour appearance. On request, the colour management engine converts the source RGB or CMYK values to the destination RGB or CMYK values, so that the colour appearance is preserved.
In addition to the source and destination profiles, a conversion also demands specification of a rendering intent. Rendering intents are simply methods of handling out-of-gamut colours - that is, colours present in the source that the destination is physically incapable of reproducing. Colour management offers four different rendering intents.
Perceptual rendering fits all the source colours into the colour gamut - the range of reproducible colours - of the destination, preserving the overall relationships between colours, and hence the overall colour appearance. It's typically used for photographic images that contain important out-of-gamut colours.
Saturation rendering converts saturated primary colours in the source to saturated primary colours in the destination, even if the hues are somewhat different. It's typically used for business graphics such as charts and graphs.
Relative colorimetric rendering converts white in the source to white in the target - we judge colours in relation to what we see as white, so prints look “wrong” if the white in the source is rendered as something other than paper white in the destination - then adjusts all the in-gamut colours accordingly, and dips out-of-gamut colours to their nearest printable hue. It's typically used for flat colour such as vector graphics as well as for photographic images that don't contain important out-of-gamut colours.
Absolute colorimetric rendering reproduces the colour of the source white in the output. It's primarily useful for proofing, since it enables side-by-side matching of colours on different stocks such as glossy and newsprint. For example, if you want to make an inkjet printer emulate newsprint, absolute colourimetric rendering makes the inkjet lay down some yellow and black ink in the paper-white areas to simulate the grayish-yellow tone of the newsprint as compared to the brighter white inkjet paper.
Mellow Colour are making waves with their colour quality management systems, more>>>
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