Ultra-Brief History of Colour Management

In The Beginning

In 1861, James Clerk Maxwell produced the first "trichromatic" colour photograph using his famous insight that a full spectrum of colour could be reproduced by the admixture of red, green and blue light. The a priori reason for this is rooted in the physiology of the human eye. Cone cells in the human retina are sensitive to red, blue and green light and generate electronic nerve impulses to the brain through a complex, little understood mixing process which results in the sensation of colour. Maxwell's discovery is at the root of all modern imaging systems.

In 1931 the C.I.E. (Commission Internationale de l'Eclairage) standardized illuminants for judging colour ("D65" daylight) and through a carefully controlled process actually plotted numerically every colour in the visible spectrum using averaged results from a group of human observers with excellent colour vision - the "standard observer." This work led to the modern "colour space" in Photoshop® known as LAB colour.

Modern Colour Management

LAB colour is the basis of the "colour engine" that Adobe Photoshop® and other photographic programs use to translate and interpret the colour produced by various devices. Device colour has also been standardized in ICC (International Colour Consortium) profiles, text files which describe device colour for use in colour management systems.

During the 90's Photoshop used a "device" colour space based on a colour computer monitor. Photographers and others wishing to get accurate colour would print out a file and then compare the printed result with the image of the file on their monitor. Their individual monitors were then adjusted to closely match the printed output. This worked fairly well but as everyone's monitor was slightly different, anyone dealing with files from different sources (e.g. printers) experienced difficulties, and typically readjusted colour on their own monitors, which they knew from experience they could trust. Another problem was that

although Photoshop had introduced some colour management capability, it was little understood by average users and also the colour space, being based on a monitor, turned out to be inferior to the expensive "closed-loop" drum scanning systems printers and service bureaus used. These sophisticated traditional systems used a larger colour space than the early Photoshop colour space and created superior CMYK colour separation files for print.

Adobe also took notice of these inferior results and fixed the problem with the introduction of the Adobe RGB (1998) colour space, which was a much larger colour space (ie. it could describe a broader range of the visible spectrum). Another aspect of this colour space was that it was device independent, not based on the limited abilities of any monitor to reproduce colour. Since this development, photographers and designers have had the tools to match the best drum scanner separations.

Cameras that can capture files in the Adobe RGB (1998) colour space therefore have the capability of capturing a broad range of colour ideal for converting to CMYK print files or to sRGB files for the web and other screen-based presentations.

Two Common Mistakes Regarding Adobe RGB (1998) Files

Although Adobe RGB (1998) files are the best practical photographic master files for all purposes, they are often misunderstood, which leads to two mistakes:

1) JPEG files for the web should never be made from Adobe RGB (1998) files directly. Always convert to sRGB (a smaller colour space suitable for the web and screen-based presentations) first. Then make your jpegs.

2) In the conversion process to sRGB, never "Assign" (Photoshop language) sRGB to files originating in the Adobe RGB (1998) space. This is the equivalent of "lying" to your colour management system, and will result in desaturated colour and degraded contrast. Always "Convert" (Photoshop language) the file.

--- Don Cooper

The Adobe RGB 1998 Colour Space (above) plotted in LAB, as compared to the much smaller monitor-based sRGB Colour Space seen below. Ambiguous RGB values like R 255, G 0, B 0 are mapped to unambiguous LAB values in these colour spaces by the "colour engine" in colour management systems. As the Adobe RGB 1998 Colour Space is much larger than sRGB, many of its colours will be "out of gamut" (beyond sRGB's ability to describe). This problem is solved by rendering intents.

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