by Mike McNamee Published 01/02/2010
Metameric Index or Colour Constancy Index
The first question is, why should we care? Well for starters, the eye is very sensitive to greys that are not neutral and is most sensitive of all in the mid- to three-quarter tones (ie mid-grey up to very light grey). For this reason we have always reported our metameric index as that measured on a neutral 50% grey swatch, going between tungsten light (called Illuminant A) and daylight (as defined by so-called D65 conditions). Given that greys are the most sensitive tones, the most common observation is that a poor monochrome inkjet print looks magentabiased in your tungsten-lit living room, turning to a rather unpleasant olive-green when you carry the print into the daylight-lit conservatory. Although we tend to call the effect 'metamerism' it is more correctly termed 'colour inconstancy'. This is because the print is being viewed on its own, metamerism is, more correctly, the difference you see between two colours which match in one room, but not in the other. To put that another way if a print is mounted on a grey card and the greys of the print match the grey card mount in one room, but not in another, that is metamerism. Colour Inconstancy is more complex because it is the perception of neutrality in one room and the lack of it in the other, ie only one tone is needed - a print without a mount.
As you might imagine, the scientists at Epson, Canon, Hewlett Packard and a host of other research institutes have laboured long and hard to reduce colour inconstancy and although it is an inevitable feature of using multiple inks to create a gamut of colours, the ability to mess about with the ink mixtures, as they are fired onto the page, has enabled the scientists to optimise Look Up Tables (LUTs) for colour constancy. As users what we see is a continual improvement, stretching forward from the Epson 2000P (and what a dog that was!) to the present-day UltraChrome ink sets with CII values of four- and five-fold better performance.
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