CIE system of colorimetry

For about 100 years there has been an international system for colour specification – it’s called the CIE system. The acronym comes from Commission Internationale de L’Eclairage.

This system is based on the notion of additive colour mixing – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/

Since it is possible to mix together three primary lights and make a wide gamut of colours (though not, of course, all colours) the principle is that the amounts of these primaries that an observer would use to mix togther to match a colour is a useful specification of that colour. We refer to these amounts as tristimulus values. One could imagine a visual colorimeter whereby an observer would try to match a colour that is to be specified by adjusting the intensities of three primary lights that are mixed together – once a match is obtained then the tristimulus values would define or specify the colour. All that would be necessary would be to able to decide on a set of primaries and manufacture the visual colorimeters so that they are very consistent from one device to the next. It would be a little clumsy though to have to use one of these visual colorimeters. But in principle it could work.

Fortunately the CIE does not require the use of such visual colorimeters since in 1931 the CIE measured the trismumulus values that observers made when matching various colours. These were averaged to create the so-called CIE standard observer.  And here’s the really clever bit. Having defined the CIE standard observer it is possible to calculate the tristimulus values (the amounts of the three primaries that an observer would use to match a colour) without any further observations. All that is required is that we know the amount of light at each wavelength reflected by a sample or (in some cases) emitted from a device such as computer display and then – by using our knowledge of the CIE standard observer – it is possible to calculate the tristimulus values.

So what were the primaries. If you have read my previous post, What is a colour primary – http://colourware.wordpress.com/2009/07/08/what-is-a-colour-primary/ – you’ll know that the choice of colour primaries is somewhat arbitrary. Well, in fact the original determination of the standard observer what carried out in England using red, green and blue primaries. But the data obtained were later modified to refer to a different set of primaries known as X, Y and Z. It was necessary to make this adjustment because using any set of real primaries it was impossible to match any colour with mixtures of the primaries; using RGB meant many colours could be matched, but not all. So a set of so-called imaginary primaries was conceived which could – in theory – be used to match all colours. So the tristimulus values of the CIE system are known as X, Y and Z. 

In fact, it didn’t really matter which set of primaries was used; the CIE system was concerned with colour matching. If two samples have the same tristimulus values then they would be a visual colour match no matter which set of primaries was used. So the choice of primaries really was not critical.

Today many instruments are commercially available – colorimeters, reflectance spectrophotometers, radiometers) – that, with the use of software, allow the CIE XYZ values to be measured; these instruments are extremely valuable in many industrial and commercial applications. The CIE system is still very much alive today, though many users often prefer to use one of the more advanced colour spaces – such as the CIELAB colour space – which was defined by the CIE in 1976 and whose values are very easily calculated from the CIE XYZ values.  For further information about the CIE please visit their web site – http://www.colour.org/

4 thoughts on “CIE system of colorimetry

  1. Hi!

    this is the web address of the CIE: http://www.cie.co.at/

    About the primaries, they were not so arbitrary. Of course they always are, but the point of the XYZ primaries was to avoid the negative values necessary to give coordinates in the CIE RGB system when a sample to match is outside of the CIE RGB gamut.

    The example of the visual colorimeter you give is very real: there are some RGB colorimeters like this!

    Another thing the 1931 XYZ system did was to make imaginary primaries such as X & Z dont contain any luminance: all the luminance is “contained” by the Y primary. So Y is also a direct measure of the luminance of the colour.

    A

    1. You’re right about the two design elements in the XYZ primaries; they both wanted to ensure that Y corresponded to luminance and that XYZ were always all-positive. However, there is still an infinite set of possible candidates for XYZ that could be chosen that would meet these criteria. My main point was that, although this is true, it doesn’t matter.

      1. yeah that’s right! in 1964 CIE made the new U’V’W’ primaries, they have the same characteristics, but they generate a bit more uniform colorspace.

        I always wonder why we don’t use them instead of XYZ… But true, it doesn’t make much difference, and no difference at all for most applications.

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