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The CIE - "La Commission Internationale de l'Éclairage"

The International Commission on Illumination (CIE) is an organisation devoted to international co-operation and exchange of information among its member countries on all matters relating to the art and science of lighting. Its membership consists of the National Committees in 39 countries and of 11 individual members. The objectives of the CIE are:

  • To provide an international forum for the discussion of all matters relating to the science, technology and art in the fields of light and lighting and for the interchange of information in these fields between countries.
  • To develop basic standards and procedures of metrology in the fields of light and lighting.
  • To provide guidance in the application of principles and procedures in the development of international and national standards in the field of lighting.
  • To prepare and publish standards, reports and other publications concerned with all the matters relating to the science, technology and art in the fields of light and lighting.
  • To maintain liaison and technical interaction with other international organisations concerned with matters related to the science, technology, standardisation and art in the field of light and lighting.

The CIE is recognised as the authority on all aspects of light and lighting. As such it occupies an important position among international organisations.

 
 


CIE colour systems

In 1931 the CIE developed the XYZ colour system, also called the "norm colour system." This system is often represented as a two-dimensional graph which more or less corresponds to the shape of a sail. The CIE colour standard is based on virtual primary colours XYZ (tristiumuls values) which do not exist physically. They were originally derived from the colour matching stimuli R, G and B (red, green, blue: 700, 546.1 and 435.8 nm) such that the XYZ values are positive for any colour which was not possible with RGB.

  • X = 0.49 R + 0.31 G + 0.20 B
  • Y = 0.17697 R + 0.81240 G + 0.01063 B
  • Z = 0.01 G + 0.99 B

X, Y and Z are, however, purely theoretical in contrast to colour gamuts such as RGB or CMYK (cyan, magenta, yellow, black). These primary colours have been selected such that colours which can be perceived by the human eye lie within their colour space. The XYZ system is based on the response curves of the eye's three colour receptors. Since these differ slightly from person to person, the CIE has defined a "standard observer" whose spectral response corresponds more or less to the average response of the population. This objectifies the colourimetric determination of colours. The three primary colours of the CIE XYZ reference system call for a spatial model with coordinates X, Y and Z, which can be drawn as a chromaticity triangle. To arrive at a two-dimensional diagram (see Fig. 1), this chromaticity triangle is projected into the red-green (X/Y) plane.
This is only meaningful, however, if appropriate standardisation is performed at the same time which allows the lost value Z to be read from the new two-dimensional model. This is achieved by introducing the chromaticity coordinates x, y and z.

  • x = X / (X + Y + Z)
  • y = Y / (X + Y + Z)
  • z = Z / (X + Y + Z) = 1 - x - y

A colour is not fully defined by its chromaticity (x and y). A brightness coefficient also needs to be specified as well.

 
CIE1931 xy colour plane
Fig. 1: The CIE1931 XY colour plane
  Fig. 2: The CIE1976 u'v' colour plane
     
  The CIE 1976 uniform colour space

Although the x, y chromaticity diagram has been widely used, it suffers from a serious disadvantage: the distribution of the colours on it is not uniform. This is illustrated in Fig. 3: Each of the short lines in this figure joins a pair of points representing tow colours having perceptual colour difference of the same magnitude, the luminances of all the colours being the same. Ideally these identical colour differences should be represented by lines of equal length. But it is clear that this is far from being the case, the lines being much longer towards the green part of the spectral locus, and much shorter towards the violet part, than the average length.
In Fig.4 a different chromaticity diagram is used, in which the lines of Fig. 3 are shown again. It is immediately clear that the variation in the length of the lines, while not eliminated, has been much reduced; in fact, the ratio of the longest to the shortest line in Fig. 4 is only about four to one, instead of about twenty to one in Fig.3. The chromaticity diagram shown in Fig.4 (and in Fig. 2) is known as the „CIE 1976 uniform chromaticity scale diagram", commonly referred to as the „u', v' diagram". It is obtained by plotting u' against v' , where:
The u', v' diagram is useful for showing the relationships between colours whenever the interest lies in their discriminability.
Chromaticity diagrams have many uses, but, as they show only proportions of tristimulus values, and not their actual magnitudes, they are only strictly applicable to colours all having the same luminance. In general, colours differ in both chromaticity and luminance, and some method of combining these variables is therefore required. To meet this need, the CIE has recommended the use of one of two alternative „colour spaces". The first of these spaces to be considered is the „CIE 1976 (L*u*v*) colour space" or the „CIELUV" colour space. It is produced by plotting, along three axes at right angles to one another, the quantities:



      For Y/Yn > 0.008856:                                                For Y/Yn <= 0.008856:
where u'n , v'n are the values of u', v' for the appropriately chosen reference white. Here, L* denotes the lightness of the colour, ranging from zero (black) to 100.
In order to calculate the „General Colour Rendering Index" colour distances in a similar colour space are used: The „1964 Uniform Colour Space". It is based on the u, v chromaticity diagram, which is very similar to the u', v' diagram (u=u', v=v' ). In this colour space the lightness is presented by W* ; U* and V* are again the chromaticity co-ordinates:

 
 
  Fig. 3: Uniformity within the CIE1931 XY colour plane
 
  Fig. 4: Uniformity within the CIE1931 u'v' colour plane
   
 

The CIE L* a* b* colour space

While the human eye at first registers three colour stimuli relating to red, green and blue light, in a further (brain-) processing stage three sensations are generated: a red-green sensation, a yellow-blue sensation and a brightness sensation. These sensations are used to develop the „complementary colour system" or „CIE L*a*b* colour system". It is based on differences of two elementary colour pairs: red-green, yellow-blue as well as the brightness „pair" black-white (Fig. 5). Hue and chroma are defined by the coordinates a and b which can have both positive and negative values. The third characteristic, brightness, is represented vertically by means of a brightness scale designated L* with scale values ranging from 0 (black) to 100 (white). The formulas for the transformation of xyz to CIE L*a*b* are:

     For Y/Yn > 0.008856:                                                For Y/Yn <= 0.008856:

and

 
 
  Fig. 5: The CIE L*a*b* colour space, based on the differences of two elementary colour pairs red-green (a) and blue-yellow (b). The brightness is taken into account by the L value that ranges from 0 (black) to 100 (white).
  8 CIE test colours
 

The General Colour Rendering Index

In 1965 the CIE recommended the procedure of measuring and specifying colour rendering properties of light sources, based on a test colour sample method. This rating consists of a General Colour Rendering Index (R
a) which may be supplemented by a set of Special Colour Rendering Indices. The derivation of the colour rendering indices is based on a general comparison of the length of colour difference vectors in the CIE 1964 Uniform Colour Space. To apply this method the resultant colour shifts for suitably chosen test colour samples must be calculated. In order to do that, first the CIE 1931 XYZ - values of various test colours must be determined for both the illuminant to be tested and a reference illuminant. The next step is to transform these tristimulus values into the 1964 CIE uniform colour space. A set of eight test-colour samples, specified by their spectral radiance factors, is given in the CIE reference. An RGB approximation of these colours is given in Fig. 6. Reference illuminant
At the committee meeting in Evian (1962) the CIE agreed that there is no need to include any reference illuminant realisable in practice in the specification. Nevertheless, the appraisal of colour rendering properties of a light source shall always be referred to a reference illuminant, which may be defined mathematically. This reference illuminant shall be of the same or nearly the same chromaticity as the lamp to be tested. Unless otherwise specified, the reference illuminant of light sources with a „correlated colour temperature" below 5000 K shall be a Planckian Radiator and from 5000 K onwards a series of spectral power distributions of phases of daylight. The „correlated colour temperature" is defined as follows: "The temperature of the Planckian Radiator whose perceived colour most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions". The chromaticity difference (
DC) between the lamp to be tested (uk, vk) and the respective reference illuminant (ur, vr) can be calculated as:

where u and v denote the colour coordinates in the 1960 CIE uniform chromaticity diagram. If the chromaticity difference between the lamp to be tested and the reference illuminant is greater than a given maximum tolerance of 5,4*10-3 µK-1 the resulting Colour Rendering Index may be expected to become less accurate.

  Fig. 6: RGB approximations of the 8 CIE test colours used to calculate the colour rendering index (Ra)
 

Different colour rendering under differnet white light sources

Yttrium aluminium borate (YAB) is a well-known host for metal ions. Rare-earth and transition metal doped materials can be used as lasers because of their outstanding optical properties and have been investigated. In YAB, The Y3+ ions are located in trigonal prismatic sites with D3 symmetry, the metal ion is co-ordinated to three oxygen atoms from the borate top and bottom layers, where the two triangles are slightly rotated against each other. The Al3+ sites show almost perfect octahedral symmetry. Upon doping, the rare earth ions replace yttrium ions whereas transition metal ions such as Cr3+ are substituted for Al3+. For most of the optical applications of the YAB system the luminescence properties of the doped materials are of importance. The co-doped materials Ho,Nd:YAB, Ho,Cr:YAB, Nd,Cr:YAB and Ho,Nd,Cr:YAB reveal interesting colour rendering properties. In this context it is their absorbance, which gives rise to their colour change. Holmuim and Neodymium sesquioxides already show slight colour differences between daylight and fluorescent tube illumination. This perceptual colour difference is enhanced drastically when these elements are used as dopants in the YAB host. Furthermore, their combination in the same host increases the variety of colours - an effect which cannot be achieved by simply mixing the oxide powders. Chromium shows strong absorptions in the visible range. In combination with Ho3+ and Nd3+, Cr3+ enhances the crystals’ hue. Ho:YAB, for instance, is yellowish brown in daylight and pink when illuminated with a type 840 fluorescent tube. If Cr3+ is added, the crystal will still look pink in the latter case but will be dark green in daylight. An example of the different chromaticity co-ordinates of the crystals is given in Fig. 7.

 

chromaticity of the doped YAB crystals under different white light sources

  Fig. 7: Ho,Nd:YAB crystal chromaticities in the CIE1931(xy) chromaticity plane. The numbers indicate the illumination with the different light sources: 1: fl. lamp type 32, 2: fl. lamp type 33 3: fl. lamp type 840 4: fl. lamp type 960 5: white LED, 6: tungsten lamp, 7: daylight CIE D5500
 

Crystal colours

The colours of Nd,Ho:YAB; Cr,Ho:YAB; Nd,Cr:YAB and Cr,Nd,Ho:YAB under illumination with seven different white light sources are shown in Fig. 8. The materials change their colour upon illumination with different white light sources so dramatically that they could be used for light source discrimination; the microcrystalline powders on the other hand could be used as novel coating materials with a colour changing effect that depends on the spectral distribution of the illuminating light source.
A very good overview and detailed descriptions on measuring colours are given in R.W.G. Hunts book "Measuring Colour", Ellis Horwood Publishers (1987) and in the technical reports obtainable at the CIE. More references on this subject and references on YAB can be found in the paper given below.

Literature:

Links:

 

crystals under different white light sources

  Fig. 8: Ho,Nd:YAB; Ho,Cr:YAB and Nd,Cr:YAB crystals under illumination with three different white light sources: a fluorescent lamp type 840, a tungsten lamp, and a white LED