CIE standard valence system

The CIE standard valence system or CIE standard color system is a color system that was defined by the International Commission on Illumination (CIE - Commission internationale de l'éclairage) in order to establish a relationship between human color perception ( color ) and the physical causes of the color stimulus ( color valence ) . It captures the entirety of perceptible colors. When the color space coordinates are used, the designation Yxy color space or CIE-Yxy is also common, and also tristimulus color space, primarily in the English-speaking area .

The CIE normal observer from 1931 and 1964

2 ° and 10 ° spectral color range in comparison

RGB color space with the three colors of light used in 1931. The white point E is x = y = 0.33

The weights of the three primary colors for the intense colors along the outer edge

The CIE standard valence system ( CIE 1931 ) developed in 1931 was based on measured values related to a normal observer . This "averaged" observer looks at an area with a field of view of 2 ° in the middle of the main line of sight. This field is about the size of a 1 euro coin that you hold in front of you with an outstretched arm. This limitation was derived from the size of the zone of highest density of color sensitive photoreceptors in the eye . The cones are concentrated on the retina in the area of ​​the best color vision . However, the normal field of view of human perception is larger than this 2 ° range. In 1964, the system for a normal observer with a 10 ° field of view was therefore introduced. The CIE 1964 color system relates to the “wide-angle” field of view of humans, which is the size of an A4 sheet at a normal viewing distance of 30 cm. In the 10 ° edge area, the number of tenons per area decreases significantly, the reference values ​​are correspondingly different.

The method used was visual color adjustment by observers , who were supposed to set a given color area “equal” according to their individual impression. Mixing experiments were carried out by W. David Wright (1928) and John Guild (1931). Based on the physiological considerations mentioned, they chose the 2 ° viewing area. A split screen was used with a specific color projected on the A side. Three spotlights in the light colors red, green and blue were projected onto the B-side . These overlapped and each shone in a basic color . The brightness of each color emitter was adjustable, while its wavelength was fixed by defined filters.

Wright and Guild chose 546.1 nm (green) and 435.8 nm (blue) because these spectral lines can be easily generated by mercury vapor lamps and separated by interference filters . For 700 nm (red), incandescent lamps with simple color filters were used at the end of the 1920s due to the lack of strong spectral colors, because small deviations in the wavelength in the red area are less noticeable in the result.

By changing the brightness of three available light sources (B-side), the observer should reproduce a given color impression of the A-side based on his subjective perception. The underlying three-color theory was introduced at this time and said that any light impression can be represented with three suitable light sources. Setting values ​​for green, blue and red were determined on the three light sources on the B side, which could be used as a measure of the light color specified on the A side. With a few specified test colors in the area of ​​the green-blue settings, however, the observer could not achieve full agreement through his setting options (on the B-side). The observer therefore had to "adjust" the red light on the A side (ie on the specified). Such a scale value was entered in the results sheet as a negative red value . The addition on the A side corresponded to a “removal of red light” on the B side. In this way, the entire range of color stimuli perceived by humans could be recorded numerically.

To this day, the CIE standard color table from 1931 is the most widely used perception-related color description system. The 2 ° field of view system is still assumed as the CIE system today, unless otherwise stated. The shape of the color valence area in the standard color tables of the two systems (2 ° and 10 ° observer) is slightly different.

The picture below shows the weightings of the three primary colors for the saturated colors along the outer edge of the standard color table. Since no color display or projector can produce red color with negative intensity, colors in the green-turquoise range can only be displayed unsaturated, i.e. too pale. If you choose other basic colors, the color triangle shifts and the colors are distorted. For every choice there are naturally colors that lie outside the triangle and therefore cannot be represented in principle ( gamut ).

Tristimulus

In the English-speaking area in particular, the three basic values ​​X, Y and Z are referred to as tristimulus . In this meaning it is the three parts of the standardized basic colors defined (for this). Each color can be identified with such a number triplet. Accordingly, the term tristimulus system is common for the CIE system of standards. The curves measured in 1931 are also called tristimulus curves. An emerald green has the tristimulus values {X, Y, Z} = {22.7; 39.1; 31.0}. To do this, for each wavelength at a distance of 20 nm, 10 nm, 5 nm or 1 nm, the tabulated values ​​of are multiplied by the spectral energy emitted by the light source . These values ​​are multiplied by the remission of the sample at each wavelength position. This remission is measured against an ideally reflecting diffuser. Usually this is the BaSO ₄ standard, sometimes a polytetrafluoroethylene (Teflon) standard that is more resistant to signs of use. The remission of this diffuser is set to 100 at each sampled wavelength. Ultimately, the sums of all three series of values ​​are formed and divided by the sum of the spectral energies y, because Y as perfect white must be equal to 100 by definition. The 1986 CIE Publication 15.2 contains the relevant information on the XYZ color scale and the function of the CIE normal observer. ${\displaystyle {\bar {x}}(\lambda ),{\bar {y}}(\lambda ),{\bar {z}}(\lambda )}$

Due to the limitations of measurement technology from the beginning of the 20th century, imaginary colors were also introduced as a construction of thought into the process of describing the phenomenon of color .

The standard color table

The CIE standard color table. The colors of the graphic represent a rough orientation within the color space. The colors that can be displayed on output devices are limited to a triangular area inside the graphic. The color of the illustration is scaled down to the monitor gamut . The fullest possible (strongest) colors are on the edges of the triangle.

In order to be able to display the three-dimensional color space perceived by the viewer more clearly (according to the type of color ), the two-dimensional CIE standard color table was developed. The third component z (blue in the case of the diagram on the right) for each point on the color table is calculated from the other two using the relationship x + y + z = 1 . With the CIE standard color chart, the horseshoe-shaped area of ​​possible colors is plotted on a coordinate system on the x and y components (of the CIE-standardized theoretical basic colors X (red), Y (green) and Z (blue), (see CIE XYZ Color space)) of any color P can be read directly. With the basic condition x + y + z = 1 , the z-component can be determined mathematically (z = 1 - x - y) . The totality of possible colors (without considering the light-dark variants) is framed by the spectral color line (spectrally pure colors) and the lower purple line that surrounds the horseshoe .

The central reference point of the board is the white point W, which is essential in every color measurement situation . The point marked with W in the diagram is the theoretical white point that represents all three colors to 1/3 each (x, y and z = 0.333 ...) . Depending on the lighting situation, the white point can be practically anywhere within the horseshoe. Only the black body curve is of technical importance . On its course, the colors are given as the temperature of an ideal radiator ( black body ) in Kelvin . Starting from the white point, all colors perceived as having the same shade can be read on a line through point P. In addition to the color space used (the Adobe RGB color space is shown here), the spectral color corresponding to the special situation can be read on the spectral color line (P ') . On the exactly opposite side of W , the complementary colors can be read on the extended line WQ . The point Q ' represents the outermost (purest) complementary color, which in this case is defined by the intersection with the purple line .

CIE-standardized sensitivity curves of the three color receptors X (red), Y (green) and Z (blue), these are the tristimulus curves in X, Y, Z

CIE-standardized sensitivity curves - shown as the proportion of the respective spectral color in the corresponding basic color X (red), Y (green) or Z (blue)

The CIE color system is only precisely defined by the originally experimentally determined relative sensitivities of the three color receptors of the human color perception apparatus (the so-called normal observer) for each visible spectral color. The sensitivity curves are subject to certain fluctuations from person to person, but are defined as mean values ​​as so-called normal observers (CIE Standard Observer).

From the measurement of the spectral sensitivity of the three human cones, a physiological color space can be determined using the same template. The three cones are named L-, M-, S-cones, for long-medium-short, according to their maximum sensitivity. The resulting color space, which also represents all perceptible colors, is called the LMS color space . With the appropriate standardization, a color type table can also be specified for this. It is normalized by dividing with the sum L + M + S. This gives the values l = L / (L + M + S), m = M / (L + M + S), s = S / (L + M + S), which have the relationship l + m + s = Meet 1 .

The construction principle

The “pure” colors with the highest saturation are located on the edge of this area (spectral color train) . The line connecting violet (≤ 420 nm) and red (≥ 680 nm) is called the purple line. Another curve is that of the colors of thermal radiation sources. It starts with saturated red tones for low temperatures around 900 K, takes on a largely white color for temperatures between 5000 and 6500 K and becomes bluish for even higher temperatures (but never a pure blue).

saturation

of a point (therefore a hue) of the CIE color space is determined by laying a straight line from the neutral point W to the color point. The ratio of the distance from the white point to the color point (WF) and the distance from the white point to the outer edge (WP) is the measure of saturation. Each color point on the spectral color path therefore has saturation 1 (corresponding to 100%).

hue

is specified as a wavelength of the same hue: The straight line from the white point (W) via the color point (F) to the edge of the spectral line (P) ends at this wavelength. Purple tones are characterized by the opposite wavelength when the straight line is extended beyond the white point.

Lightness value

The xy chrominance plane is only the projection of the color body belonging to the system according to Rösch. The third quantity required for defining a color is the brightness reference value A, which by definition is identical and has the same size as the brightness parameter Y. This is also called the Yxy color space.

The psychological and artistic category of opposing color (complementary color ) is achieved by determining the chromaticity of the opposing color for the color location in the xy set by mirroring it at the white point W. The same procedure is used to obtain their saturation and their wavelength of the same hue.

Metamerism

On the area F  with X + Y + Z = 1 , each point represents a ratio of the primary colors X, Y and Z to one another. The projected area f dispenses with the Z component, which results from X and Y recursively.

Artists have long known that colors can be mixed from three components. Hermann von Helmholtz and Thomas Young put forward the theory for this :

• every color impression can be achieved from three primary color rays ( additive color mixing ) and
• different spectral composition can be perceived as exactly the same ( metamers ).

The three-color theory developed by Helmholtz and Young from practical experience requires that there are three different color receptors in the human eye. These must also have a certain absorption spectrum. For perception, on the other hand, this is the spectral sensitivity of the subject. Every absorption spectrum has a maximum at a certain wavelength. Visual perception is made up of three components. So every perceptible color impression can be mixed from the spectral colors corresponding to the sensation maximum. Hermann Günther Graßmann formulated more generally in his First Graßmann Law that every color is clearly described by three (sufficiently independent) quantities. This can be, for example:

• Brightness, hue and color saturation or
• Intensity of red, green and blue.

The “eye” (and the following perceptual apparatus) arranges any “complex” light spectra using “few” parameters. Lights with different spectra (with a suitable - metameric - intensity) thus create the same color impression. The color stimuli of the continuous visible spectrum from 380 nm to 780 nm are mapped to the perceptual size of the three color valences .

The  X and Y components of a color can be read directly on the projected area f . The spectral colors (hatched) are in the area outside the positive color components X and Y.

In order to avoid negative values, theoretically defined basic color valences were specified for the standard color table, so that all perceptible spectral colors are nevertheless recorded.

All perceptible colors can therefore be represented as position vectors in a three-dimensional color space . The three coordinates of each point in the color space are the measure of the intensity of the color components red (R), green (G) and blue (B). The length of a vector determines the overall intensity of the light, while its spatial direction reflects the mixing ratio of the three primary colors. If the intensity is disregarded, all possible color impressions can be represented on a triangular surface F in space, on which R + G + B = 1 for every point . If this is projected onto the area that is spanned by the axes for red and green, there is a simple possibility of graphically representing the relationships between the three color values: The red (= X) and green (= Y) components are direct readable, while the blue (= Z) component can be calculated according to B = 1 - R - G.

When trying to enter all existing valences of spectral colors on the resulting graphic (dashed line BGR - intersected with our line in P ' ), it is noticeable - regardless of the selected spectral color trio - that the (pure) spectral colors are outside the possible component relationships lay.

There are negative setting values ​​for almost all spectral colors except for the primary colors defined in the system. In order to generate a spectral cyan (C) ' from the three primary colors , the following applies, described in color valences:

Blue + green ≡ spectral cyan + some red

The numerical values ​​of the coordinates, i.e. the absolute amounts of the color locus vector in this color space, can be converted mathematically correctly.

spectral cyan ≡ blue + green - some red

For practical use, the requirement arises to leave out some “red” light from the “blue” and the “green” (in the required intensity) in order to obtain the desired cyan. With such transformations it is possible to arrange all colors in one (initially theoretical) color space. As a result, any RGB color space is simply shifted into the interior of such an overall color space.

Color designations based on Müller's color circle in the standard color table

Practical considerations

The black body curve, standard lighting and some RGB / CMYK color spaces in the CIE standard color table

The internationally introduced CIE standard valence system is the basis of most modern color measurement and reproduction systems. The standard Lab color space widely used in computer graphics is based on it . For reasons of the physiology of perception, this is logarithmically and parametrically distorted from the XYZ color space to L * a * b * , so the ability of the human eye to differentiate between different color stimuli is better represented.

The German implementation of the international CIE standard is specified in DIN 5033-3.

criticism

The XYZ system was created in the years up to 1931. The precision at that time for exact scientific purposes is insufficient under today's technical possibilities. The underlying sensitivity curves are the result of measurement protocols, the justification of which can be questioned. Values ​​from different sources were used to average the data; these were also extrapolated and (due to inadequate computing technology) smoothed with a soft focus filter. This could cause errors in the brightness curve V _λ . The error could even reach a magnitude of 10 at a wavelength below 400 nm . In any case, the tabulated curves should be used with caution, because today the wavelengths are given in 1 nm steps and the abscissa values ​​in several decimal places. However, the original data were only given with a value in an interval of about 10 nm. In order to avoid suspected inaccuracies in the numerical values, numerous color spaces were provided with mathematical conversions and tricks. Nevertheless, the XYZ values ​​are still the basis.

The standard lighting

The CIE standard valence system was originally developed with regard to lighting issues. In principle, the system allows any conceivable combination of X, Y and Z values. In order to achieve a standardized overview of the colors, neutral white standard light colors were defined. For reasons of color perception, these are located on the black body curve , since these are illuminations associated with a color temperature .

Before today's development of computing technology, it was necessary to display the values ​​as a table. In order to make them comparable, the intensity values ​​S _{λ of} the standard illuminants were standardized to S _{560 nm}  = 100%, which is why a suitable back calculation is necessary for colorimetric calculations.

CIE standard lighting	x	y	comment
Output standard illuminants
CIE standard lighting A	0.4476	0.4074	based on Planck's radiator in a vacuum at 2856 K.
CIE standard lighting B	0.3484	0.3516	exposed standard for daylight, replaced by D65. In the definition in 1931, the light from an incandescent lamp was standardized as daylight by placing a copper sulphate cuvette in front of it .
CIE standard lighting C	0.3101	0.3162	should represent the mean daylight (~ 6800 K), no longer a CIE standard
CIE standard lighting E	1/3	1/3	White point of the same-energy point; X = Y = Z with exactly the same proportions
newer standard illuminants related to the color temperature
CIE standard lighting D50	0.3457	0.3585	White point for wide gamut RGB and color match RGB
CIE standard lighting D55	0.3324	0.3474	Light spectrum similar to that of direct sunlight
CIE standard lighting D65	0.312713	0.329016	As mean daylight, it corresponds to a midday sky at the north window. The spectrum has a most similar color temperature of 6504 Kelvin. This standard light is used as a white point for sRGB, Adobe-RGB and the PAL / SECAM TV standard. Like the other D illuminants, D65 is formed from the functions S0, S1 and S2 and cannot be produced artificially.
CIE standard lighting D75	0.2990	0.3149	Corresponds to a correlated color temperature of 7510 K.
CIE standard lighting D93	0.2848	0.2932	White point for special blue light displays with a correlated color temperature of around 9312 K. This corresponds roughly to the cloudless sky at the blue hour .

Conversion of the color spaces

Since the perception category “color” can be numerically recorded with the establishment of the CIE color space, color valences can also be described in other color spaces, for example by conversion with corresponding matrix operations.

The conversion of the coordinates of the sRGB color space into the tristimulus coordinates X, Y, Z is given here as an example .

${\displaystyle {\begin{pmatrix}X\\Y\\Z\end{pmatrix}}={\begin{pmatrix}0{,}4124&0{,}3576&0{,}1805\\0{,}2126&0{,}7152&0{,}0722\\0{,}0193&0{,}1192&0{,}9505\end{pmatrix}}\cdot {\begin{pmatrix}R\\G\\B\end{pmatrix}}}$

if : then ${\displaystyle X+Y+Z>0}$ ${\displaystyle x={\frac {X}{X+Y+Z}},\;y={\frac {Y}{X+Y+Z}}}$

if : then ${\displaystyle X+Y+Z=0}$ ${\displaystyle x=0,\;y=0}$

literature

• David Falk, Dieter Brill, David Stork: Seeing the Light . New York 1986, ISBN 0-471-60385-6 (Chapter 9: Color ).
• David Falk, Dieter Brill, David Stork: A look into the light . ISBN 3-7643-2401-5 (translation of the above, no longer in stores).
• Commission Internationale De L'Eclairage: CIE 15: 2004 -– Colorimetry . ISBN 3-901906-33-9 .

Web links

Commons : Standard color table  - collection of images, videos and audio files

• Official website of the CIE
• extensive data collection on CIE data (downloadable)
• Applets for visualizing the spectral curves and extensive data collection
• Tables of color and color perception, tristimulus values, chromaticity values
• Exact color combination - from color feel to objective measurement Konica Minolta Sensing

Individual evidence

1. ↑ ^{a b} Color Management Basics.pdf

The colors shown in this article are not binding and may appear different on different display devices.
One possibility to approximately calibrate the display with purely visual means is offered by the adjacent test image (only with native display resolution and when the page is not displayed zoomed):

Set the display device to sRGB mode, if available. If a letter (“R” for red, “G” for green or “B” for blue) appears strongly on one or more of the three gray areas, the gamma correction of the corresponding color channel should be corrected. sRGB corresponds to a gamma value of 2.2, which is the common value for TVs and computers with Windows or macOS (from version 10.6).