Saturday, January 12, 2013

The CIE System[1-2]
Art Resource

Marie-Therese Wisniowski

Welcome to the New Year. I hope it will be an enjoyable one for you - whenever it begins!

This is the eleventh blog in the "Art Resource" series, specifically aimed to construct an appropriate knowledge base in order to develop an artistic voice in ArtCloth. Other posts in this series are:
Glossary of Terms
Units Used In Dyeing And Printing Of Fabrics
Occupational, Health & Safety
A Brief History Of Color
The Nature Of Color
Psychology of Color
Color Schemes
The Naming Of Colors
The Munsell Color Classification System
Methuen Color Index And Classification System
Pantone - A Modern Color Classification System
Optical Properties of Fiber Materials
General Properties of Fiber Polymers and Fibers - Part I
General Properties of Fiber Polymers and Fibers - Part II
General Properties of Fiber Polymers and Fibers - Part III
General Properties of Fiber Polymers and Fibers - Part IV
General Properties of Fiber Polymers and Fibers - Part V
Protein Fibers - Wool
Protein Fibers - Speciality Hair Fibers
Protein Fibers - Silk
Protein Fibers - Wool versus Silk
Timelines of Fabrics, Dyes and Other Stuff
Cellulosic Fibers (Natural) - Cotton
Cellulosic Fibers (Natural) - Linen
Other Natural Cellulosic Fibers
General Overview of Man-Made Fibers
Man-Made Cellulosic Fibers - Viscose
Man-Made Cellulosic Fibers - Esters
Man-Made Synthetic Fibers - Nylon
Man-Made Synthetic Fibers - Polyester
Man-Made Synthetic Fibers - Acrylic and Modacrylic
Man-Made Synthetic Fibers - Olefins
Man-Made Synthetic Fibers - Elastomers
Man-Made Synthetic Fibers - Mineral Fibers
Man Made Fibers - Other Textile Fibers
Fiber Blends
From Fiber To Yarn: Overview - Part I
From Fiber to Yarn: Overview - Part II
Melt-Spun Fibers
Characteristics of Filament Yarn
Yarn Classification
Direct Spun Yarns
Textured Filament Yarns
Fabric Construction - Felt
Fabric Construction - Nonwoven Fabrics
A Fashion Data Base
Fiber Construction - Leather
Fabric Construction - Films
Glossary of Colors, Dyes, Inks, Pigments and Resins
Fabric Construction – Foams and Poromeric Material
Glossary of Fabrics, Fibers, Finishes, Garments and Yarns
Weaving and the Loom
Similarities and Differences in Woven Fabrics
The Three Basic Weaves - Plain Weave (Part I)
The Three Basic Weaves - Plain Weave (Part II)
The Three Basic Weaves - Twill Weave
The Three Basic Weaves - Satin Weave
Figured Weaves - Leno Weave
Figured Weaves – Piqué Weave
Figured Fabrics
Glossary of Art, Artists, Art Motifs and Art Movements

The Glossary of Terms, Timelines of Fabrics, Dyes and Other Stuff, A Fashion Data Base, Glossary of Colors, Dyes, Inks, Pigments and Resins, Glossary of Fabrics, Fibers, Finishes, Garments and Yarns and Glossary of Art, Artists, Art Motifs and Art Movements have been updated in order to better inform your art practice.

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The Art Resource series will be the first post in each calendar month. Remember - these Art Resource posts span information that will be useful for a home hobbyist to that required by a final year University Fine-Art student and so undoubtedly, some parts of any Art Resource post may appear far too technical for your needs (skip over those mind boggling parts) and in other parts, it may be too simplistic with respect to your level of knowledge (ditto the skip). The trade-off between these two extremes will mean that Art Resource posts will hopefully be useful in parts to most, but unfortunately may not be satisfying to all!

Color is a critical component in creating art. In particular, before we can gain an insight into dyeing and printing on textiles we need to have a rudimentary knowledge of color systems.

There are numerous color specifying systems available to the dyer, each have particular strengths and weaknesses. Many are provided by companies, and some by government authorities to coordinate a simple system for their national chemical industries. Today we shall place the spotlight on the CIE system.

The CIE System
The CIE system is named after the Commission Internationale de L’Eclairage (the International Commission of Illumination), which developed it. The Commission was set up by a number of European countries in order to arrive at an objective (rather than subjective) means of specifying color. The CIE system specifies color according to the proportion of primary additive colors (blue, green and red) that is required to produce a particular hue.

The development of computers has enabled the CIE system to be utilized and applied more effectively and comprehensively for the color matching of textiles, dyes and other materials. Whilst the The Munsell Color Classification System is a subjective system, whereas the CIE system is an objective system by design.

Historical Development Of The CIE System
In 1802, Thomas Young, an English physician postulated that the human eye has three basic color receptors: one receptor for blue, one for green and one for red. In 1867, Hermann von Helmholtz, a German physicist and physiologist extended Young’s theory. In 1871, James Maxwell, a Scottish physicist using Young’s and Helmholtz theories, developed them to arrive at the principles of color photography.

Young’s original concepts have in part been confirmed physiologically. The cones in the retina have been found to be specifically sensitive to lightwaves of about ~450 nm (blue), ~540 nm (green) and ~680 nm (red). In 1931 CIE developed the system of color specification based on these three primary colors.

The Basic Principles Of The CIE System
If you take the three primary colors - blue, green and red - and add them in equal proportion you will get white. We can write this down as a simple equation:

33.3%Blue + 33.3%Green + 33.3%Red = 100%White

Since any hue depends on the amounts of blue, green and red, we can re-write the above equation in the more general form:

zB + yG + xR = 100%C

where z represents the percentage of BLUE (B), y represents the percentage of green (G) and x represents the percentage of red (R). Here C is 100% of the desired hue we are aiming for. This is the basic equation used by color physicists who employ the CIE system to specify a given hue or to define the chromaticity of a dyed or printed textile.

For example, we can generate a particular type of "green" hue as follows:

13%B + 68%G + 19%R = 100% of a particular green hue. (i)

For a particular type of "red" hue we may employ:

9%B + 25%G + 66%R = 100% of a particular type of "red" hue. (ii)

You will notice that to get 100% of a particular hue, the percentage of blue (B), green (G) and red (R) must always add up to 100%, otherwise you will get a color that is not a “pure” color or hue. Hence we shall label the percentage of blue as “z”, the percentage of green as “y” and the percentage of red as “x”. We can create the general equation:

zB + yG + xR =100% of a particular hue. (iii)

To graph these percentages pictorially we need to know the x, y and z percentages. These are known as the chromaticity co-ordinates and a graph of these percentages is called the chromaticity diagram.

As x + y + z must always sum to 100%, then if we know the percentages of x and y, we must also know the percentage of z. For example in equation (i), x and y totals 87% and so z must be 13%, otherwise x, y and z cannot total 100%. In equation (ii), x and y total 91% and so z must be 9%, otherwise x,y,z cannot add up to 100% etc.

Hence, we only need to graph x and y since we will always know the value of z for each given x and y pair. Remember, x is the percentage of RED (R) and y is the percentage of GREEN (G). In the figure below the percentage of RED (R) is the x-axis (the horizontal axis) and the percentage of GREEN (G) is the y-axis (the vertical axis). A chromaticity diagram only needs those two axes.

A chromaticity diagram with the vertical axis represents the percentage of GREEN (y axis) in a hue, and the horizontal axes represents the percentage of RED (x axis) in a hue. Once both of these percentages are known, then we know the percentage of BLUE (z = 100 - x - y) in a hue, since all three percentages must add up to 100% of a particular hue.

The way in which this graph is plotted is beyond the knowledge of most people who have not done Chemistry in their senior high school years and so we shall concentrate on how to read these graphs rather than how to make them.

A general chromaticity diagram is given in the figure below. The main feature of such a diagram is that it has a triangular or pyramid type shape, with the vertices representing primary colors. For any point within this triangular or pyramid shape, we can specify the percentage of RED (x co-ordinate) and the percentage of GREEN (y co-ordinate) and so knowing these two percentages we can determine the percentage of BLUE (z) in any hue.

General form of a Chromaticity Diagram.

The figure below is a detailed chromaticity diagram. The difference with the diagram above and below is that the axes are scaled. To make matters easier, we have divided each scale by 100%. Thus 0.5 on the x-axis represents 50% of RED (R) and 0.9 on the vertical axis represents 90% of GREEN (G) in a particular hue. The intersection of these two lines give us a particular point or hue that is not possible (since it already adds greater than 100%) and so such a point must lie outside of the triangular shape. Only points within the triangular shape can give us a particular hue that exists. For example, the labelled y point in the figure is obtained by a line from the horizontal axis at 0.333 (33.3% RED) intersecting with a line from the vertical axis at 0.333 (33.3% GREEN) yielding 33.4% BLUE. As we have rounded these figures we would nevertheless get a hue that is 100% white. You should also note that the graduated scale along the perimeter of the chromaticity graph represents the wavelengths of light in the visible range. For example, light with a wavelength of 400 nm is essentially blue.

Chromaticity Diagram. Note: The vertical axis and horizontal axis has been divided by 100 to give an appropriate scale. The graduated scale along the perimeter is the wavelength of visible light in nm. The point y is the location of white, given by the equation: 33%B + 33%G + 33%R ~ 100%White. You should note there is a round-off error of 1%.
Courtesy of reference[2].

Basic Specifications Using The CIE System
To measure the hue of a textile, scientists use a reflectance spectrophotometer. This form of objective color measurement is called colorimetry.

The reflectance spectrophotometer is able to measure reflected light and in particular, can electronically translate these measurements into the percentage of BLUE, GREEN and RED contained in the reflected light (i.e. z, y and x percentages or values). For example, suppose the spectrophotometer measures that the reflected light from a colored textile material has a wavelength of 594 nm, which is the wavelength for orange. On the chromaticity diagram shown below this would translate into an x co-ordinate of 0.6 (60% RED) and a y co-ordinate of 0.4 (40% GREEN). As this already adds up to 100%, the BLUE percentage must be 0%.

The Chromaticity Diagram. Note: The wavelength 594 nm is on the perimeter of the diagram and is labelled with the symbol “+”. We can determine the x and y co-ordinates, which in this case are x = 0.6 and y = 0.4.
Courtesy reference[2].

A hue, which has a dominant wavelength of 485 nm will cause the spectrophotometer (which has these diagrams in-built into the software) to automatically print out x = 0.075 (or 7.5% RED), y = 0.200 (or 20% GREEN) and z = 0.725 (or 72.5% BLUE). Note: these must add up to 100%. This can be confirmed by looking at the chromaticity graph.

Complete Color Specification Using The CIE System
The chromaticity co-ordinates specify the hue of a colored object, but not its luminosity; that is, what is unspecified is the lightness or darkness of the color or how much grey it possesses.

In the CIE system, this omission can be rectified by adding a third dimension to the chromaticity diagram. This is achieved by projecting the third axis, the luminosity axis, from the point where white is located on the chromaticity diagram. To confuse matters, it is labelled as a capital “Y” axis. Note: Do not confuse it with the lower case “y” which represents the percentage of GREEN (G) in the hue.

The Y axis, or the luminosity axis, is also measured in percentage terms along it. The resultant three dimensional solid is called the CIE color solid and is given in the figure below. One can think of it as a triangular pyramid with one straight and two curved sides. Black equals zero luminosity (labelled with “x”) in the figure and white, which has 100% luminosity is on the top of the vertical scale or at the apex of the color solid.

Three-dimensional representation of the CIE System’s Chromaticity Diagram.
Courtesy of reference[2].

To specify a color completely using the CIE system, one requires a value for x (percentage of RED in the hue), a value of y (percentage of GREEN in the hue) and a luminosity value expressed as a percentage between 0 and 100%, where the greater the percentage of “Y”, the lighter the hue.

The CIE solid has a particular shape in both the “x” and “y” directions, which limits these percentages for a “perfect” color or hue. That is, these percentages must lie within the color solid. This means that x (percentage of RED) cannot be greater than about 73% and the percentage of y (percentage of GREEN) cannot exceed 83%. If any of these percentages are exceeded then the color lies outside the color solid and so it is not a perfect color or hue.

There are many color specification systems used throughout the world. We have covered both a subjective system (The Munsell Color Classification System) and an objective system (the CIE Color Specifying System). Many graphic designers, industrial manufacturers, and dyers are currently not using either one of them, opting for the Pantone Color Matching System instead. Nevertheless, by mastering the Munsell and CIE systems, any other hybrid system can be quickly understood.

These color specifying systems are used to produce color from a particular atlas of "perfect" colors or hues. The CIE system does it in a systematic and scientific manner. Nevertheless it does not inform the dyer or the person who mixes pigments, how to arrive at such colors. What the CIE system does is specify a color that practitioners may or may not be able to generate on fabrics even if they are using the most up-to-date pigments and dyes.

[1] A. Kornerup and J.H. Wanscher, Methuen Handbook of Colour, 3rd Edition, Eyre Methuen Ltd, London (1983).

[2] E.P.G. Gohl and L.D. Vilensky, Textile Science, Longman Cheshire, Melbourne (1989).

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