Introduction
Color is an important aesthetical aspect of coated objects. Different persons have different perceptions of a specific color. Therefore, it is important to have an objective way to characterize colors of coatings, as well as to have a tool to quantify differences between colors. The good news is that there is a standard system that is used all over the globe: the CIELAB color system. This article is about the fundamentals of this system and how it is used in coating industry.
The CIELAB color space
The 3-dimensional CIELAB color space.
The color of (coated) objects is visualized and specified by using the CIELAB color space, introduced in 1976. This 3-dimensional color space is built-up from three axes that are perpendicular to one another.
The L*-axis gives the lightness: an ideal white object has, by definition, an L* value of 100 and the L* value of a black object is 0. All shades of grey are on the L*-axis.
Chromatic (‘real’) colors are described by using the two axes in the horizontal plane. The a*-axis is the green-red axis and the b*-axis goes from blue (-b*) to yellow (+b*).
Each color can be represented by a color point (L*, a*, b*) in the color space; L*, a* and b* are the color coordinates of the color point.
The asterisk (*) symbol of L*, a* and b* indicates that the color coordinates are defined in the new color system; it is the follow-up of the older Lab color system. The new system is now used to specify colors, even though often the simplified notation of the color coordinates, without the * symbol, is used.
Color differences
Consider two colors, 1 and 2, that have color coordinates L*1, a*1, b*1 and L*2, a*2, b*2, respectively.
The difference in lightness between the colors is quantified by ΔL* = L*1 – L*2, the green-red color difference is quantified by Δa* = a*1 – a*2 and the blue-yellow color difference is quantified by Δb* = b*1 – b*2.
The total color difference between two colors, ΔE*ab, is the distance between the color points of the two colors in the CIELAB color space. The color difference can be calculated by using the mathematical theorem of Pythagoras:
A small ΔE*ab value implies that the colors are close to each other. The concept and specification of colors, as well as quantifying color differences, is important when a color must be matched or when a new batch of a paint must have a color that is close to the color of the standard paint, the reference. The color coordinates of the batch and the reference are measured and the total color difference ΔE*ab is calculated. But, how well must the color of the new batch match with the color of the reference? The answer to this question is a criterion in the quality control of a new batch of paint. Depending on the industry and application, the criterion for color approval can be strict, for example ΔE*ab < 0.3, or much milder, for example ΔE*ab < 1.5.
In the daily practice of those who work with paints, the notation of a color difference is often simplified as ΔE* or ΔE.
Hue and chroma
Hue is the color tone or color name of a color. Chroma is the amount of saturation of a color. Colors of high chroma are said to be clear, bright or brilliant. Dull (pastel) colors have a low chroma.
Hue and chroma can be visualized and quantified by using the a*b*-plane of the CIELAB color space.
Hue angle and chroma in the a*b*-plane.
The hue of a color is specified by its hue angle hab (without * symbol) in the a*b*-plane, given in degrees (°). The hue angle range starts, by definition, at the positive side of the a*-axis and goes counter-clockwise. This implies that red has a hue angle of 0°. A full circle goes from 0° to 360°. Cyan blue (an organic blue pigment with color index PB 15), for example, has a hue angle of about 240°. The hue angle of a color can be calculated from the color coordinates:
The chroma of a color is quantified by C*ab; it is the distance of the color point to the L*-axis. The chroma value of a color can be calculated from the color coordinates by using Pythagoras’ theorem:
A saturated (or brilliant or pure) color is represented by a color point that is far away from the lightness axis. The color point of a pale (or dull or pastel) color, a color of low saturation, is close to the L*-axis.
So-called achromatic colors, from black via grey to white, have a chroma value of zero; color points of achromatic colors are on the L*-axis. In coating industry, often the definition is used that achromatic colors have a C*ab below 10. Then, C*ab > 10 for chromatic colors.
Improvement of the system
While working with the CIELAB color space, color coordinates and color differences, quantified by ΔE*ab, color experts experienced shortcomings of the system. Therefore, continuous development of better systems proceeds, mainly to obtain better agreement between measured and quantified color differences with human visual perception of color differences1.
For example, it turned out that the human eye is less sensitive for color differences when considering yellow and green colors. This implies that for a certain product range, the requirement with respect to color differences can be milder for yellowish and greenish colors than for, for example, blue and violet colors.
CIE94 and CIEDE2000 are improvements, with respect to the quantification of color differences, that are used in the coating industry. Both systems are based on the same CIELAB color space that is used to quantify total color difference ΔE*ab. However, next to differences in lightness ΔL*, differences in hue and chroma, instead of using Δa* and Δb*, are used to quantify color differences in the improved systems.
In the CIE94 method, introduced in 1995, weighting factors are used to include human perception and sensitivity with respect to color differences. The color difference, quantified by using CIE94, is written as ΔE*94.
In the CIEDE2000 method, introduced in 2001, further improvements were made with respect to weighting and correction factors. As a result, a color difference calculated via CIEDE2000, written as ΔE*00, gives better agreement with human perception than ΔE*ab and ΔE*94.
Fortunately, color differences between coatings quantified by means of ΔE*ab are good enough for most coating applications.
Reference
- Book Colour Technology of Coatings, Wilhelm Kettler et al., 2016.
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