COLOR DIFFERENCE EQUATIONS
Quantification or color difference makes it possible to establish a meaningful measurement of the closeness of a color match. The basic unit is a just noticeable difference (JND). One JND is a very close match whereas three would be easily perceptible.
Color difference equations calculate the color difference in units of E, the error of the color match.
TRISTIMULUS FILTER COLORIMETERS
The Hunterlab D25 and the Gardner XL 805 are useful for color correction (shading) and quality control, where the colorants are known and there is no metamerism of any sort. The tristimulus data they provide is for only one illuminant (usually C), and therefore, metamerism cannot be identified or checked.
Sherwin Williams Macbeth MS2020/TRS-80 system, Spectroard, Macbeth MC 1500, and Applied Color System (ACS), Spectro Sensor compute XYZ values from the spectrophotometric curve of the color, making it possible to get tristimulus values for any and all of the Commission Internationale de L’Eclairage (CIE) standard illuminants, including CWF. Metamerism is detected by measuring E of the
samples under the various illuminants. If the FMC2 E is 1.0 for D65 and 2.0 for A, there is metamerism, because the color difference is not the same. Visual evaluation should confirm it.
When a nonmetameric match is needed, the trial and error method of selection of the proper colorants is very inefficient compared to spectrophotometric curve analysis. The spectrophotometric curve of a color is a plot of reflectance of light at each wavelength of the visible spectrum. It is the primary physical measurement of a color and is often referred to as the fingerprint of a color, because identical curves areidentical colors and nonmetameric.
The instrument that gives spectrophotometric curves is called a spectrophotometer. Spectrocolorimeters are spectrophotometers and when equipped with a plotter, will provide spectral reflectance curves that are used to identify colorants. Each colorant has its own characteristic spectral curve that remains identifiable in a mixture, making it possible to ascertain the colorants in an unknown. The colorants are identified by comparing the absorption maximum (reflectance minimum) of the curves of the known to that of the unknown, a method that is only as good as the library of known. If the unknown contains a colorant for which there is no curve, then it cannot be determined. This method easily outperforms experienced visual trial anderror. It can readily discern the difference between a red and a green shade phthalo blue pigment, which is difficult by visual means.
There are times when a colorant cannot be determined, because the concentration is very low, or the difference between the curves is insufficient to identify the colorant with any degree of certainty. In these cases, a trial match is made and the spectral curve compared to the unknown. It will be obvious if the wrong colorant was selected.
This trial and error method is continued until the curve indicates the proper colorants have been chosen.
COMPUTERIZED COLOR MATCHING
CCM is the best tool for matching colors. In the hands of a skilled color matcher, the quality of matches and gain in efficiency can be quite dramatic. It is a big improvement over visual trial and error and the instrumental methods mentioned previously. CCM systems are available from Diano, Macbeth, ACS, Universal Color Systems, X-Rite, and Instrumental Color Systems. All are good and all have advantages and disadvantages as far as user friendliness is concerned. None of them will do
exactly what you would like them to do, but this is true of any software that is not custom designed. Several companies, Jinwei Chemical, Sherwin-Williams, PPG, and Benjamin Moore, to name a few, have developed proprietary software to satisfy their own needs.
Color matching is a science that is best left to formulators and quality control experts. For the applicator, it can be expressed in more general terms. There are many pigments that can be used to obtain a color. All these pigments vary from one another within the same color group. For example, there are red pigments with yellow cast, orange cast, blue cast, and so forth. The pigments can be clean, meaning that they reflect light in a very limited wavelength range, or they can be dirty and reflect light in a much wider wavelength range.
Pigments are not pure compounds and many are naturally occurring compounds. This means they can vary from one production lot to another. What this all means is that the color will vary slightly from one batch of paint to another, but fortunately the human eye cannot detect very slight differences. For most applications, a E of 0.5–1.0 is an acceptable color match.
It is important to have a color standard program for consistent color match from one batch of paint to another within established limits. The company that will use the material and the manufacturer of the paint must agree on a color standard that is practical and reproducible in a paint production facility. This is necessary to maintain color consistency and reduce manufacturing errors. Consider, for example, that a paint color cannot be exactly duplicated with a printed catalog color. Similarly, some leadbearing pigmented material cannot be nonmetamerically duplicated with lead-free pigmented material.
For a new color, the paint supplier should be consulted to see if the color is feasible. They can usually determine how easy it will formulate a color match. If a color standard is already available, the paint supplier can match the established standard and submit a sample. When the sample is approved a master standard is prepared. The painted panels can be used as the master standard and working standard. Color tolerance charts can be prepared to monitor differences.
Since color standards change with time and use, the Macbeth color program ensures a consistent standard. They can also generate color tolerance charts that establish a range of color variation that is acceptable for production color matching and establish the visible limits of an acceptable color match.
If colorimetric instrumentation is used for color matching, the applicator and the manufacturer must use the same programs for color measurement (e.g., Friele, MacAdam, Chickering [FMC] II or LAB) and both machines must agree with one another. An accepted color variation should be established from one batch to another, since there is usually batch-to-batch variation. In general, using an FMC II
program, a E of less than 1.0 is acceptable for a production batch.
For visible color match system, the following parameters should be included in color control:
1. Light source to be used for viewing, that is, daylight or CWF. By using only one light source one can eliminate metamerism from one light source to another. The light should be the one that the product will normally be seen in.
2. Angle at which panels will be viewed.
3. Viewing distance from panel. At 50 ft, all colors look good. At 6 in, color matches are very difficult.
Remember, when comparing colors, that the appearance of the panels must be the same, that is, both smooth or both textured. In addition, the gloss should be the same within limits, generally ±5 gloss units.
• Appearance—the way an object looks. It involves not only color but also other visual attributes such as gloss and texture.
• Color—a visual response to light consisting of three variables; hue, saturation, and lightness. The most important appearance attribute of an object.
• Color difference—magnitude and character of the difference between two colors.
• Color tolerance—limit of color difference from a standard that is acceptable. It is generally expressed in terms of a particular color differenceequation.
• CIE LAB of L a b E—the color difference based upon the 1976 CIE color difference equation. One CIE LAB E is approximately 2 JND units of color difference.
• L—designates lightness differences.
• a—designates redness–greenness.
• b—designates yellowness–blueness.
• E—a unit of color difference. It constitutes the total differences in hue, the differences in saturation, and the differences in lightness.
• FMC2 E—the color difference based upon the Friele, MacAdam, Chickering color difference equation. One FMC2 E is approximately 1 JND unit of color difference.
• Gloss—subjective term used to describe the relative amount and nature of mirror-like (speculate) reflection. Different types of gloss are frequently arbitrarily differentiated, such as sheen, distinction-of-image gloss, and so forth. Trade practice recognizes the following stages, in increasing order on
FIGURE 5.1 Geometric metamerism.
gloss; flat (or matte)—practically free from sheen, even when viewed from oblique angles (usually less than 15 on 85 degree meter); egg-shell—usually 20–35 on 60 degree meter; semi-gloss—usually 35–70 on 60 degree meter; full-gloss—smooth and almost mirror-like surface when viewed from all angles, usually above 70 on 60 degree meter.
• Geometric metamerism—phenomenon exhibited by a pair of colors that appear to be a color match at one angle of illumination and viewing but which are no longer a match when the angle of illumination or viewing is changed, caused by gloss and/or texture differences (see Figure 5.1).
• Hunter Lab E—the color difference based upon Hunter’s color difference equation. One Hunter Lab E is approximately 3 JND units of color difference.
• Metamerism—when two or more samples match under one set of viewing conditions (kind of light source and angle of view) for an observer, but do not match under a different set of viewing conditions for the same observer. Those samples may not match for a different observer under
either viewing condition.
• Master color standard—the absolute reference color standard; a color standard panel to which all standards of a given color are compared.
• Primary working color standard—a color standard that is compared to painted items (usually) such as paint test panels and finished goods. It is used to color check everything, where the Master Standard is used only to check color standards and settle color disputes.
• Secondary working color standard—a back-up to the primary working color standard. It becomes the primary working standard when the original primary working color standard is determined to be unusable.
• Texture—nonuniform surface. Like gloss, texture affects the appearance of color.
The visual evaluation of a color match is a very subjective human judgment. If the nature of the process is not fully understood and appreciated, poor color control will result, or much more time than that is necessary will be spent in getting acceptable matches.
COLOR STANDARDS AND METAMERISM
The color standard is the most important part of a color control program. Whether the color is controlled by visual observation, color measurement, or a combination of both, it is important that the color standard and the product are not metameric to one another.
Metamerism is when colors match under certain viewing conditions and mismatch when the conditions are changed. The various viewing conditions that can causemetamerism are
• The type of light source
• The angle of illumination and angle of view
• The human observer
Metamerism can occur with all three conditions. Classification of the metamerism depends on the source.
1. Light source metamerism—colors will match under one light source (daylight) but do not match under another (incandescent).
Cause: colorants different than those in the color being matched were used.
Solution: use the same colorants.
2. Geometric metamerism—colors will match at only one angle of illumination and viewing. For example, samples will match when they are oriented so that they are illuminated at a 0◦ angle to the surface and viewed at the 45◦ angle, but will be mismatched when the viewing angle is changed to 75◦.
Cause: the gloss and/or texture is different.
Solution: control gloss and/or texture.
3. Observer metamerism—colors will be a match by one observer but not by another.
Cause: individuals have different color visions, and it becomes evident when light source metamerism exists.
Solution: eliminate light source metamerism.
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