The appearance of a finished product is often one of the most critical aspects of quality. The way it looks has a significant psychological impact on the perception of quality by the potential customers and on their willingness to buy the product. Color is the most important appearance attribute of an object. There are other attributes such as size, shape, gloss, and texture, but color is by far the most important.
Color, gloss, and texture are imparted to a product by a coating. The coating manufacturer has the responsibility for formulating an accurate color match based on the customer’s sample. The applicator also plays a role in accurate color match, since mixing, storage, application, and curing can impact color. It is necessary for the manufacturer to have a quality control program for the desired color and appearance of their product. But it is equally important to be able to detect all sources of error that affect color and appearance.
Color match is usually judged by visual evaluation in standard lighting. Some will use color difference measurement devices as well. An inspector will compare the color of the product to a color standard sample.
If there is some color difference, it will be necessary to judge if it is within acceptable tolerance levels. When there are guideline panels that represent the maximum allowable color difference limit, the task is a little less difficult. Any difference in gloss or texture will further complicate the visual process, because the color match will vary as the angles of view and illumination vary.
Usually, the coating is checked for color by the quality control lab before the customer uses it. A number of sources of error can cause a poor match.
Examples of sources of error:
1. Different color standard
2. Metameric color standard
3. Metameric batch
4. Gloss difference
5. Texture difference
6. Observed metamerism
7. Panel preparation differences
8. Substrate differences
9. Film thickness variance
10. Disagreement between color measuring instruments
In addition to these items, it is possible that the paint needs to be adjusted for accurate shading.
A color difference measurement can provide accurate numerical color match or define a difference from the specification. When measurement equipment is not used,it is hard to describe the visual difference and make an adjustment.
Color has three visual variables: hue, saturation, and lightness. Hue is a color variable denoted by red, orange, yellow, green, blue, violet, and purple. Saturation is the variable of color that denotes its purity or its degree of departure from grayness. Lightness is the variable of color that distinguishes black from white, without regardto hue, as represented by a neutral gray scale ranging from black to white.
Hue, saturation, and lightness are often called the three dimensions of color, because it takes a three-dimensional space to represent them. Lightness is distance along the vertical axis, saturation is radial distance away from the vertical axis, and variation in hue is angular distance around the vertical axis.
Color matching is the process of selecting colorants to match a given color and adjusting their concentrations until the hue, saturation, and lightness of the trial (batch) match the hue, saturation, and lightness of the given color sample.
Knowing that color has three variables, leads to the four-colorant principles of color matching. To match a color, hue, saturation, and lightness must be adjustable or controllable and can only be done with a minimum of four colorants. For example, if only one colorant—white—is used, there is no way to change it. It can only be white. By adding black as a second colorant and varying the concentration of the white and black, any number of colors, varying essentially only in lightness can be made. By
adding a third colorant, for example red, any number of colors can be made, varying in lightness and saturation but all having essentially the same red hue. By adding a fourth colorant, yellow, then all three color variables—hue, saturation, and lightness—can be varied, and a color can be matched exactly.
If a color is formulated with less than three, then the match can only be obtained when all of the colorants are exactly as they were originally. There is no latitude for production variation. If more than four colorants are used, then it becomes more difficult to determine which colorant will correct a hue difference, a saturation difference, or a lightness difference, because two colorants in a five-colorant formula will control the same variable. For example, a green made with white, black, blue, and two
different yellow pigments, needs to be more yellow. Which yellow is added? Some yellow pigments will make it more yellow but also less saturated. Other unsaturated pigments may be more effective in correcting the hue without affecting the saturation.To correct the hue, the effect on saturation must be considered.
Another reason why a five- or more-colorant formula should not be used is metamerism. Metamerism is when the colors match under one set of illuminants, observer, and viewing conditions, but do match under another. To prevent this, the match must not only contain the same colorants but also the concentration must be Color Matching and Color Control 97 the same. In a five or more colorant formula, it is almost impossible to produce batches repeatedly, with the colorants at the same concentrations. Color may not always be controlled. A four-colorant formula is unique, because only one set of concentration of the colorants will produce a match and therefore no batch-to-batch
metamerism is possible.
When matching colors by the visual approach, the color matcher selects the colorants by trial and error, experience, and knowledge of subtractive color mixture.It does not take long to learn that mixing blue and yellow together can make a green color. Colorant adjustments are made until the appearance is correct. Blue,black, or some other color is added until a match is made. A quantity of a colorant is added and the change in color is evaluated. If more is needed, the amount is estimated
by that change. This process is repeated until a satisfactory match is obtained. With good technique and accurate weighing of colorants, an experienced color matcher can make a match fairly quickly.
The problem with the visual approach is that the matches may be metameric, which may or may not be acceptable. The metamerism occurs because the colorants used are different from those that were used to produce the color sample.
There are three types of metamerism: illuminant, geometric, and observer.Illuminant metamerism is what is normally thought of when the word metamerism is mentioned, a change of light that causes a change of color.
Geometric metamerism is when the colors will match only at a certain angle of viewing and illumination. This is caused by difference in gloss and/or texture. A flat color will only match a gloss color at one viewing angle.
Observer metamerism is when colors are a match for one observer and not for another. It is caused by differences in the color vision of the observers and is quite evident when illuminant metamerism also exists. When there is difficulty in gettinga match accepted by a customer, it may very well be observer metamerism.
Dr. Henry Hemmendinger is a leader in understanding the breakdown of a color match by change of either illuminant or observer. Hemmendinger and Davidson developed the D&H Color Rule, a device to quantify the extent of observer metamerism, which is still viewed as an indispensable aide in teaching the principles of observer metamerism. Dr. Hemmendinger, also developed methods to use metameric pairs as tools to assess instrument performance. An observer can readily get a match
with highly metameric samples. The pairs chosen are noted and compared to the pairs chosen by another observer. If the pairs are the same under the same light source, the observers have the same color vision. If the pairs appear different under the same light source, it indicates a difference in color vision between the observers.
Visual color measurement is done in a light booth such as a Macbeth Spectralite.This light booth provides three light sources that are commonly used to visually evaluate color and color differences. They are North Daylight, Incandescent, and Cool White Fluorescent (CWF). The International Commission on Illumination (ICI) has standardized on North Daylight and Incandescent and given them the designations illuminant D65 and illuminant A, respectively. CWF has no standard as yet.
The important feature about the booths is that they supply a light source that is constant. Natural daylight varies with the time of day. In addition, metamerism can be evaluated very readily, because two different light sources are provided. Two ASTM (American Society for Testing Materials) standard practices—D1729, Visual Evaluation of Color Difference of Opaque Materials and D4080, Visual
Evaluation of Metamerism—are methods that recommend the use of light booths.
When matching a color where metamerism is obvious, the match under daylight may look like it requires red to correct it, but under incandescent it needs green. When there is no concern about metamerism, color match is done under one light source, preferably the one under which it will be viewed. When metamerism cannot be tolerated, other colorants are selected and tried until the combination does not produce a metameric match. This trial and error method can be very time consuming and is outmoded by methods using color measuring instruments and computer color
matching (CCM) systems.
COLOR MEASUREMENT EQUIPMENT
Intelligent use of color-measurement instruments will increase the efficiency of matching colors. They measure color difference in terms that allow description of hue, saturation, and lightness, and also quantify these differences. This makes it possible to estimate the amount of colorant to add to correct the color.
Colorimeters measure color in terms of three numbers called tristimulus values— X, Y, and Z. These values are the amounts of three primary colors—red (X), green (Y), and blue (Z)—needed to match or specify a color under illuminating and viewing conditions as standardized by the ICI. Each color has its own tristimulus values, and identical values are identical colors. Differences in tristimulus values between colors can be analyzed to determine the direction and magnitude of the color difference.
To facilitate the translation of the color difference from numbers to the more meaningful visual descriptive terms and quantify color difference, the XYZ values are converted into the Hunter L, a, b Scale. This transformation is simple, and all modern instruments offer it as an option. L is lightness–darkness, a is redness–greenness, and b is yellowish–bluish. If the difference between the trial values and given color in L, a, b values was −0.5, −0.5, and +0.5 respectively, the trial values indicate that
the color is darker, greener, and yellower than the given color. If the color difference is unacceptable, then the trial must be corrected by adding white to make it lighter,adding the bluest colorant to make it bluer or less yellow, and adding the reddest colorant to make it redder or less green.
By keeping records of the readings and the addition of the colorants, it is possible to estimate the amounts of the colorants for a correction. For example, one pound of blue lowered b from +0.5 to +0.25. The goal is to minimize b. Therefore, if one pound of blue changed b by 0.25 units, then another pound of blue would lower b to 0. The other colorants are estimated similarly.
Some instruments indicate color difference in the tristimulus values. The color difference is in terms of XYZ ratios, expressed as percent between the values of the batch and the standard. XYZ ratios of 100, 100, 100 indicate that the values of the standard and the batch are the same and the colors match. Any values other than 100 indicate that a color difference exists. It is much more difficult to translate these values into visual descriptive terms as opposed to L, a, b values.
As an example, green reads 101.5, 100.5, 100.5. The Y ratio, which is a measure of lightness, is greater than 100 indicating the batch is lighter. In order to determine the hue and saturation difference, it is necessary to compare the relative values of XYZ ratios. The hue of a green can only be yellower or bluer so the Z ratio is the hue indicator. However, it must be compared to the X ratio that is the saturation indicator for a green. In the example, Z is lower than X meaning that the batch is yellower.
For saturation, the X ratio must be compared to the Y ratio and since the X ratio is greater than the Y ratio, the batch is less saturated or grayer. Another way of looking at this is that there is more red than needed. Red is the complementary color of green. Complimentary colors when mixed produce gray. The logic is similar for analyzing the remaining category of colors blue, yellow, and red.
XYZ values are the basic measurement of color, and for this reason estimating a correction to a batch is easier and more reliable than other color scales but still requires experience. A green reads 110, 110, 110 and is made with white, blue, yellow, and black. An addition based on experience of one pound of blue, one pound of yellow, and 0.1 pound of black is made. The result of the addition was 105, 104, 105 and another addition is necessary to correct the match. To estimate the amount of blue,
the ratio affected the most is used to compute the addition.
Although this can be determined experimentally by adding some blue by itself and seeing which ratio has changed the most, it is more expedient to use the following rule. The bluest colorant will lower the X the fastest, the reddest colorant the Y, and the yellowest the Z. Therefore, X will be used to estimate the amount of blue, Y the black, and Z the yellow. One pound of blue lowered the X ratio by 5 and to get 100,another pound is added. A pound of yellow will lower the Z by 5 so another pound
of yellow is needed. The black lowered Y by 6, so 0.06 pounds more is needed.
This method works very well, but experience is needed for the first addition and all colorants must be added simultaneously or too much of the colorants will be estimated. Once a batch is shaded using this method, and the readings and colorant additions are recorded, the information can be used for succeeding batches. This has been called the “historical” method and has proven to be very successful.
This method would not have been possible without color measurement. The most difficult decision to make when matching a color is deciding when to stop.
Viewing colors that differ and judging whether or not it is a close enough match is very subjective. No two people see color alike. It is a rare occasion when total agreement will occur, and this is especially true when colors are metameric. Every individual has his or her own idea as to what is an acceptable match, and this may change from day to day because of deadlines imposed to get the job done. This is where instruments perform well. Measurements are objective
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