Establishing Instrumental Color Difference Tolerances for Your Products - an12_05 Follow

Color is a very important aspect of products for consumers. The appearance of a product is perceived (often correctly) to be related to its quality. This is true in almost all industries. From cookies to vinyl siding, customer buying decisions are often based on product color, making it important to bakers and extruders alike.

The color of a product may be judged generally to be “acceptable” or unsatisfactory,” or it may be judged in more detail to be “too light,” “too red,” or “too blue.” Such judgments can be made visually or instrumentally based on a perceived difference between an ideal product standard and a sample.

When this difference is quantified, tolerances can be established.
Tolerances are limits within which a product is considered acceptable. Any product falling outside the tolerances is unacceptable. Having good tolerances in place for each product allows you to make quick and easy pass/fail or ship/don’t ship decisions. When tolerances are established instrumentally, they may be expressed in any of the color scales or indices available with the instrument. In order to set
tolerances, an ideal or close-to-ideal product standard is required, as well as a variety of products that have already been determined to be acceptable or unacceptable.

There are two levels of visual color differences that are used to establish color tolerances:
• Minimum perceptible difference, which defines a just-noticeable difference between standard and
sample.
• Maximum acceptable difference, which is the largest acceptable difference between standard and
sample.

Perceptible vs. Acceptable Differences

Manufacturers are generally concerned about the maximum acceptable color difference rather than a minimum perceptible difference, and color tolerances are usually based on the maximum acceptable difference. Any difference larger than that would cause the sample to be rejected.

In the end, agreement between the buyer and seller on acceptability criteria is necessary for establishing product color tolerances and purchasing specifications.
A step-by-step process for establishing color difference tolerances is outlined in the rest of this Applications Note.

Step I. Establish a Standard

The first step in implementing a tolerancing program is to establish a standard that represents the ideal color for a particular product. In theory, the established manufacturing process should be capable of producing this color the majority of the time.

Tolerances should be established separately for each product color, so you will need a product standard for each color. It is normal to have difference tolerances for different colors. (It is also typical to find that your tolerances must be tighter to provide acceptable results for darker colors and lower-chroma colors.)

In a customer/vendor relationship, the standard representing the target color may be submitted by a designer or customer. This submission is then matched by the vendor’s manufacturing process and returned to the customer for approval. This begins the process of color communication.

On the other hand, when the color evaluation is being driven by internal quality concerns, it is most effective to use a standard that represents the process average. This can be accomplished by selecting a physical specimen from the center of the population or by averaging the measured results of a group of
specimens to determine a numeric mean. An example of determining a colorimetric mean is shown below. Many HunterLab products (ColorFlex, DP-9000, EasyMatch Coatings, EasyMatch OnLine, EasyMatch QC, MiniScan XE Plus, Universal Software) can automatically average samples for you.

Use of the Mean for Establishing a Standard
Sample ID L* a* b*
1 20.22 0.64 -11.22
2 20.70 0.66 -11.02
3 20.73 0.61 -10.99
4 20.38 0.60 -10.82
5 20.75 0.49 -10.62
6 20.37 0.50 -10.43
7 19.99 0.54 -11.20
8 20.15 0.52 -10.81
9 20.29 0.59 -10.55
10 20.48 0.57 -11.22
11 20.50 0.64 -10.72
12 19.87 0.70 -11.02
13 20.81 0.59 -11.22
14 20.31 0.53 -11.18
15 20.56 0.61 -10.57
Average 20.41 0.59 -10.91

Care should be taken to preserve the color of physical standards by minimizing the influence of light, temperature, contamination, and other aging factors. A system may be established whereby duplicate standards are created and stored until needed. The amount of change in a current, or “working,” standard over time can be determined by comparing it to a stored, and theoretically pristine, duplicate. If an instrument is being used to measure color, the current instrumental reading for the standard can be compared to the previously assigned values. As suggested by the SAE J1545 Recommended Practice, if a working standard has deviated by the greater of 0.2 color difference units in ΔL*Δa*Δb* or
ΔL*ΔC*ΔH* or 0.1 times the tolerance range, the standard should be carefully evaluated and possibly replaced with a back-up. The worksheet below details an example evaluation of such a standard.

Step II. Visually Evaluate Pass/Fail

Once the product standard is established, a “pass” or “fail” rating can be assigned visually to any specimen that is compared to that standard. Results should be reported in a fashion similar to those shown below, including complete information on the particular conditions under which the specimens were evaluated.

Since specimens may vary from the target color in terms of lightness, redness/greenness, or yellowness/blueness, it may be helpful to employ physical standards which deviate along the tolerance perimeters for these three axes. An example of this type of tolerancing arrangement is shown below.

Visual Deviations from Target in Lightness/Darkness, Red/Green, and Blue/Yellow

To be useful, these visual evaluations must be as repeatable and reproducible as possible. The parameters listed below must be carefully controlled. Similar parameters are listed in ASTM Standard D1729. A light booth can be a useful tool for establishing a standard light source (such as incandescent, fluorescent, or daylight), angle of illumination, and angle of viewing.

Conditions to be Controlled for Visual Evaluation

1. Spectral quality of the light source
2. Intensity of the light source
3. Angular size of the light source
4. Angle of incidence (the angle from which light strikes the object)
5. Angle of viewing (the angle at which the object is viewed)
6. Background color.

Step III. Make Instrumental Measurements
Modern color measuring instruments (spectrophotometers and colorimeters) use the CIE colorimetric mathematical model to relate the human perception of color to instrumental response. Colorimetric scales such as CIE L*a*b* and CIE L*C*h can be derived from this mathematical model to serve as useful tools in communicating and quantifying color and appearance. The numbers obtained describe the nature and magnitude of the difference in color between standard and sample in a way that is meaningful to a human observer. 

Instruments not only provide an objective, numerical measurement system, but they can also often discriminate or “see” small color differences better than the average human observer and can do so repeatably. In other words, instruments are more accurate and more repeatable than humans. Another recognized advantage to using instrumentation is the agreement on specimen readings between different instruments, which is better than the agreement between visual assessments by two different human observers. This function, known as reproducibility, easily expands the capability for communication of color between different manufacturing facilities. 

When comparing results for different specimens measured on different instruments, specific sample handling techniques and instrumental settings should be defined and used. Adherence to a defined method will reduce the error associated with sample presentation and instrumentation. Some of the parameters to be considered and standardized in test method development are listed below.

Criteria for Color Measurement Methodology
• Instrument geometry: 45°/0° or diffuse/8° (sphere)
• Sample preparation, including taking opacity, translucency, etc. into account
• Sample presentation, including instrument port size, etc.
• Color scale or color difference scale
• Illuminant
• Standard observer
• Standardization mode
• Sample averaging.

The recommended practices of various professional trade associations (such as ASTM, TAPPI, and AATCC) are available in the literature.

When comparing measurements made on different instruments, the best results are obtained for color differences when the instrument group contains units of similar geometrical design. For instance, it would not be advisable to compare results obtained on a 45°/0° instrument to those obtained using a
diffuse/8° instrument. For recommendations of ways to maximize inter-instrument agreement, refer to the Applications Note titled “Maximizing Inter-instrument Agreement.”

When it is important that two or more instruments of similar design read the “same” values for a group of specimens, the technique of hitch standardization may be employed. This process involves naming one instrument as the reference, or “master,” unit and mathematically adjusting the secondary, or “slave,” units to match. In this way, two or more instruments can be “hitched” together. For more
information on this process, see the Applications Note titled “HunterLab’s Guide to Hitch Standardization.”

Instrument diagnostics provided by the instrument manufacturer should be run on a regular basis. Adherence to a schedule will ensure confidence in the measurements and allow early detection of instrumental problems.

Once the instrument and method are in place, it is time to gather and read a group of samples. To be most effective, this group should be large enough to provide some statistical credibility (i.e., twenty or more samples) and should include samples that pass, as well as samples that fail when visually rated.
Specimens that are unacceptable assist in finding the numerical tolerance boundaries. To verify and refine the initial product tolerances, expand the data base by collecting more samples.

Next, rate the acceptability (in terms of pass or fail) of each sample by visually comparing it to the physical product standard as described in Step II (if you haven’t already). Then, measure each of these samples on the instrument and determine its color difference relative to the product standard. And example data summary is shown below.

Step IV. Establish the Tolerances
There are several types of tolerances you may establish and several methods for doing so, which are outlined below.

Tolerance Type 1: Rectangular Tolerances
Rectangular tolerances are the simplest type of tolerances and are shown in a form similar to the example given below. All three components of the color scale (such as L*, a*, and b* or L, a, and b) should be toleranced.

(See attached for the complete article with illustrations)