Quality 101: Connecting the Dots of Color Measurement
June 7, 2011
Even though industries as varied as automotive, aerospace and pharmaceutical manufacturing do not seem to have much in common, they all share the basic need to measure color accurately and reliably so their products adhere to customers’ specifications.
Whether the challenge is to measure metallic paint on the side panel of a car or the upholstery of an airliner seat, instrument manufacturers have come up with sophisticated tools that can help virtually any company to learn and use the language of color for quality control.
Just like a workman needs the proper tools to complete a project, manufacturers should first spend time to understand from their customers exactly what needs to be measured and to what accuracy. Some common questions include:
Which color scale does the customer use? Instruments assign numerical values to the three basic elements of color: hue, chroma and value. There are three common standards that communicate a particular color in the vast universe of possible colors: CIE Xyz, CIE L*a*b* and CIE L*C*h°.
What level of spectral resolution does the customer require? For instance, RGB instruments can only give a relative nonstandard value, colorimeters give one of the above mentioned standard values with moderate accuracy, and a 31-point spectrophotometer gives all the standard color values, plus a full reflectance curve data, at a high level of accuracy.
What system of tolerancing does the customer use and how tight is the tolerance? Some common tolerancing systems include CIELab, CMC Lab, DE 2000 and HunterLab. Selecting the appropriate system of tolerances is critical because it guards against release of nonspecification product and false rejects. Wide open tolerances may require only a simple RGB instrument, while tight tolerances may require a 31-point spectrophotometer.
How smooth is the surface being measured? Is it smooth like a car panel or relatively rough like the upholstery of a car seat? Some textured surfaces such as cloth are angularly sensitive, meaning that the color measurement is affected strongly by the orientation of a piece.
What type of illumination is the customer using? Many colors undergo a common color phenomenon called metamerism when viewed under different light sources. Two color samples may appear identical when viewed under incandescent illumination but appear very different under daylight. In general terms, customers choose a standard illuminant that best represents the source that will light their products where they are sold. Some standard illuminants include A, C, D50, D65, F2, F7 and F11.
Answers to some of these questions may be found by asking how the customer monitors and controls color quality, but often manufacturers need to implement additional color measurement techniques that apply to their particular processes. Many instrument makers provide free advice on the latest methods of measuring color that are tailored to the type of material involved, tolerances and cost of the quality control program.
For instance, instrument manufacturers have developed relatively low cost, noncontact in-line devices that measure paints and coatings, plastic injection molded and extruded parts, and even rough surfaces, such a brick, so companies can now afford to continually monitor processes at a particular piece of equipment on the factory floor, rather than relying only on occasional sampling in the laboratory.
In the past, manufacturers that use lab-confined spectrophotometers would take samples only at the beginning, middle and end of the production runs due the effort and expense involved in the measurement process. New 45/0 degree geometry spectrophotometers can accurately control the quality of color by measuring a wide range of wet and dry samples without touching test surfaces.
In addition to targeting samples precisely, the new generation of 45/0 degree geometry spectrophotometers have advanced optics and sensors that accurately measure gloss as well as colors. Using computer software, the instrument can compensate for the difference in appearance between dry and wet samples.
Instrument makers have increased the sophistication of spectrophotometers by adding more illuminators and sensors than 45/0 degree geometry instruments so that automotive suppliers and manufacturers can now measure metallic and interference paints that have proven so popular with the vehicle-buying public.
But spectrophotometers certainly are not the answer for every color measurement application. In cases where precise color measurement is not required, other families of instruments can represent the most cost-effective solution for the control of color quality. For instance, colorimeters are simpler and less expensive instruments that use red, green and blue filters that emulate the response of the human eye to light and color. These instruments are used effectively for sorting and quick in-line checks for less exacting jobs.
A good visual analogy to compare the scale of resolution of a colorimeter with a spectrophotometer is this: if a colorimeter measures on the scale of inches, a spectrophotometer will measure on the scale of ¹⁄₁₆ of an inch. There are many applications in everyday manufacturing where a colorimeter is perfectly adequate to the task and a spectrophotometer would be considered overkill.