Measurement: Colors, Colors, Colors
Manufacturers concerned with making products in the exact color specified by a customer may use spectrophotometers and colorimeters to track quality, but they can easily overlook the need to do proper visual evaluation of colors and color differences as part of quality control.
At some point in the supply chain, quality control personnel need to evaluate how the colors of products look when they are placed under the same illumination as where they are sold or used-illumination that is often specified by the customer. And there isn’t any substitute for the human eye when it comes to making those judgments.
The good news is that manufacturers can take inexpensive and practical actions to assess the visual color quality of products. Best practices for controlling the visual quality of color fall into three main areas: quality and intensity of light sources, the color acuity of those who are responsible for checking products, and clear procedures for preparing samples for quality checks and maintaining records.
The Eye of the BeholderSome manufacturers mistakenly believe that evaluating colors visually is entirely subjective, based only on the opinions of observers. But over centuries of study, scientists and engineers have developed theories and methods that make visual evaluation of colors reliable and reproducible enough to be considered objective measurements.
Best practices in visual color evaluation start with proper illumination at the factory and knowing what illumination customers will use to evaluate products. Manufacturers of color measurement equipment have developed light booths that simulate a number of illumination conditions, such as natural sunlight at different times of the day, the average fluorescent illumination that a shopper may encounter in a store and the lighting a consumer may experience at home with incandescent lights.
Some manufacturers may use filtered tungsten halogen lamps to accurately simulate how daylight looks in the morning, at noon or in the evening because the perception of colors changes under natural sunlight at different times of the day. Filtered tungsten halogen provides diffuse illumination, making it better suited for evaluating directional samples, such as precious gems and metals; metallic, pearlescent or glossy paints and plastic samples; and high sheen fabrics or directional textiles.
Manufacturers also may need accurate simulations of the average fluorescent illumination that one may find in a retail outlet. Most shoppers today view products under fluorescent lamps that have spikes of energy levels among certain green wavelengths that alter how colors are perceived.
Often, light booths are equipped with incandescent bulbs to simulate what customers may see in their homes and ultraviolet lamps to replicate how paints and coatings that contain optical brightening agents (OBAs) would appear under sunlight. Natural daylight contains ultraviolet rays that are invisible to the human eye, but the rays are converted into visible light when they strike certain chemicals used to brighten colors.
CIE Publication 51 sets international standards of controlling ultraviolet light that can cause metamerism, an optical phenomenon where a pair of samples might match in color under one light source, but appear different under another light source. Lab personnel can also adjust the amount of ultraviolet light in the test stations, depending on how the sample should be tested.
The Right IntensityIn addition to the quality of light, test samples need to be exposed to a controlled intensity of light.
International standards committees such as the ASTM, ISO, DIN, and BAM have outlined some best practices illuminating light, medium and dark colored coating samples when evaluating their colors.
For instance, one standard states that the illumination may be as low as 50 foot-candles, or 540 lux, for viewing very light materials and as high as 200 foot-candles, or 2,150 lux, for viewing very dark materials.
Both lux and foot-candle are measurements of the intensity of light that is brought to bear on a defined area. One lux is defined as one lumen uniformly distributed over an area of one square meter. A typical office may have illuminations in the 320 to 500 lux range, while direct sunlight on a clear day may be measured at more than 100,000 lux. A foot-candle is defined as one lumen uniformly distributed over an area of one square foot, with one foot-candle equaling the power of approximately 10.8 lux.
Companies can determine the lux or foot-candles that are falling on a sample with a light meter or with adjustments in the light booth itself.
After manufacturers determine the right light sources to illuminate test samples, they should check whether their quality control personnel and lab technicians can see colors well enough to perform their duties properly.
Many people cannot see the entire range of visible colors. Experts say nearly one in every 12 males and one out of 255 females do not have normal color vision. In addition to heredity, color vision can be affected by a person’s age, health and prolonged exposure to sunlight.
There are simple and inexpensive tests that indicate how well a person can see colors, such as the Ishihara Color Vision Test and the Farnsworth-Munsell 100-Hue Test. The FM Hue test is specified as the proper test in international standards for visual assessment of plastics, coatings and other materials to distinguish between individuals who have poor, normal or exceptional color vision.
Describing a ColorThe third leg of an effective color evaluation program is to establish standard procedures to get consistent results that have practical limits on what colors are acceptable. Many companies cannot operate under a rule of “no perceptible color differences” because products that meet such a standard would be too costly to manufacture.
To provide quality products at a profitable price, manufacturers need to establish standards on what colors differences are acceptable and explain those standards clearly to their quality control personnel.
For example, companies should document:
how samples are prepared for evaluation;
how samples should presented for viewing; and,
how observers should conduct their assessments and report results.
Some tips on viewing include:
remove everything but the sample and standard from the light booth;
determine the angle that samples will be illuminated and viewed, generally either at a 0 degree angle or a 45 degree angle. If a viewing geometry is specified, then that is the arrangement that must be used by all supply chain partners; and,
test observers should not wear brightly-colored clothing while evaluating samples in a light booth and obviously must not wear colored glasses or colored contact lenses.
When they are reporting their results, test observers should try to use a very limited vocabulary. For example, when describing the sample’s color compared to the standard, the observer should identify if the sample is lighter or darker (lightness difference); yellower, bluer, redder, greener (hue difference); and more or less colorful (chroma difference).
Further, the magnitude of the observed color difference may be described in terms such as slightly, moderately, and extremely. A best practice in reporting results is to use simple, consistent vocabulary to describe color differences. For example, “Sample is lighter and redder, needs to be darker and greener.” The most pronounced color difference element should be mentioned first.
Part of standard procedures is to have a regular method of keeping records, such as:
the light booth and lamps used to view the sample;
the condition of the lamps when the sample was evaluated;
whether equipment has been calibrated and certified as working properly; and
whether observers have been tested for color vision deficiencies.
All of this information and more can be recorded automatically by more advanced light booths on the market. Lab personnel can conduct their tests and record results on a personal computer connected to the light booth. The software automatically maintains all of the important records, which can be sent electronically wherever they are needed or printed out to accompany the physical test sample.
Manufacturers that maintain good records on their visual tests can solve problems much quicker and easier when they occur because quality control personnel can determine if visual evaluation tests were performed correctly. For instance, lab technicians can create files on some software on the market that tell brand owners or specifiers exactly the sequence of lamps that were used in a test to ensure that the wrong lamps were not accidentally selected to cause a false result.
Even though visual evaluations of colors aren’t tracked by numerical result like spectrophotometer readings, they are still valid tests that manufacturers should conduct to make sure they satisfy customer specifications. Q
A free online version of FM 100 Hue Test is available at www.xrite.com/custom_page.aspx?PageID=77 . The online version isn’t exact enough to be used for testing purposes, but it is an entertaining test that gives an indication of a person’s ability to match hues.
Tech TipsColor evaluation is not solely objective.
Filtered tungsten halogen lamps can simulate how daylight looks in the morning, at noon or in the evening.
In reporting results, test observers should use a very limited, very specific vocabulary.