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Human perception still integral to color testing; instrumentation can't detect subtleties

Taking on ever greater aesthetic standards, colors and color matching are becoming an integral component of plastic manufacturing. As with all aspects of molding, the inherent drive is towards standardization and the greatest level of repeatability. In striving for these elusive goals, the use of complex testing machinery has been accepted as a means of acquiring quantifiable, precise results. But in the instance of color, research by the color concentrate supplier Teknor Color Co. (Pawtucket, RI) suggests that the best results from color testing are borne out of the joint efforts of humans and machines.

While machinery provides precise quantifiable results, there are many subtle nuances to color detection for which the eye is more adept. Two prime examples are gloss and surface differences, which the eye is more suited to detect. As a null detector, the eye can catch very subtle differences in color with the ability to differentiate between 10 million shades in some instances.

The eye is only effective under certain circumstances, however. Assuming there’s no color blindness, eyes should only view colors, especially bright ones, for 20 seconds. Fatigue in color-sensing cells is diminished the most if color viewing is limited to 10-second intervals. The background on which a sample is viewed also has an effect. A neutral gray like Munsell N7 is ideal, and this situation is further enhanced if it’s placed in a light booth.

For more precise color testing, instrumentation has been developed. Using photosensitive detectors to measure sample reflectance, these machines emulate their natural counterpart, the human eye. The earliest example of such technology is the tristimulus colorimeter. Using a stable light source and a photoelectric cell detector, this device uses filtered illumination to pinpoint three shades: red, green, and blue. Then the detector cell quantifies the intensity of the reflected light.

Fairly rudimentary, colorimeters are currently limited to quantifying whiteness or yellowness in natural polymers or the brightness value of white pigments. Supplanting colorimeters are diffuse sphere spectrophotometers. These devices use indirect illumination for greater accuracy and replace the photoelectric cell with a monochromator. They measure the entire range of the visible spectrum at 400 to 700 nm intervals.

Paired with computers, these devices perform color tolerancing with software like CIELab from the Commission Internationale de l’Eclairage. A three-axis coordinate system plots lightness/darkness, red/green, and yellow/blue. These tolerances create a cube that assumes humans perceive visual differences in areas of hue, chroma, and lightness/darkness equally.

To bridge the gap between visual and instrumental testing, a new tolerance scheme, CMC, has been developed. Instead of cube, tolerances are plotted in an ellipsoid, which is said to best approximate human perception.

Four factors hinder instrumental evaluation, and make it obsolete when it’s not paired with human perception. Metamerism is one such factor and involves varying results depending on the light source. A sample may appear one way in natural light, but under fluorescent light, it’s perceived differently. Fluorescence refers to samples absorbing energy in the UV range and converting it to visible light, causing error in detection. Directionality of color is found with metallic or pearlescent pigments and causes different results depending on part orientation. Finally, specular gloss can saturate a detector and interfere with detection.

Teknor Color Co., Pawtucket, RI

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