In the eye of the beholder

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The human eye is a powerful instrument that can distinguish over 350,000 shades. While this color sensitivity unquestionably enhances the human experience, it can also be a source of conflict between suppliers and customers where color consistency is vital.

Sabrina Köhler, who runs the colorimetric laboratory at the Hohenstein Institute in Bönnigheim, Germany, says the majority these color conflicts can be avoided with instrumental and visual color analyses, even when target values are on different materials and surfaces.

In product development and manufacturing there are often no normative values that can be used to define acceptable color deviations. Therefore, it is important to stipulate the acceptable range of color tolerances from the initial product concept through development and through ongoing manufacturing. To achieve the best possible color consistency, instrumental and visual color analysis is needed at every stage.

Instrumental color measurement uses a spectral photometer to determine the spectral reflection of test samples. The process examines which portions of white light are reflected by a material sample, establishing color perception in the human eye. Scientists at the Hohenstein Institute then derive tristimulus values and color chromaticities (degrees of whiteness) from the spectral data according to different internationally used formulas. Color differences between control and reference samples and pass/fail standards can also be determined with this process. The testing can be done on all types of textiles, glass and plastics as well as on paints, varnishes and coatings applied to carrier media.

Many companies supplement instrumental analyses with visual color analyses for continuous quality assurance. Specially trained employees evaluate the color effect under controlled lighting conditions. Köhler advises caution with visual color analysis, however, noting that not all employees are suitable for visual color evaluation due to their visual acuity. Approximately 8 percent of men and around 0.5 percent of women are affected by a color vision deficiency, usually manifesting as an inability to distinguish between red and green hues.

The Hohenstein Institute offers a special eye test for employees in color quality assurance. The Farnsworth-Munsell 100 hue test requires subjects to place 85 color caps in the correct order of hue according to the color wheel. A computer analysis of the results looks for any deviations from the ideal sequence. If the test shows no serious deficiencies in color vision, there are no objections to the visual sign-off. As eyes change with age, the tests have to be repeated regularly.

Source: Hohenstein Institute

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