1. Color Measurements
The human eye can detect very small differences in color, but it cannot assign numerical values to those differences. Visual comparisons between two colors include whether the sample color is lighter or darker, redder or greener, or yellowier or bluer, than a standard color. This forms the basis of the internationally standardized CIEL*a*b* tristimulus color system1. Without going into any details regarding color theory, or the mathematics involved with it, we can define
L*: Black/white axis. High values mean lighter (whiter), low values mean darker (blacker) colors. Scale goes from 0 (black) to 100 (white).
a*: Red/green axis. Positive values are red, negative values are green. Scale goes from +127 (reddest) to -128 (greenest) colors.
b*: Yellow/blue axis. Positive values are yellow, negative values are blue. Scale goes from +127 (yellowiest) to -128 (bluest) colors.
1.1. L*a*b* Color Coordinates:
‘Hunter Lab tristimulus coordinates are sometimes used in the U.S. The values are similar, but not identical, to CIEL*a*b* numbers.
2. Color Control
Color differences are calculated from the individual L*, a*, and b* coordinate differences between a color standard and a color supposedly matching the standard. Such color differences are named Delta E*, or DE* for short.2
The equations for calculating L*, a*, b*, and DE* were set up to reflect a color difference of 1.0 DE* to be the smallest possible color difference seen by an untrained, “average” person.
DE* values are being used as a quality control measure for color differences in the North American powder coating manufacturing and application industries:
|0.0 to 1.0
||Called “commercial match.” Accepted generally by custom coaters and low cost general metals applications.
|0.0 to 0.5
||Narrower color difference; specified often by OEMs in telecommunications, higher-end metal furniture, appliance, heavy equipment, military suppliers, and architectural industries.
|0.0 to ~0.3
||Very narrow color difference; specified by OEMs in automotive and some appliance industries.
3. Color Standard Maintenance
• Keep at least two color standards for each color.
• One standard—called reference standard—should be kept in a non-transparent paper enclosure (envelope or similar) and locked away at all times in a cool, dry, and dark place. Major OEMs and paint manufacturers keep reference standards in refrigerated units to slow down aging (which causes yellowing) of the finish.
• Do not keep reference standards in transparent or non-transparent plastic wrappings or enclosures; plasticizer in the plastic could migrate into the finish of the standard and cause changes in gloss or color.
• The second standard—called working standard—should be used in production and quality control for color comparison against new lots of the same color. This working standard should also be kept in a non-transparent paper enclosure (envelope or similar) and locked away at all times in a cool, dry, and dark place when not in use.
• The working standard should be checked against the reference standard periodically to assure color consistency between the two standards.
• Working standards may get dirty or grimy over time from dust and fingerprints, which affects color readings. Such standards can usually be cleaned by gently using water, liquid dishwasher detergent, and a soft sponge or soft paint brush no scrubbing with hard brushes or similar abrasive media. Allow the cleaned working standard to dry out for at least a couple of hours or preferably over night—before using it again. This cleaning method works fine for all gloss ranges and several textures, except for sandpaper-like textured (sand texture) standards.
• If the working standard cannot be fully reconstituted (cleaned) anymore, and the color difference between the working standard and the reference standard becomes greater than about 0.2 to 0.3 DE* units, replace it with the reference standard and ask the paint supplier for a new reference standard. Better yet, discard both standards and asks the paint supplier for two new standards, otherwise a possible color difference between the old reference standard and the new reference standard has to be factored in with each color check.
4. Special Tips for Color Checks
• Do not store color data in the memory (database) of the color computer! When checking colors against computer stored standards, voltage and amperage fluctuations from starting or stopping energy-hungry machinery in the vicinity of the color computer can result in fluctuations of the light source illuminating the test sample. This can result in faulty readings of up to 0.3 DE* units! Therefore, always compare test panels from newly delivered paint lots directly against the working standard, taking at least two comparison readings.
• Calibrate your spectrophotometer maybe every 4 hours, or as often as the operating manual or laboratory procedures demand
• ALWAYS (!) use visual comparisons under specified light sources (light booth), or natural light, to verify DE* readings. Some spectrophotometers show very poor readings near mass tone colors e.g. very dark blue or green colors.
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