Measuring the Color and Appearance of Special-Effect Paint

Manufacturers that design, formulate and apply coatings have struggled for years to find a rigorous and easy-to-use method that reliably measures the color and appearance of special-effect paints. Companies tried to relate measurements taken with handheld spectrophotometers that collect colorimetric data from several in-plane angles to events that occurred in production processes, but these instruments, by their nature, could not collect the essential data points to yield reliable results. Other companies have tried to skirt the problem by creating a coarseness index or “sparkle” metric that relies on photographic images taken in-plane, but this method is easily confused and insufficient for rigorous analysis.

altExisting laboratory-based instruments can accurately characterize special-effect paints, but the devices are bulky, take hours to process a single sample, and operate only in a sheltered environment. To meld the best of both worlds, X-Rite Inc., applied established principles of quantum electro dynamics, optics, and statistics to essentially simplify the way that laboratory spectrophotometers perform their work.

A New Approach

Engineers employed some fundamental scientific principles to create a three-dimensional mathematical model for any special-effect paint that can be used as a distinguishing fingerprint for designers, paint manufacturers, and their end-users. Production personnel now can immediately identify and troubleshoot defects that are not detected using other methods. The mathematical model can be applied to virtually any product that uses pigmented and flaked ingredients for its color and appearance: automotive paint, metallic printing inks, nacreous pigments in plastics, textured and patterned fabrics, prints on glossy paper and even cosmetics.

The new method detects and quantifies what is essentially a 3 dimensional spectral curve by adding more sensors and illuminators to a spectrophotometer to gather information out of the plane of illumination relative to the test surface. Using this technique, X-Rite was able to unravel in two days a problem involving matching parts coated with effect paints that troubled a major automaker for more than two months. X-Rite coined the term xDNA — similar to the way every person has a unique DNA structure — to describe its three- axis technology that includes an advanced spectrophotometer and software package that interprets data.

X-Rite’s new spectrophotometer uses two illuminators and 11 sensors that measure 31 bands of the visible spectrum, from blue representing the shortest waves in the 400 nanometer range to red representing the longest waves at the 700 nanometer range. The illumination sources are gas-filled tungsten lamps color corrected to approximately 4000°K that flash intense white light at 15° and 45° angles to nominal of the test surface. Light emanating from the test surface is collected at 10 angles from spectral angles at -15°, 15°, 25°, 45°, 75°, 110°, 25°az90, 25°az-90, 60°az125.3, and 60°az-125.3. The “az” notation refers to the azimuthal rotation from aspectral reference.

BiDirectional Reflectance Distribution Function

The foundation of the new method rests on the BiDirectional Reflectance Distribution Function (BRDF), a function first proposed at the College of Optical Sciences at the University of Arizona in 1977 that is used today in applications as wide-ranging as analyzing climate change on Earth, the fabrication of semiconductors and computer chips, and the computer-generated graphics seen in movies. Observers can use the BRDF to learn much about the material nature of an object by directing light of known characteristics onto the test surface, then measuring and analyzing the returning light. Under the BRDF, light coming back from the object must, by definition, have encoded in it the transformation that it underwent inside the object. Because of the law of conservation of energy, the energy of the illuminating light must equal the energy of light reflected, refracted, absorbed and scattered.

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