Visual and instrumental correlation of sparkle by the magnitude estimation method

Abstract

Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XX Most real surfaces and objects show variation in appearance with viewing and illumination directions. Besides angular dependency, they also show spatial variation in color; i.e. they exhibit some sort of texture. Of the surfaces we see, surfaces colored by special-effect pigments produce several complex visual effects, like change in color and lightness with viewing and illumination angles, and effects like sparkle and gloss on other textures.In the last two decades, different commercial devices have appeared to help ensure the proper characterization of materials with special-effect pigments. However, the instrumental characterization of sparkle is currently available only by a commercial device integrated into a multi-angle spectrophotometer. As it is difficult to find complete open original studies about the sparkle effect for designing and calibrating this commercial instrument, the main objective of this work was to check whether a good visual and instrumental correlation exists between the sparkle that the observer perceives and the sparkle value provided by the device using some subsets of goniochromatic samples with different types of special-effect pigments and colors. Visual assessments were made by a conventional magnitude estimation method in a directional lighting booth, which belonged to the same company owner of the sparkle instrument, in different geometries and at distinct illuminance levels.The results revealed that there was a good visual correlation of the sparkle-grade value. By separately analyzing the factors used in its instrument algorithm, such as sparkle intensity and sparkle-area values, it was clearly shown that the correlation was not good or simply did not exist. Consequently, and perhaps as regards choice of new special-effect pigments, such as synthetic mica and other future ones, we generated herein even more questions about current mathematical algorithms, and only recognized calculating this texture effect at the industrial level.

Review of instrumental inter‐agreement study of spectral and colorimetric data of commercial multiangle spectrophotometers

“…The visual appearance of a material with these special‐effect pigments is very attractive since the color appearance changes with changes in the illumination and observation directions (goniochromatism). In addition, they provide visual texture effects (gloss, sparkle or glitter or glint, coarseness or graininess, pearliness, etc), and therefore it requires a complex instrument to characterize these materials completely, and to propose efficient models of visual and instrumental correlation for detection, scaling and discrimination (including tolerances). Thus, in many cases, a conventional optical set‐up, typically used in classical spectrophotometers or based on color‐imaging systems, both for diffuse or directional geometries, is not completely efficient for color management in these industries (automotive, etc.…”

Evaluating the Graininess Attribute by Visual Scaling for Coatings with Special-Effect Pigments

“…To measure the sparkle effect, the sample is illuminated directionally at 15 • , 45 • and 75 • , and counted to the normal direction from the sample surface. Three parameters are obtained to characterize sparkle: sparkle intensity (Si), sparkle area (Sa) and sparkle grade (S G ) [7,8,13]. To measure the graininess effect, the sample is diffusely illuminated by an integrating sphere.…”

Graininess characterization by multidimensional scaling

“…Sparkle intensity is specified as the intensity of the small bright light spots in relation to the intensity of the surrounding less bright. Sparkle area and sparkle intensity are combined in the representative sparkle attribute called sparkle grade (6,7,13). To measure the graininess effect, the sample is diffusely illuminated by an integrating sphere.…”

“…However, there are no standards like ISO, ASTM or DIN which propose the mathematical and optical algorithms required to measure and calculate the sparkle or graininess effect implemented by the BYK-Gardner company. Therefore, it is very important to visually validate the sparkle and graininess effects in psychophysical experiments as these texture effects are important for the visual discrimination of many materials and quality control (13).…”

“…Figure 3): L1 : L * = 60, G [3.5, 8] L2 : L * = 45, G [4, 9.5] L3 : L * = 35, G [4, 11] L4 : L * = 25, G [4, 12.5]L5 : L * = 15, G[4,13] …”