Multi-angle Color Measurement

1. Visual evaluation of effect coatings

1.1 Multi-angle color evaluation

As metallic finishes show a lightness change with different viewing angles, the sample needs to be tilted to create the same effect during visual evaluation. This effect is also referred to as “light-dark flop”. The bigger the lightness changes between the angles of view are, the more the contours of an object will be accentuated. In order to observe color travel of interference finishes, the panel should be moved to allow increasing or decreasing the angle to the light source (Fig. 1).

1.2    Visual effect evaluation

In addition to color changes our total perception is also influenced by the effect of the metallic flakes or other sparkling pigments. This effect changes with the lighting conditions, for example direct sunlight versus cloudy sky (Fig. 2).
A sparkling or glitter impression can be observed under direct sunlight. This effect is often described with different words such as sparkle, micro brilliance or glint and is generated by the reflectivity of the individual effect pigment. 
Therefore, it is influenced by the

• Flake type and size
• Concentration level of the effect pigment
• Orientation of the effect pigment
• Application method 

The sparkle impression changes depending on the illumination angle.

Apart from the sparkle effect under direct sunlight, another effect can be observed under cloudy conditions, which is described as coarseness or salt and pepper appearance. This visual graininess can be influenced by the flake diameter or the orientation of the flakes resulting in a non-uniform and irregular pattern. The observation angle is of low relevance when evaluating graininess [1, 2].

Image 1 A - Visual evaluation of traditional metallic finishes / B - Visual evaluation of effect coatings with color flop

Image 2 Sparkle under direct sunlight vs. Graininess under cloudy sky

2. Multi-angle color measurement

ASTM, DIN and ISO standards define multi-angle color measurement to objectively describe the color of metallic finishes. Research studies show that a minimum of three, and optimally five viewing angles are needed [3, 4, 5, 6, 7]. The measurement geometry for multi-angle color measurement is specified by aspecular angles. The aspecular angle is the viewing angle measured from the specular direction in the illuminator plane. The angle is positive when measured from the specular direction towards the normal direction (Fig 3) [8, 9].
Directional illumination is used versus circumferential illumination because circumferential illumination minimizes the contribution from directional effects such as the Venetian blind effect and surface irregularities. Thus, averaging of the circumferential illumination would cause the measured color values of two specimens to be the same, while visually the two specimens would not match. For color QC, the colorimetric data L*, a*, b* (or L*, C*, h°) and delta E* can be used. The tolerances are usually higher for the near specular (15°, 25°) and the flop angle (75°, 110°) than the 45° tolerance. In order to have a unique tolerance parameter independent of color, weighted factors have to be used. Therefore, automotive companies often have set specifications on delta E’ based on DIN 6175 using 3 or 5 angle instrumentation.
Another useful index is the flop index, a measure of the change in lightness of a metallic color as it is tilted through the entire range of viewing angles [10].
In the last years a new generation of special effect pigments has become more and more popular. For some of these new pigments the color travels over a wide range. In order to fully capture the color travel of these interference pigments it is necessary to add viewing and illumination angles. To keep the whole procedure practical for industrial use with a portable spectrophotometer it was determined that an additional angle behind the gloss e.g. -15° is of benefit (Fig 4 and 5) [9, 11].

2.1 Multi-angle color and effect measurement with BYK-mac i

Traditional 6-angle color measurement calculates color values by averaging the spectral reflection over the entire illuminated spot and therefore cannot differentiate between the color of the basecoat and the reflection of the aluminum flakes. As a consequence, two effect finishes can have the same color values with a 6-angle spectrophotometer, but visually appear very different. The visual difference is a result of the flake effects.

To characterize the impression of effect finishes under different viewing angles and illumination conditions, the BYK-mac i spectrophotometer objectively measures the total color impression (Fig. 6):

• Multi-angle color measurement (6-angles) clearly defines the light-dark as well as color flop behavior of effect finishes
• Sparkling and Graininess control with a high-resolution CCD camera simulates effect changes under direct and diffuse lighting condition

Image 3 Schematic of a multi-angle spectrophotometer

Image 4 Color travel of an interference pigment

Image 5 Measurement of color travel behind the gloss at -15°

Image 6 Schematic for multi-angle color and effect measurement

3. Effect Measurement

3.1 Sparkle measurement under direct illumination

The sparkle impression changes with the angle of illumination. Therefore, the BYK-mac i spectrophotometer illuminates the sample under three different angles 15°/45°/75° with very bright LEDs and takes a picture with the CCD camera located at the perpendicular.
The pictures (Fig. 7) are analyzed by image analyzing algorithms using the histogram of lightness levels as the basis for calculating sparkle parameters. The overall impression of sparkle can be described with the one-dimensional parameter sparkle grade. A sparkle grade is defined by sparkle area and sparkle intensity (Fig 8). As the one-dimensional sparkle grade can have the same value with different ratios of sparkle area and intensity visually acceptable tolerances need to be defined based on comparing a sample to a standard. Therefore, the sparkle data are also displayed in a difference graph (Fig. 8). A new sparkle tolerance equation ?S was developed with several auto OEM makers, paint and pigment manufacturers based on visual correlation studies. As a guideline the weighted total color difference equations were used resulting in an elliptical tolerance model.
The human eye is less critical to a change within a sparkle grade than it is to a change from grade to grade. Therefore, the longer axis of the ellipse is towards the sparkle grade lines. To use the model as a Pass/Fail tool for paint batch or part QC, the total sparkle difference between sample and standard is calculated: ΔSparkle

3.2 Graininess measurement under diffused illumination

Graininess is evaluated by taking a picture with the CCD camera under diffused lighting conditions, created by a white coated hemisphere. The picture (Fig. 9) is analyzed using the histogram of lightness levels whereby the uniformity of light and dark areas is summarized in one graininess value.
A graininess value of zero would indicate a solid color, the higher the value the grainier or coarser the sample will look under diffused light.
 

Image 7 Low and high sparkle (glint)

Image 8 Visualization of sparkle area and intensity

Image 9 Low and high graininess (coarseness)

4. Influence of flake size on sparkle and graininess

Sparkle and graininess data give information on flake size and concentration levels. The sample below (Fig. 10) shows a silver finish with three different flake sizes (25 µm - 34 µm - 54 µm). Visually, the silver finish with the coarser aluminum pigments appears more sparkling under direct illumination and more “grainy” under diffused lighting.
The BYK-mac i measurement data correlates with the visual judgment: sparkle area, sparkle intensity and graininess increase with flake size. 

Image 10 BYK-mac i effect data for silver finishes with different flake sizes

5. Influence of flake orientation on total color impression

Besides flake types and concentration levels, the comparison of sparkle area at 15° and 75° illumination gives information about flake orientation.

5.1 Influence of paint application method 

In order to increase paint efficiency, the basecoat application is changing to 100% electrostatic application. Metallic finishes containing coarser aluminum flakes will show more non-parallel oriented flakes. The result will be a lower light-dark flop and more sparkling at a low grazing illumination angle. In the following example the basecoat of the car body was applied 100% electrostatically and the bumpers were painted with a bell / pneumatic application. The total color difference using the mean ΔEDIN was acceptable.

Yet, visually, the car body was sparkling considerably more than the bumper. The BYK-mac i measurement data reflects the visual impression clearly evaluating the Sparkle 75° data. The Sparkle 75° measurement evaluates the aluminum flakes which are non-parallel oriented; therefore, the main changes can be seen in an increasing sparkle area (Fig. 11).

5.2 Influence of rheology additives

Flake orientation can also be influenced by the paint formulation, e.g. the rheology additive. As fine aluminum flakes have more edges and consequently more light is scattered, the orientation is more important for coarser pigments. The use of an optimized rheology additive will result in a better light-dark flop and less sparkling at lower grazing angles. 
In the following example (Fig. 12) a waterborne system was evaluated using three different rheology additives: a standard system, an acrylic thickener and the BYK-Chemie wax additive AQUATIX®. Visually, the three panels look the same under direct illumination at a steep angle. When comparing at a lower grazing angle, the system using the BYK-Chemie wax additive shows less sparkling.
BYK-mac i measurement data correlates with a visual judgment. The sparkle area for the system with wax additive at 75° is smaller than for the two other systems. As Sparkle 75° evaluates flakes which are non-parallel oriented, this clearly shows that by using the BYK-Chemie wax additive AQUATIX® the orientation of the aluminum flakes is improved.

Image 11 BYK-mac i effect data showing the influence of the application method

Image 12 BYK-mac i effect data shwoing the influence of the rheology additive

6. Summary

The introduction of more and more new effect pigments requires new innovative measurement technologies to capture the total color impression. It is no longer sufficient to measure color only under different viewing angles. The color perception is also changing with different lighting conditions (direct and diffused illumination) and therefore, so-called sparkle and graininess effects need to be objectively described.
BYK-Gardner’s BYK-mac i spectrophotometer (Fig. 14) together with the data analysis software smart-chart is a comprehensive tool to objectively control total color impression in the lab and at the production line.