Special-Effect Pigments - Color and Perception

The pigments can be roughly categorized as follows.

• Metallic pigments consist of very thin metallic flakes or wafers that act as miniature mirrors. Their intensity changes according to the angle from which they are viewed. Maximum light intensity is achieved near the "gloss", the angle at which the incident light is reflected. Minimum is experienced at an angle far away from the gloss.
  Factors affecting this flip-flop effect are the shape and size of the pigment particles and the way they are laid down within the paint.2,3

• The most requested class of effect pigments at the moment is basically a mica flake coated with single or multiple thin layers of metal oxide. The interplay of colors produced by these pigments is due to the layered structure of the metal oxide, which is also responsible for the rich deep glossy effect.2,3

• Striking optical effects are achieved by combining specially selected coated transparent flakes of silica. The flakes are produced in a uniform thickness, which gives purer interference colors and gloss. Brightness of color and a conspicuously angle-dependent viewing effect provides outstanding styling potential.3-5

• Coated aluminum-oxide flake pigments offer a brilliant sparkling effect, a phenomenon previously unseen in pigments of such small particle size.3-5

• Opaque multiple-layer pigments produce rich interference colors that depend strongly on the viewing angle.6,7

• The optical effects of liquid crystals are due to the helical structure exhibited by highly crosslinked liquid-crystalline structures of organic molecules. The pigments are transparent and produce their angle-dependent effects on colored undercoats.8,9

Interference - Color by Physics

Light impinging on a pigment is partly reflected from its surface and partly refracted through it. This causes wavelength shifts, with individual rays of light intermingling and so causing the familiar interference. When wave crest meets wave crest, the amplitude becomes more intense. A wave trough meeting a peak partially cancels the amplitude, though total cancellation practically never occurs.

As an example, a classical interference pigment such as an Afflair type pigment consists of a flake of mica surrounded by a layer of titanium dioxide. When white light falls on this type of pigment, some of it is reflected at the surface of the pigment, also creating a phase shift of half a wavelength. The remainder of the white light is transmitted through the titanium dioxide layer and meets the surface of the mica, where it is again partially reflected. This reflected component emerges from the pigment parallel to the first reflected component. Since it had further to travel through the titanium dioxide layer, the wavelengths of the second light component will be back in phase with those of the first. As a consequence, the light waves will be intensified and thereby produce the color reactions as perceived by the human eye. By calculating the shift under simple and ideal conditions, it is possible to arrive at an equation for maximum reflection and hence also for the resulting color. The elements of this equation are the thickness of the refracting titanium dioxide layer, its refractive index and the angle of illumination. As the thickness of the layer increases, the color of the Afflair pigment changes from silver-white, through yellow, red and blue, to green.

Starting from a flat angle of incident light (relative to the horizontal surface of a test panel), the maximum shifts to the longer-wave region as the angle of illumination becomes steeper. For example, Blue Pearl changes from violet to greenish blue as the angle is altered from flat to steep (see Figure 1).12 Such color shifts are indicative of interference phenomena at work13,16,17 and are thus typical of these kinds of pigment. If the illumination angle is kept constant and the viewing angle altered, again a color shift is perceived (see Figure 2). Applying the aforementioned interference pigments to a white undercoat will, at more than 30 degrees from the gloss angle, produce the color that is complementary to the immediately reflected color as it is reflected from the white undercoat (see Figure 3).

Two fundamental color shifts can be discerned in interference pigments.

• The first is the shift produced by varying the illumination angle and keeping the aspecular angle constant (i.e., 15 degress from the gloss angle). This is the typical interference shift.

• The second effect is when the illumination angle is kept constant (i.e., at 25 degrees or 45 degrees) and the aspecular angle is varied. In this case (depending on the illumination angle), the typical color curves are produced which, as the viewing angle increases, tend towards the same or almost the same point. This phenomenon is known as aspecular shift.17

Visual Judgement of Effect Pigments

A real object, such as an automobile, has numerous angles and contours. When the sun is high in the sky, horizontal surfaces reflect directly upwards; the observer sees the effect away from the gloss. It is interesting to note that different color effects are produced depending on whether the observer is standing on the side of the gloss nearest to the light source in the cis position or "behind the gloss" in the trans position (see Figure 5).

With the sun in the high position, light rays strike the sides of the automobile at a shallow angle and the viewpoint is a long way away from the gloss. When the sun is low in the sky, on the other hand, light impinges on the sides of the vehicle at a steep angle; at the same time, horizontal surfaces such as the roof and hood are illuminated from a flat angle. Again, a variety of color effects are achieved depending on how tall the observer is and how far he/she is away from the automobile.

Multi-Angle Measurements - Absolutely Essential

The immediate question is: what features must the spectrophotometer have to enable this type of testing to be carried out? The answer to this question can be derived from points previously made. First, there must be a variable light source. In a spectrophotometer this is achieved through pre-set angles. Second, the observation point must also be variable. This is again achieved through pre-set angles.

This is the most efficient way of measuring the color dynamics that result from the combination of geometries and pigments.

Spectrophotometers - An Angle-Dependent Overview

The Phyma WICO 5&5 System measures under 22.5 degrees and 45 degrees illumination, offering 67.5 degrees, 0 degrees, -22.5 degrees, -45 degrees and -67.5 degrees viewing and a large measuring spot. The five pre-sets between 0 degrees and 180 degrees (relative to the horizontal) provide virtually ideal pigment definition.

The Murakami GCMS-3 provides complete definition in 1 degree steps. The instrument features illumination from 16 degrees to 180 degrees and observation up to 196 degrees (transmission).

Totally variable reflectance and transmittance measurements from 0 degrees to 360 degrees can be achieved with the GON 360 from Instrument Systems.

For comparison of results, the Zeiss GK 311/M multi-angle instrument was used, though the instrument is no longer manufactured. In it, it is possible to set both the angle of illumination and the viewing angle independently of one another in 5 degrees steps. The minimum illumination angle is 25 degrees (horizontal) and the maximum viewing angle 155 degrees.

The Results - More than Just Numbers

Afflair 97225 Ultra Blue Pearl
This transparent pigment is one of the most frequently used blue pigments. Its powerful blue reflection color changes from blue-violet when illuminated at a flat angle to blue-green when the angle is steeper and when viewed near the gloss. Moving from "near gloss" to "remote from gloss" causes the color to become duller irrespective of the angle of illumination. The Zeiss GK 311/M clearly shows this color change - as an interference shift as the angle of illumination is varied from 25 degrees to 70 degrees, and as aspecular shift at 25 degrees and 45 degrees illumination as the effect angle rises above 15 degrees. The angles change clockwise from 0 degrees to 180 degrees relative to the horizontal. The 45 degrees illumination angle has its equivalent specular angle at 135 degrees; measurement with a 15 degrees effect angle thus occurs at 120 degrees.

The 45 degrees effect line (aspecular shift) for Afflair 97225 can be demonstrated in the X-Rite MA68II beginning with 15 degrees up to a 110 degrees effect angle. The Macbeth 642 gives similar results, except that it only begins measuring at a 25 degrees effect angle. The Optronik MultiFlash, too, only starts at a 25 degrees effect angle, though comparison is difficult as the measuring geometrics change from the cis to the trans position. Conspicuous during measurement of the transparent pigment types is their "swing" to the 115 degrees geometry. With its circular illumination, the Minolta CM512m3 can barely produce the full color effects of Afflair 97225, having, as it does, only three measuring geometries (see Figure 8).

Colorstream Ti-Pigment
This pigment is one of the second-generation innovative types based on thin flakes of silica (SiO2). The color changes from violet to green at a constant effect angle of 15 degrees. The effect lines (aspecular shift) under 25 degrees and 45 degrees illumination are tending toward the same point.

It can be clearly seen that the aspecular shift at 45 degrees shows good agreement in both instruments (see Figure 5a). However, it also becomes clear that the instrument with the fixed illumination angle of 45 degrees cannot adequately display the broad color bandwidth of the pigment.

ChromaFlair 190
This pigment changes from yellow to green via red, violet and blue when the angle of illumination is changed while the aspecular angle is kept constant. When the illumination angle is kept constant, the color changes to dull.

The area that is described by the color changes can be well presented in the a*b* diagram. Although the CIELAB system, strictly speaking, is only applicable for showing color differences, it is nevertheless useful for demonstrating color effects. The Zeiss GK 311/M cannot clearly register visual impressions of color. Since its design does not permit illumination angles flatter than 25 degrees, the visual impression of yellow at < 25 degrees can no longer be measured.

While all of the measuring instruments register the color change at 45 degrees illumination or observation in more or less the same way, the positions of the data points with the Minolta are much more conspicuous here than with Afflair 97225. This instrument changes the angle of illumination and the aspecular angle simultaneously, as shown in Figure 10.

Magic Purple ED 1480
BASF's Variocrom pigments are comparable with the ChromaFlair pigments from Flex Products. Starting with yellowish-red, the color of this pigment shifts toward red-violet when the angle of illumination is made steeper while the aspecular angle is kept constant. Under constant 25 degrees illumination the effect line (aspecular shift) at 30 degrees from gloss intersects the interference line (interference shift) at geometry 35/130 degrees and forms a common point of different intensities.

Clearly to be seen with this pigment is the similarity of measurement data obtained with the various instruments, though the Minolta does stand out on account of its different measuring geometrics. Also, the Optronik MultiFlash using flat illumination far exceeds the results of the other instruments.

Helicone 624
Visually, an intense green is observed with flat illumination, changing to an intense red as the angle of illumination becomes steeper. Changing the observation angle away from the gloss causes this pigment to tend toward dull.

The Zeiss GK 311/M produces an interference line with pronounced arc shapes with yellow as transition color in the region of 45 degrees illumination, while the other instruments only "begin" with yellow and give measurement data tending toward dull as the aspecular angle increases.

The Issues - and a Pragmatic Solution

Summary

Therefore, any measuring instrument used must be able to measure both these types of shift. Only then will judgement of the pigment be optimum and precise. Illumination and observation angles should therefore be introduced as close as possible to the horizontal, so as to better mimic the actual visual impression through the measurement data.

Acknowledgement
We wish to thank to Minolta, GretagMacbeth, Optronik and X-Rite for kindly providing the measurement data.

References