Without color the world would be a boring place. The ability to see colors is a gift we should treasure. That being said, why can we see the world in so many colors?
Figure 1: With color the ripe oranges can be seen from far away (picture: pixabay)
The human perception of color is possible as the eye developed additionally to the light sensitive rod cells some new specialized retinal cells – the so-called cone cells. The evolution of these cells began, when primates switched their activities from night to daytime and began to search for ripe fruits and different kind of plants. Color became a new advantage for surviving.
A healthy human eye nowadays can see many different color shades, from yellow to violet with different saturations and lightness levels. In total we can detect more than 10 million colors.
Our visual acceptance for color differences changes dramatically from chromatic to achromatic / pastel-like colors. Our eyes are much more tolerant when comparing colors in chromatic areas than for example in the grey area. Scientists saw a need in differentiating both color types - chromatic and achromatic– to create color tolerances which fit best to our visual perception.
The fact that we accept more color deviation in colors with higher C*-values than color deviations in color areas with lower C*-values was already examined in the beginning of the last century by visual assessment tests of David MacAdam and later further developed by the International Commission on Illumination (CIE Commission internationale de l’éclairage). The so-called tolerance ellipses (figure 2) are smaller and more “spherical” when closer to the achromatic midpoint and wider when the color turns more into the chromatic direction on the outer lines. The ellipses are also longer in the chromatic direction and narrower in the hue direction: this means the human eye accepts more deviation in Chroma than in Hue.
Figure 2: The graph shows the CMC tolerance ellipsoids in the CIELAB space.
Because of this fact tolerances for achromatic / pastel-like colors should be set "spherical" in the 3-dimensional color space. L*, a* and b* are the values which need to be controlled. For chromatic colors the elliptical tolerance based on L*C* and h°- values is the best decision.
But when is a color defined as chromatic? The first version of DIN6175-2 and also some automotive OEM propose to use LCH-tolerances when the C*-value of the color is larger than 18. For dark colors, when the L*-value is smaller than 27 the proposal is to use also LCH-tolerances when the C*-value is higher than 10.
Figure 3: Examples for achromatic (top row) and chromatic (bottom row) solid colors measured with BYK-mac i (only 45° angle is presented).
The soft rose and the dark olive on the right side are borderline between L*a*b* and L*C*h°. For these borderline colors it is more or less the same whether one explains the color deviations with L*a*b* or L*C*h°.
For a chromatic red it is more natural to explain color deviations in L*C* and h° and not with L*, a* and b*. The sample on the right side is less chromatic, instead of saying the red is greener compared to the red on the left side.
Figure 4: Solid colors measured with BYK-mac i (only 45° angle is presented)
When looking at effect colors this topic becomes even more complex. An effect color can show high C*-values at the flash / near specular angle (15° angle) and low C*-values at the flop angles (75° and 110° angle). In these "mixed" chromatic / achromatic cases the color deviation should be explained for the specific viewing angles. In the following example, the sample is less chromatic near the flash angle and more green and yellow at the flop angle.
Figure 5: Pearl colors measured with BYK-mac i (only 15° angle and 110° angle are presented)
High chromatic colors in the automotive industry are a challenging topic despite allowing larger tolerances. On the one hand, it is obvious that these vivid colors are special which is why OEM designers are always on search for them. A new high chromatic color on the street is immediately catching the eye. The color and the shape of a vehicle are the first things we detect and a high chromatic candylike looking color is often used when presenting a new car model.
On the other hand, these colors are very difficult to handle from the technical point of view. The pigments used for high chromatic colors are much more expensive and often show a lower weather stability. In addition, since the hiding power of the pigments is not sufficient, a higher pigment amount is needed in the formula to reach the demands of the OEM specification. But when the pigment load goes higher, the cost increases and the color layer adhesion becomes weaker. One challenge after the other needs to be overcome.
But not enough yet. With new application processes like spraying transparent basecoat layers on top of a normal basecoat new chromatic colors can be created like the famous Mazda Red 46V. Also tinted clearcoats lead to new chromaticities in colors. And again, more new challenges are created with these methods: these new colors are even more challenging in regard to adhesion and matching in other technologies or for plastic parts. Let’s not even talk about the challenge to allow spot repair.
There is no time to get bored in the world of colors as there are always new challenges to master.