When comparing two samples with the same pigmentation, but different degrees of gloss, the one with the higher degree of gloss is visually perceived as darker and more saturated than the matte sample. For this reason, it is very important to measure both surface effects separately.
Otherwise, the visible difference may be interpreted as a difference in color, although the real difference is in the gloss. For the manufacturer, the changes necessary to adjust the color are different from a gloss correction. The hue (color) is influenced mainly by the type or amount of pigmentation. Gloss is dependent on the utilized matting agents and baking temperature.
The images in Figure 1 illustrate how light is reflected off surfaces of differing gloss. On the first surface, part of the incident light ray is directly reflected and is influencing the image forming quality of the surface, i.e., how glossy and brilliant a surface appears. In the case of smooth, high gloss surfaces, the law of reflection is valid: angle of illumination = angle of reflection. The image of a reflected object can be seen distinctly. In the case of rougher surfaces, the light is not only reflected in the direction of specular reflection, but also diffusely reflected in other directions.
The angle of illumination highly influences the measurement results. In order to evaluate the whole range from high-gloss to matte surfaces, three different angles of illumination are defined, which means three different measuring ranges (Fig. 2).
In order to differentiate the gloss of the samples, it is necessary to select the appropriate measuring geometry. First, the test specimen is measured with the 60° geometry. The 60° geometry should be used if the gloss reading is between 10 and 70 units. If the 60° gloss is higher than 70 units, the 20° geometry will be advantageous. If the 60° gloss is lower than 10 units, the 85° geometry should be used.
The part of the incident light which is not directly reflected penetrates the coating and is absorbed or diffusely scattered by the pigments within the mass of the material. This scattered light exits the coating and is uniformly scattered in all directions. The diffused component causes the color impression.
Visual perception of color is very subjective and has shortcomings. One such shortcoming is that with increasing age, the eye gets fatigue and the lens turns yellowish; consequently, an elder person judges colors to be more red and more yellow. Secondly, our mood influences our color perception. Such things as bad traffic, an auto accident, or a disagreement may affect our visual impression differently than if we are in a good mood. It is also proven that color vision is dependent on gender. The inherited types of color vision defects are far more common in men than in women, with a sexlinked, recessive gene causing them. Men who inherit a single such gene will show some form of color deficiency. Women, on the other hand, must inherit the gene from both parents to be affected. Only about 0.5% of women have defective color vision, while approximately 8% of men will show some form of deficiency. The most common defect is the red/green blindness. One should take into consideration that the phrase “color blind” is misleading. Humans who see only shades of gray are very rare. Thus, the first step for an objective color control procedure is to standardize the observer.
Reflected light from a colored object enters the human eye through the lens and strikes the retina. The retina is populated with three different types of lightsensitive receptors: one which reacts to red light, another to green light, and a third to blue light. Together they stimulate the brain to produce the impression of color. To determine the sensitivity of the receptors, systematic visual tests were done by the CIE (Commission Internationale de Éclairage) in 1931 and 1964. Based on the results, the 2° and 10° observer were standardized (Fig 3), representing a small and large field of view, respectively. When viewing a sample, the eye integrates over a large area, which correlates best to the 10° observer.
Color also changes with lighting conditions. Different light sources cause different appearance: daylight represents a sunny day without clouds, incandescent light simulates a warm atmosphere, like at home, and a cool white light simulates a department store atmosphere. Therefore, standard illuminants have to be agreed upon and used. The prerequisite of a light source to be usable for color evaluation is to continuously emit energy throughout the visible spectrum (400 to 700 nm). The CIE standardized light sources by the amount of emitted energy at each wavelength, which equals the relative spectral power distribution.
The most common illuminants are D65 to simulate
natural daylight, illuminant A to represent incandescent
or tungsten light, and F2 or F11 representing a fluorescent
department store light source (Fig. 4).
Light source and observer are defined by the CIE and their spectral functions are stored within color instruments. Optical properties of an object are the only variables that need to be measured. Modern color instruments measure the amount of light reflected by a colored sample. This is done at each wavelength and is called the spectral data.
For example, a black object reflects no light across the complete spectrum (0% reflection), whereas an ideal white specimen reflects nearly all light (100% reflection). All other colors reflect light only in selected parts of the spectrum. Therefore, they have specific curve shapes or fingerprints, which are their spectral curves.
In the graphs on the next page, typical spectral curves for a red, blue and green sample are shown (Fig. 5).
Color systems combine data from three elements: light source, observer, and object. They are the tools to communicate wand document color and color differences. The system, which is recommended by the CIE and widely used today, is the CIE Lab system (Fig. 6)
It consists of two axes, a* and b* which are at right angles and represent the hue dimension or color. The third axis is the lightness L*. It is perpendicular to the a*b* plane. Within this system, any color can be specified with the coordinates L*, a*, b*. Alternatively, L*, C*, h° are commonly used. C* (Chroma) represents the intensity or saturation of the color, whereas the angle h° is another term
to express the actual hue.
To keep a color on target, a standard needs to be established and the production run is compared to that standard; a typical customer / supplier situation. Therefore, color communication is done in terms of differences rather than absolute values. The total change of color, ΔE*, is commonly used to represent a color difference.
The same ΔE* value can be obtained for two sample sets, and yet they look completely different. To determine the actual change in color, the individual colorimetric components ΔL*, Δa*, Δb* or ΔL*, ΔC*, ΔH* need to be used.
The calculation and interpretation of the differences are determined as shown in Figure 7 below. The color differences must be agreed upon between customer and supplier. These tolerances are dependent both on demands and on technical capabilities.
Until now, two separate instruments – a gloss meter and a spectrophotometer – were needed to measure color and gloss of an object. Today’s modern technologies allow for both measuring systems in one instrument.
by Sandra Weixel, Geretsried, BYK-Gardner