It is no secret that color is critical to the cosmetic industry. Color affects consumer emotions and drives up to 95% of purchase decisions (Pigments in Cosmetic Formulations - Effect Pigment Symposium 2020). Color's importance weighs so heavily on the cosmetic success that the industry uses a scientific approach to ensure product quality.
When measuring color, typically, a spectrophotometer is used. The instrument measures the spectral transmission or reflection of a specimen under one illumination and one viewing geometry. This geometry is typically diffuse illumination with a detection angle 8 degrees from the normal of the specimen, but other geometries exist, such as 45-degree illumination (annular or circumferential) and 0-degree detection.
The Behavior of Colorant in Cosmetics
Before we dive further into measurement instrumentation and techniques, it is essential to look at the use of colorants in cosmetics. Major colorants are categorized as dye and pigment. A dye is soluble in water or oil, while pigment stays as a particle. Pigment generates light reflection in a cosmetic product. The human eye captures reflected light and recognizes it as the color. Different types of pigments create a different appearance of a product. Absorption pigment reflects light of a specific wavelength while absorbing other light, creating the matte color effect. On the other hand, effect pigment like pearl luster pigment gives a shimmering and glittering impression, enhancing color expression in a product. Pearl luster pigment consists of a substrate such as mica and coating such as Titanium dioxide (TiO2). Incident light is reflected in multiple layers, which creates a pearl-like shinning effect. Substrate Layer of metal oxides Furthermore, it can create interference pigment by changing the thickness of the coating. Using the same coating material like TiO2, reflection, and interference from different coating thicknesses creates various color expression.
The beauty of effective pigment is that it can create a different appearance by angle. For simple evaluation of such pigment, three primary angles, face, flash, and flop, are used.
Face: The color of a surface observed in line of sight along the normal to the surface. The surface is directly "facing" the observer. Flash: is an angle near the specular angle, usually as 15 or negative 15 degrees. This would be near the highlight viewed on the surface.
Flop: is the color observed at a large angle to the specular is called "Flop color," When the flop color is different from the flash color, the specimen "flops" This is the color you will notice a sample's contour curves away from the normal. Effect pigment can have what is known as a lightness flop on the Flop Index. The flop index, FI, essentially describes the lightness difference between flash and flop angle, with the other terms used as scale factors. FI = 2.69 (*+,° ∗ /*++0° ∗ )+.++ *2,° ∗ 0.34 This example uses 15 degrees aspecular as the flash angle, and 110 degrees aspecular as the flop angle, and 45 degrees is normal to the sample. Note that different organizations use different angles for flash and flop angles. flop index of 0 correlates to no change in lightness, and in the perception of solid color at all angles. It appears matte if no glossy clear coat is provided.
A flop index of 10 correlates to a moderate flop between lighter and darker appearance with illumination and observation angle. On the left is a lipstick shade with a low flop index, while the lipstick on the right has a higher flop index. There is also a phenomenon known as a color flop. Color flop typically occurs with interference pigments found in cosmetic applications. Such pigments have a distinctive hue and chroma shift with varying illumination and observation angle.
The particle is composed of multiple layers with different indices of refraction to produce interference—the large difference in index of refraction results in large surface reflection at the boundary between layers. The light reflected from the second boundary layer then interferes with the light reflected from the first boundary layer. The light reflected from the second boundary layer then interferes with the light reflected from the first boundary layer.
Depending on the index of refraction, the layer thicknesses, and the incident angle, the pathlength light travels through the layer will change. The pathlength will determine the wavelength-dependent interference, where constructive interference results in the interference (or resonant) color. Color impression from interference pigment comes primarily through two parts, 1, as described interference, and 2, the background material. The background can absorb or scatter. Backgrounds with large amounts of absorption have nearly a black background and result in the interference color dominating color perception. A heavily scattering background (such a white) will have a much larger influence on the total color perception than a black background. Backgrounds with both absorption and scattering will influence the color perception somewhere between white and black.
How an Instrument Can Capture the Behavior of Effect Pigment
As mentioned at the beginning of this paper, Spectrophotometers work great for measuring materials that are not gonioapparent. However, effect pigments cannot be characterized by this simpler geometry. For this reason, goniospectrophotometers are used. Gonio is ancient Greek for "angle." And a goniospectrophotometer is a spectrophotometer where multiple illuminations and viewing geometries are used during measurement. A multi-angle spectrophotometer is a simplified goniospectrophotometer. Multiple illuminations and or viewing angles are used, but they are fixed. This reduces the size, complexity, and amount of data to process compared to laboratory spectrophotometers.
The portability and repeatability make multi-angle instruments great for quality control applications and research & development. The several angles are described in ASTM E 2194 (2014), published by a US standardization committee, American Standard Test Method International. The angles are defined as an angle from specular light, aspecular angle, and As15, 25, 45, 75, 110°. ASTM E2539 (2014) also describes an angle for measuring pearlescent paints, and the angle is As-15. Konica Minolta Sensing's Multi-Angle Spectrophotometer CM-M6 has one illumination angle and six detection angles. The image shows the aspecular angles in red for the CM-M6, which are -15, 15, 25, 45, 75, and 110 degrees.
So far, we have only discussed one illumination angle and 6 observation or measurement angles. The CM-M6 has two illumination angles and 12 observation angles to take two sets of measurements for the same sample. The sample is first illuminated from the left and measured at 6 aspecular angles (relative to the specular angle on the right). The sample is then illuminated from the right and measured at the same 6 aspecular angles; of course, this is relative to the left's specular angle.
The measurements are then combined into a single set of 6 aspecular measurements that compensate for the sample's curvature.
Measurement time:
• Measurement time in double path mode is longer than single path mode, but increased repeatability of measurements on curved surfaces.
• On flat surfaces, single path measurements are as reliable as double path measurements and can reduce measurement time.
Sample Evaluation Comparing with the Target
Below is a measurement example of a nail polish that contains pearlescent pigment;
One target and two samples were measured at 6 angles. Three graph data show L*, a*, b* respectively. Sample 1 shows very close data with the target in L*, a*, b* at all angles. On the other hand, Sample 2 shows a noticeable difference in L* at -15° and 15°. and a remarkable difference in b* at -15° and 15°. Below are visual images of the target and samples on a table rotated at angles. The center specimen is the target. Sample 1 on the left and sample 2 on the right. You notice that the materials are showing different looks by angles.
The color shift difference in data might indicate that the effect pigments or other morphology parameters' orientations are not similar enough between the target and Sample 2.
Conclusion
Effect pigment is one of the distinctive materials to differentiate the appearance of cosmetic products. In the development of those products containing effect pigment, color needs to be evaluated from different angles. A portable type multi-angle spectrophotometer made this measurement work remarkably easy compared to using a conventional large gonio spectrophotometer. Still, portable multi-angle spectrophotometers in the market were relatively heavy and bulky, so that an operator needs to hold an instrument with both hands for measurement. Konica Minolta CM-M6 was designed to be lightweight and compact so that measurement can be done even with a hand. Its high optical technology achieved multi-angle optical geometry in a compact body. Simultaneously, it realized an ideal measurement area size for small samples and samples with a limited flat area.
The CM-M6 provides measurement data with high stability and accuracy, which researchers and designers in the lab and operators at quality control can count on.
About Konica Minolta Sensing
Konica Minolta Sensing is ‘The Standard in Measuring Color, Light & Display’. Konica Minolta provides advanced optical technology that precisely measures the elements of color and light. Our products are a staple in research and manufacturing environments, helping organizations to meet product quality and operational goals with less waste, time, and effort.
This commitment to creating value for customers is the core principle behind the Konica Minolta brand. It’s also the driving force behind the high level of quality and precision built into each of our products and why we’re the technological leader in color and light measurement solutions today.
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