Optics & Photonics News - July/August 2010

colorants, Lochhead and a graduate student, Laura Anderson, found that scientists are developing composite makeup particles that diffract, refract and reflect light off the wearer’s skin, rather than covering the skin with an opaque barrier. Not only do these particles let the natural skin color pass through to the outside world—albeit with enough blurring to hide small surface imperfections—but they can even produce new hints of color for a radiant glow.

The diagram below illustrates what happens when light hits one of these coated mineral particles, shown in cross-section. According to Morante, the particles—which are no larger than 10 µm in diameter for optimum feel and performance in the makeup— scatter light away from the eye so that the small wrinkles aren’t apparent.

Some foundations, according to Lochhead, contain tiny alumina platelets, some treated with metal oxides and others coated with silica spheres. These structures bounce the light around inside the wrinkle—total internal reflection— and soften the look of the wrinkle without the use of opaque pigments.

Simple light reflections are another way to hide wrinkles. Since the wrinkle is depressed from the skin surface, light-reflective particles at the base of the wrinkle will make the crevasse reflect more light. “That will make the wrinkle

Glass and mineral cosmetic particles can be coated with titanium dioxide and other substances to produce iridescence, or colors that appear to change with the angle of view.

Younger cosmetic wearers may have more blemishes than wrinkles, and composite particles can help them too. Researchers at Avon Products Inc., according to their patent application, embedded sub-micron-sized alkyl silane-treated titanium dioxide particles into silicone gels. The embedded particles increase the refractive index of the gel, which then scatters incoming light away from the blemish underneath.

Another blemish-hiding makeup formula uses two kinds of 10- to 20-µm-wide alumina platelets: one treated with metal oxides and the other covered by titanium-coated spherical silica. “Together, the two different platelets form the mosaic, which gently reflects light and matches the natural color of the skin,” wrote the team from Color Access Inc. of Melville, N. Y. (U.S.A.), which patented the idea. Still other concealers described in patent applications use tiny glass beads or platelets coated with nylon or even silver.

Iridescence and pearlescence

Glass and mineral cosmetic particles can be coated with titanium dioxide and other substances to produce iridescence, or colors that appear to change with the angle of view. When applied to the skin, such particles form a semi-transparent layer that creates phase-shifting and interference effects, and some wavelengths of light are attenuated more than others. Iridescence happens in nature—in pearls, mollusk shells, butterfly wings, peacock feathers and some insect exoskeletons.

Pearlescent pigments are generally shaped like flakes instead of spheres. Originally, tiny platelets of mica were coated with metal oxides with a higher refractive index than the mica substrate. Since natural mica tends to be somewhat irregular and can scatter light too much, cosmetics companies have been making artificial platelets of materials such as borosilicate.

Color researchers manipulate the types of substrates and coatings to enhance the reflection and interference of certain wavelengths. At L’Oréal, scientists have attempted to create

appear less deep, and it will fool the eye into thinking that the skin surface is more even than it is,” she said.

“It is very much an optical illusion,” Draelos said. “You can create some very interesting optical effects with very small particles of pigment.”

[ Cross-section of hypothetical makeup particle ]

Surface reflection
c Decreased
and diffused
Diffused inner reflection
c Increased

EMD Chemicals

TiO2 Talc Al2O3

}

Transmission
c Increased
and diffused

Nick Morante Cosmetic Consultants

Cosmetics made of multilayered particles reflect and transmit diffuse light to conceal wrinkles or impart different colors to observers at different viewing angles. (Inset) Cosmetic-grade pigment of TiO2-coated mica particles.