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Measuring soft focus properties of cosmetic filler particles: a new technique to measure soft focus properties of cosmetic filler particles has been established to improve upon traditional methods.

THE SOFT FOCUS effect is the ability of cosmetic filler particles to reduce the visible signs of wrinkles or fine lines on the skin. Such an effect is achieved optically by the interaction of visible light with the filler particles, whereby the light is diffused as it passes through the cosmetic. The ability of a material to exhibit a soft focus effect is determined by many factors, including the chemical makeup of the material, particle size, morphology and particle orientation.

When light impinges on a material, it is either absorbed, reflected or transmitted. For white pigment particles, the absorption in the visible light range is minimal. Transmission of light through a film consists of two components:

* Specular transmittance due to light passing through the material without interaction and

* Diffuse transmittance that stems from forward scattering of light.

Likewise, the reflectance consists of:

* Specular reflectance due to coherent scattering of light on the surface, e.g., mirror reflection and

* Diffuse reflectance that stems from backward scattering of light. (see Figure 1)


When dispersed in a cosmetic formulation an optimum soft focus material will possess a combination of high total transmittance and high diffuse transmittance. High total transmittance will ensure a more natural look is obtained as the light is able to pass through the cosmetic and reflect from the skin, maintaining the natural skin tone. In addition, a high proportion of diffuse transmittance means that the light passing through is scattered in many directions by the particles, allowing reflection to occur from many points on the skin. This feature results in a softening of the fine features of the skin to produce a natural translucent finish. In contrast, reflection from the filler particles is undesirable. Diffuse reflection leads to a whitening effect as the natural skin tones are not visible, producing a "caked-on" look. This is true for pigment particles such as titanium dioxide, which possesses a high diffuse reflection component and a high opacity. In addition, specular reflection leads to a high luster, or unnatural sheen, similar to an oily film.

The ability of particles to scatter light depends on their refractive index ([n.sub.p]) and the refractivc index of the medium ([n.sub.m]) in which the particles are dispersed. The greater the refractive index difference between the particles and the medium, the greater the amount of scattering. Hence a small difference in refractive index will lead to almost no scattering and the particles will not be visible in the medium, as is the case with mica in most cosmetic bases. In contrast, a large difference will result in almost all scattered or diffuse light, such as with Ti[O.sub.2]. This suggests that a material with a high refractive index with respect to the cosmetic base should be selected to maximize the amount of diffuse light. There is a trade-off however, as a large refractive index difference increases the opacity or reflectivity (R), as given by a form of the Fresnel equation:

R = [([n.sub.p] - [n.sub.m]).sup.2]/[([n.sub.p] + [n.sub.m]).sup.2]

and hence leads to an unnatural appearance. Therefore, an optimum refractive index is needed to ensure that a good combination of transparency and diffuse light is achieved.


Traditionally soft focus measurements have been performed by dispersing the powder in nitrocellulose lacquer, casting into a grim and drying. Nitrocellulose was often selected as the base as it has a refractive index similar to most cosmetic oils and because a solid film is obtained after casting and drying. However, there are many problems associated with such a technique. One, the method of film casting does not always ensure consistent film thicknesses are obtained, leading to poor reproducibility. Two, drying of the film leads to shrinkage and possibly to the development of cracks or stresses in the film. Films of around 10/[micro]m in thickness are typically cast in order to reduce such drying stresses. However, this can lead to film thickness problems if the soft focus particles are larger than this size.

To overcome these issues, a method has been adopted where the powder is dispersed in an oil medium that possesses a refractive index close to typical cosmetic oils (1.4 to 1.6). Soft focus measurements can be performed by placing the sample in a quartz cuvette with an optical path length of up to 20 [micro]m, as typically used in the sunscreen industry. This overcomes the need for a dried film but allows comparable film thicknesses and particle concentrations as used in the nitrocellulose films. If particle sizes are large, making the formation of a 20 [micro]m film difficult, a slight modification to the technique may be used, in accordance with Beer's Law. The logarithm of the transmittance is proportional to the volume fraction of particles (V) and the optical path length (L), as given by:

ln ([l.sub.t]/[l.sub.o] [varies] V.L

The same value of transmittance should be obtained if the value V.L is kept constant, thus allowing a longer path length to be used as long as the sample concentration is diluted accordingly. Compliance to Beer's Law is shown in Figure 2 (AL 5-10), where the same transmittance is obtained for the two measurement conditions:

i) 1 mm path length with a concentration of 0.39 wt%;

ii) 20 [micro]m path length with a concentration of 17 wt%.


Samples were prepared by dispersing the dried powder in octyl stearate using an ultrasonic bath and roll milling. All samples were prepared at 17 wt% to agree with the work conducted by Emmert (1996). The 17 wt% samples were analyzed using a quartz cuvette with a 20 [micro]m path length. Alternatively, if the 20 [micro]m films were difficult to form, the sample was diluted (by volume concentration) by 50 times and a quartz cuvette with a path length of 1mm was used. Measurements were performed using a Cary 300 UV-visible spectrophotometer fitted with an integrating sphere, over the wavelength range of 800-400 nm. The total and diffuse transmittance values of the suspensions were measured with an octyl stearate background. Two Alusion samples (aluminium oxide) AL 1-5 and AL 5-10, boron nitride, talc, silica and nylon 6 were compared.


Figure 3 shows a comparison of the total transmittance values, divided into diffuse and specular components, for the different samples at a wavelength of 550nm. Silica shows essentially no diffuse transmittance and is not visible in the medium, as the refractive index of silica is virtually the same as that of octyl stearate. Talc and nylon 6 also show a low diffuse transmittance for similar reasons. As a result, silica, talc and nylon 6 are poor choices for soft focus materials. Boron nitride shows a high diffuse component but the total transparency is low, meaning that much of the light is reflected by the particles and not directly by the skin. The two Alusion products show a good combination of total transmittance and diffuse transmittance. A higher diffuse component is observed with the smaller particle size due to the larger number of particle edges giving rise to light scattering. The total transmittance is lower with the smaller particle size as the larger number of particles means that the light travels through the suspension with more interaction.



A new technique has been established to measure the soft focus properties of cosmetic filler particles. The suitability of Alusion as a soft focus material has been demonstrated through comparison with the other materials. It possesses a superior combination of total and diffuse transmittance, providing a more natural looking soft focus effect.


Emmert, R. (1996) "Quantification of the Soft-Focus Effect," Cosmetics and Toiletries, 111, 57-61.

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Author:Cukrov, L.M.; Robinson, J.S.; Innes, B.; McCormick, P.G.
Publication:Household & Personal Products Industry
Date:Aug 1, 2003
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