Perception of Translucency and Glossiness: Influence of the Optical Scattering Properties on Sensory Evaluation

Abstract

In the cosmetics industry, fine particles suspended or dispersed in a medium are widely used. Optical properties of the medium can be an indicator for evaluating the performance of cosmetic product, such as the ultraviolet protection capacity of sunscreens and also its appearance. However, the relationship between the optical properties and the appearance of the product has not been clarified. In this study, dispersions of fine particles with titanium dioxide or zinc oxide were used as scattering medium to clarify the relation between their scattering properties and changes in visual perception. For visual perception, sensory tests with human panelists were performed using a custom-designed apparatus to inspect the samples and evaluate the two visual sensory quantities, “glossiness” and “translucency”. The sensory test employed Thurston’s one-pair comparison method, and a total of 18 panelists were asked to evaluate the samples. The sensory tests showed that the translucency became stronger as the scattering coefficient decreased. On the other hand, only the samples in the group with the highest scattering coefficient showed a slightly low glossiness. In contrast, the other samples showed little difference in glossiness due to the difference in scattering coefficients. The results of this study indicate that the appearance of cosmetics products can be influenced by controlling the scattering properties of the medium.

1. Introduction

Designing and marketing aesthetic products is of growing importance in markets where many basic needs of consumers have been satisfied. As core product attributes, such as quality and functionality, become increasingly indistinguishable with a lack of uniqueness, manufacturers are shifting their focus or efforts away from concrete product characteristics towards less tangible features such as aesthetics [1,2].

Nowadays, the research objectives in cosmetics are obviously to create products that are always more innovative and more successful, and above all, to perfectly come up to the sensory expectations of consumers. Consumers generally choose a cosmetic cream for its functions or its promise of efficiency but are mostly seduced by the pleasure it brings to them, especially in terms of texture. Therefore, over the last few decades, sensory analysis has been developed and used to describe and quantify texture characteristics of cosmetic products [3,4,5,6,7,8,9,10,11].

Timm et al. [6] investigated various suspensions of cosmetic powders concerning the perceived skin feel after application. Their results indicated that cosmetic powder particles should be small with a rather irregular shape to lubricate the tribological contact between fingertip and skin surface to have a more “powdery” skin feel or tactile perception. Despite the existence of tactile perception [7,8,9,10], to our knowledge, there exists little research on the evaluation of the appearance or visual perception of the cosmetics themselves. Dubuisson et al. [11] provides valuable data evaluating the appearance of “glossiness” for cosmetic products. They used the Spectrum Descriptive Analysis method to discriminate and characterize aqueous phases of cosmetic oil-in-water emulsions containing xanthan gum. Based on their research, according to xanthan gum concentration, there is consistent linear relationships in solution for the appearance of glossiness. However, while the result presented in their study is valuable when the contents are matched, another concern is that the data are not versatile enough for application to emulsions used in a variety of cosmetic products.

Emulsions are colloidal dispersions of a liquid in another immiscible liquid stabilized using a surfactant and/or solid particles [12,13]. In the cosmetic industry, scattering medium with fine particles suspended or dispersed such as emulsions are widely used in color cosmetics, skincare, and personal care products. Takahashi et al. [14] revealed that the particle size distribution in dispersion changes simply by changing the stirring speed in the manufacturing process of cosmetic products. They also found that the optical properties of the dispersions changed with the change in particle size distribution. As an application of their technology, Asano et al. [15] succeeded in improving the sun protection factor (SPF) of cosmetic creams by varying the stirring speed. The optical properties are values that determine the propagation of light within a semi-translucent and scattering medium. Therefore, it can be an indicator of the performance of a cosmetic product, such as SPF, and would be an indicator of the appearance of the product. If the relationship between the optical properties and appearance can be clarified, it will provide a useful indicator for controlling the appearance of various products.

The purpose of this study is to clarify the relation between the scattering properties and the change in visual perception by using dispersions of fine particles with titanium dioxide or zinc oxide as scattering medium. As for visual perception, behavioral studies were conducted with human panelists with a custom-designed apparatus to inspect the samples and evaluate the two visual sensory quantities “glossiness” and “translucency”.

2. Materials

There are three optical properties, which characterize the light propagation in the scattering and absorption medium with μ_a being the absorption coefficient [mm⁻¹], μ_s being the scattering coefficient [mm⁻¹], and g being the anisotropic factor to decide the scattering phase function [16,17].

When changing the absorption coefficient, it is necessary to consider a complex system that includes a wavelength spectrum because color has a great impact on visual perception. Here, we simply focus on the relation between the scattering coefficient and visual perception. The material was adjusted to have an absorption coefficient close to 0.

The scattering properties of the scattering medium with particles vary not only with the constituents but also with the manufacturing process [14,15,18]. Among them, it is known that even with the same constituents, changes in the stirring speed can also change the scattering properties, along with changes in the particle size distribution. In this study, as shown in

Table 1. Particle material and velocity of samples (Nos. 1–10) to change the scattering coefficient with stirring speed.

Sample No.	1	2	3	4	5
Particle	TiO2	TiO2	TiO2	TiO2	TiO2
Stirring speed [m/s]	5	10	20	30	40
Sample No.	6	7	8	9	10
Particle	ZnO	ZnO	ZnO	ZnO	ZnO
Stirring speed [m/s]	5	10	20	30	40

and

Table 2. Components of the sample Nos. 1–10.

Material	Mass Concentration [wt%]
Purified water	96.2
Particle	3.0
Thickener, carboxymethyl cellulose	0.8

, samples with variations in scattering properties were prepared using two types of particles (titanium dioxide and zinc oxide) and changing the stirring speed while keeping the other constituents the same. By changing the stirring speed 5 to 40 m/s (refer to Table 1) of the two particle types in five different ways, 10 different samples were prepared. In addition, the samples with high scattering coefficients containing a large number of particles or nearly the limit of particles just before becoming lumps with a paste-like formation were prepared, as shown in

Table 3. Stirring machine and speed of the samples to make highly scattering coefficient medium.

Sample No.	11	12
Stirring machine	Filmix-40 (FM-40)	Homogenizing Disper Model 2.5 (DH-2.5)
Stirring speed	40 m/s	3000 r/min
Stirring time [s]	300	300

and

Table 4. Components of the sample Nos. 11–12.

Material	Mass Concentration [wt%]
Purified water	8
Particle (TiO2)	30
Thickener aqueous solution, carboxymethyl cellulose 2%	50
Emulsifier (HC-60)	2
Ethanol	10

. The prepared samples were stirred slowly several times with a stick before transferring them to the containers for sensory testing and scattering coefficient measurements. During this process, we confirmed that there was no change in the dispersion state of the sample, such as precipitation. A photograph of a sample created through these processes is shown in Figure 1.

The particles used were titanium dioxide (MT-100AQ, primary particle diameter of 15 nm, TAYCA CORPORATION, Osaka-shi, Osaka, Japan) and zinc oxide (FINEX-50W, primary particle diameter of 20 nm, Sakai Chemical Industry Co., Ltd., Sakai-shi, Osaka, Japan) with hydrophilic surfaces. A thin-film spin system high-speed mixer (FILMIX^®FM-40L, PRIMIX Corporation, Awaji-shi, Hyogo, Japan) was used for samples Nos. 1–11 and a Homodisper (Homogenizing Disper Model 2.5, PRIMIX Corporation, Awaji-shi, Hyogo, Japan) for sample No. 12. The relation between mixer stirring speed and particle size distribution has been reported previously, and it was also reported that the stirring process maintained dispersion condition and that little precipitation was observed even after one month of making the dispersion [14].

3. Method

3.1. Measurement Method of the Scattering Properties of Scattering Medium

An instrument developed in our previous work [19,20] was used to measure the optical properties, and it can non-invasively measure the optical properties of semi-transparency materials. This measurement instrument utilizes the Reflection Spatial Profile Method (RSPM), and a schematic diagram of the instrument is shown in Figure 2. Structured light is irradiated onto the measurement sample, and the spatial profile of the reflected light is measured by a cooled charge-coupled device (CCD) camera. Since the spatial profile of the reflected light is dependent on the optical properties of the sample, the principle is that the absorption and scattering coefficients can be measured by inverse analysis.

For the inverse analysis, it is necessary to fix the anisotropy factor g. The anisotropy factor g varies with particle material and particle size distribution [21]. Therefore, the samples used in this experiment, shown in Section 2, are expected to have different values of g for each sample. Since it was difficult to measure the g value in the samples, to examine the relation between scattering properties and appearance, we decided to use the reduced scattering coefficient μ_s′ [22] shown in the following equation:

μ_s′ = μ_s (1 − g),

(1)

where, the value of g was assumed to be 0.8 for all the samples.

3.2. Constructed System for Visual Perception Test

Figure 3 shows the experimental setup for visual perception. In the system, to restrict panelists’ view, a dome/sphere-like isolation system was constructed for sample viewing. With such a system, the panelists can view only the samples to be tested inside the dome thus eliminating the disturbance from outside. The dome was constructed with a styrofoam hemisphere (diameter is 50 cm) whose inside was painted with black paint (JSC-3, JAPANSENSOR Corporation, Minato-ku, Tokyo, Japan). Light from a double-arm LED light source (LED-W60, ARMS system Co., Ltd., Setagaya-ku, Tokyo, Japan) with the same light intensity was used to illuminate the samples to be compared at an angle of 30 degrees and the reflected light from the samples was adjusted to reach the panelist viewing through a hole made in the dome. To make the point light source, an aperture (aperture diameter is 1 mm) was placed immediately after the light source. The panelist was instructed to look only at the sample while minimizing the external disturbances from all the other surroundings influencing the view. Panelists were asked to report the result of the comparison of the samples orally.

Thurston’s one-pair comparison method was used for analyzing the results and will be briefly discussed in the next section.

3.3. Thurston’s Paired Comparison Method

Thurston’s pairwise comparison method was employed in the sensory evaluation experiments in this study. We will provide a brief description of Thurston’s pairwise comparison method [23,24] and overall protocol used in the study.

Thurston’s pairwise comparison method is a method for scaling the sensory states of the elements that form a population. In our example, the elements are medium with different optical properties, and the sensory states are the intensity of the two visual perceptions of the medium, glossiness and translucency. The method does not evaluate the strength of these perceptions in absolute terms, but quantifies the strength of glossiness and translucency for each element based on the results of a relative comparison of two samples extracted from the population. This method takes advantage of the fact that the human senses are not suited to absolute evaluation, but are extremely sensitive to relative evaluation. This was carried out to prevent the authors from arbitrarily impressing a sense of glossiness and translucency on the panelists. Notably, the simplest case V method of Thurston’s pairwise comparison method was employed. The details can be found in the reference literature [23,24] but here we briefly review Thurston’s pairwise comparison method and the outline of case V.

First, suppose that the intensity of luster and translucency can be expressed in terms of some scaled value. For example, let A be the scale value of translucency of an object. Since each person perceives translucency differently, the translucency values that people perceive will be distributed within a certain range. This situation is shown in Figure 4. If two samples are compared in which the translucency values A and B are far apart (as shown in Figure 4a, the distributions do not overlap), all panelists judge that the sample with the value B is more translucent. Conversely, for a pair of samples with translucency values A and C close to each other (the panelists whose distributions overlap (Figure 4b), a case appears in which the strength of translucency differs from person to person. For example, 7 out of 10 respondents would respond that the sample with a value of C is more translucent than with a value of A, while the remaining 3 respondents would respond that the sample with a value of A is more transparent than with a value of C. Of course, the numerical value of translucency of an object is not known at the sensory evaluation stage. For example, suppose 10 panelists are shown a pair of samples and asked which is more translucent. The results are scored and summarized as follows: if one (e.g., B) is more translucent than the other (e.g., C), the score is 1 point; otherwise, the score is 0. If no judgment can be made, the score is 0.5 for both. In this case, 10 points are given for A and 7 points for B as in Figure 4a. If this comparison is made for all sample combinations and the scores are obtained, a statistical process can be used to determine the relative relationship of all samples, i.e., the scale value of the visual perception of the samples.

This is an overview of Thurston’s pairwise comparison method. In particular, the method for Case V deals with the following three assumptions:

• The distribution of the translucency scale values is given by a normal distribution;
• The variance σ is independent of the size of the scale values (strength of translucency and glossiness) and has a constant value;
• In a pairwise comparison, if the difference in the glossiness or translucency scale values is the same, the strength of these visual perceptions is statistically determined in the same proportion.

3.4. Protocol of the Sensory Test

Using the experimental system shown in Figure 3, the panelists were asked to look at or fixate on the two samples placed inside the dome simultaneously while sitting on a chair, as shown in Figure 5. The panelists were asked to compare the two samples and report on the following question: choosing whether they felt a stronger glossiness (or translucency) in the left or the right sample, or whether there was no difference between the two samples. Thurston’s statistical calculation was performed with 1 point for the selected sample and 0.5 points for both if no difference was felt. A total of 66 comparative observations were carried out, where all combinations using a total of 12 samples are presented in Section 2 (C(12, 2) = 66). As the study involves perception of glossiness and translucency, each panelist was asked to carry out only one, either glossiness or translucency. This was carried out to avoid any one perception of the same sample unconsciously affecting the other perception of the panelist.

The following is the protocol of the sensory test as shown in Figure 6, where the reasons for going through the process are also given:

• Informed consent was obtained from the panelist. No details of the samples to be viewed in the experiment or the definition of visual perception to be evaluated were explained. The panelists were simply informed that they would be asked to answer questions regarding the “glossiness” or “translucency” of the substance to be viewed in Japanese;
• The flow of the sensory test was practiced with each panelist by using a mock sample;
• First, the panelist was asked to close his/her eyes for 5 s and relax. During this time, two samples were set up in the apparatus for pairwise comparisons. This process not only allows the panelist to relax during the sensory test, but also avoids the panelist from seeing the samples in an environment outside of the apparatus;
• The panelist was asked to open his eyes and to observe the sample for 3 s. The time should be short so as to induce the panelist to give an answer, rather than asking the panelist to think for a long period of time before giving an answer;
• After 3 s of observation, the panelist was asked to close his/her eyes and relax for 30 s to diminish previous samples and to focus on the comparison of only the next two samples in the experimental setup;
• Steps 4 and 5 were repeated for 33 times consecutively;
• After completing step 6 following a 5-min break, steps 3 to 6 were repeated to complete the observation of a total of 66 sets of samples. Since a total of 66 sets of samples are conducted at one time, panelists may become too fatigued to concentrate on the sensory test in the latter half of the test, so the test was divided into half sets with a break in between.

3.5. Panelists

A total of 36 panelists, 18 males and 18 females in their 20 s, participated in this experiment, with half of them assigned to the glossiness and half to the translucency tests.

The protocol was approved by the facility review committee of the Shibaura Institute of Technology (No. 21-009), and written informed consent was obtained from all of the panelists before the sensory test.

4. Results and Discussion

4.1. Scattering Coefficient of Scattering Medium Sumple Changing by Stirring Rate

The scattering coefficients of each sample shown in Section 2 were measured. Figure 7a and Figure 7b respectively shows the graphs of the stirring speed on the horizontal axis and the scattering coefficient on the vertical axis, for the particle component titanium dioxide and zinc oxide. The results were obtained using an optical property measurement system developed by the authors [19,20], and data are given at λ = 550 nm, the wavelength with the highest visual sensitivity [25] for human eyes, shown as a representative example.

These results showed that the scattering coefficient increased for both particle samples as the stirrer speed was increased. This was consistent with the results of experiments conducted in our previous study [14]. Particles in a sample have a structure that consists of not only the original particle size of the particle material (primary particle size), but also agglomerated particles of a different size (secondary particle size). Increasing the stirring speed has the effect of making the agglomerated secondary particles finer, leading to an increase in the probability density of the primary particle size and an increase in the scattering coefficient of the sample. Therefore, Figure 7 revealed that the same effect occurs even if the particles are made of different materials and have different values of scattering coefficients.

The scattering coefficients for all samples are then summarized in Figure 8. The horizontal axis is the sample number listed in Table 1 and Table 3. Sample numbers 11 and 12, which differ greatly in composition, are not shown in Figure 7 and these samples have significantly higher scattering coefficients compared to the other samples used. The scattering coefficients of the samples used in this experiment fall into three groups, with zinc oxide having very small scattering coefficients (Nos. 6–10), Nos. 11 and 12 with large scattering coefficients of different compositions, and Nos. 1–5 in the intermediate band of the three groups. The scattering coefficients of Nos. 1–10 varied depending on the stirring speed. The following sensory test was conducted using 12 samples with these scattering coefficients.

4.2. Sensory Test for Glossiness and Translucency

The sensory test results are shown in Figure 9 for both glossiness and translucency. The scattering coefficient shown in Figure 8 is obtained on the horizontal axis. The vertical axis is the value obtained by statistically processing the sensory test results for both glossiness and translucency using Thurston’s method, so that the mean value of the population evaluated becomes 0. The scale on the vertical axis indicates statistically that a difference of 1 means a difference of 1σ, as shown in Figure 4. Specifically, when two samples with a difference of 1σ are compared, statistically, 84.1% of the respondents felt that the sample with the larger value was more glossy or translucent than the sample with the smaller value.

First, chi-square statistic χ² [26] and degrees of freedom f were calculated to test the consistency of this sensory test, and both were found to be consistent at p < 0.01. In other words, as a result, the number of samples used was found to be sufficient to examine trends.

The results for translucency showed that the lower the scattering coefficient, the more translucent samples tended to be when viewed as three groups with different scattering coefficients. As mentioned earlier, a comparison of the zinc oxide sample group with a small scattering coefficient (Nos. 6–10) and the high scattering coefficient group with different components (Nos. 11–12) showed a difference of approximately 2σ, which is a sufficiently significant difference.

The difference between the groups with the same titanium dioxide compositions (Nos. 1–5) and the groups with the same titanium dioxide compositions (Nos. 11–12) was more than 1σ. This is a significant difference. On the other hand, when the results of the group of samples with Nos. 1–5 and Nos. 6–10 were compared, which were prepared at different stirring speeds, the difference falls in the range of 0.5σ to 1σ. In other words, the results suggest that there were not enough differences to clearly determine all the differences. This does not mean that there were no differences.

Furthermore, when comparing only the change in stirring speed as a small group of differences (within Nos. 1–5 or 6–10), most of the differences were within 0.5σ, indicating that there were no differences that could be considered as being within 0.5σ. In our previous study [27], the relationship between “translucency” and “scattering coefficient” was investigated by having three-dimensional (3D) images observed on a 3D display with glasses. Although there was a difference between viewing the experiment on the display and viewing the actual product samples, the results were consistent with this study: the translucency decreased as the scattering coefficient increased.

Next, we discuss the glossiness in the same way. Comparisons between group Nos. 1–5 and Nos. 6–10, which showed differences in translucency, showed that there was only a difference within 0.5σ, both between groups and within each group. In terms of glossiness, there was no clear difference in the trend. In addition, a comparison between the group of samples with the largest difference in scattering coefficients (Nos. 11–12) and the other samples showed that the difference was only about 0.8σ, even when the largest difference was obtained. In other words, although there is no clear and significant difference between the samples with the number Nos. 11–12 group and the others, it can be considered that the higher the scattering coefficient, the harder it is to feel the glossiness, but the difference is very small.

Finally, we compared and analyzed gender differences in the perception of translucency and glossiness and show the results in Figure 10 for males and females, respectively.

Both the translucency in Figure 10a and the glossiness in Figure 10b show the same trend as in the comparison mentioned above in Figure 9, indicating that the data plotted in the graphs for males and females are very close in value. In other words, there is not much difference between the ratings of these two visual perceptions for males and females. However, only in the glossiness of males, the group of Nos. 11–12 with large scattering coefficients took values at least 1σ apart from the others, indicating a clear difference.

In the evaluation of visual perception using the two keywords “translucency” and “glossiness,” it was found that when the optical properties of the scattering coefficient were varied, a strong difference was perceived for translucency, but not much difference was perceived for glossiness. This suggests that it is effective to have a low scattering coefficient to strongly impress influence transparency in cosmetics. It is important to also consider adding optical or physical properties different from the scattering coefficient to affect glossiness.

5. Conclusions

In this study, using dispersions of fine particles as scattering medium, a sensory test was conducted to investigate the relationship between the perception of “translucency” and “glossiness,” and the reduced scattering coefficient. Samples with various scattering coefficients were prepared by changing the particle material, stirring speed, and constituents. The sensory test employed Thurston’s one-pair comparison method, and a total of 18 panelists were asked to evaluate the samples.

The results of the sensory test revealed that as the scattering coefficient decreased, the translucency of the product became stronger. On the other hand, only the samples in the group with the highest scattering coefficient showed a slightly low glossiness, while the other samples showed little difference in glossiness with difference in scattering coefficients. The results of the sensory test by male and female showed no significant differences, and both groups were sensitive to translucency due to the change in scattering coefficient, but had difficulty perceiving the difference in glossiness.

In this study, the scattering coefficient that characterize light propagation in scattering and absorbing medium was treated as a topic, but the relationship with the other optical properties, absorption coefficient and scattering phase function, and visual perception has not yet been clarified, which is a limitation. In the future work, if we can control the optical properties other than the scattering coefficient and conduct similar experiments, we will be able to clarify what kind of optical properties are important for the translucency and glossiness of a material. In addition, the results of this study show that it is possible to control the translucency of scattering medium such as emulsions commonly used in cosmetics by controlling the scattering coefficient. Therefore, we believe that this result will improve the appearance of products and add value to them.