Zhang Hongyan,Wang Peng,Guo Ruoxi,Zhang Han,Zhang Lei

Tianjin Shangmei Cosmetics Co.,Ltd.,China

Abstract This paper briefly introduces the human body evaluation test method of cosmetics whitening efficacy test,the measurement method of skin melanin content,skin chromaticity and the instruments used,so as to provide ideas for scientific and rigorous test scheme design and scientific basis for whitening efficacy claim.

Key words whitening;efficacy evaluation;human body test

“Skin like snow,coagulate like grease” has always been the pursuit of oriental women,and with the improvement of consumption level,people’s pursuit of skin whitening is getting higher and higher,so that the whitening cosmetics market has become one of the main categories of cosmetics.

At present,the efficacy evaluation methods of whitening cosmetics mainly include in vitro enzyme inhibition method,cell biology method,animal experiment method and human experiment method.Among these evaluation methods,human test is the most direct and objective method to reflect the effect of cosmetics.The human evaluation test of cosmetics whitening efficacy can be divided into two types:subjective evaluation and objective instrument evaluation.Subjective evaluation can be divided into two methods:visual evaluation and volunteer self-evaluation.Visual evaluation is performed by dermatologists with the help of standard color cards of different skin colors to evaluate the skin color of the test area and record the score[1].Self-evaluation is mainly evaluated by analyzing the questionnaire filled out by volunteers.Since the subjective evaluation method has certain subjective factors,it is necessary to analyze together with the objective instrument evaluation method to provide a more complete scientific support for the whitening effect of cosmetics.

1 Determination of melanin content

From the perspective of skin physiology,the main factor affecting human skin color is the content and distribution of melanin in the skin[2].At present,most of the whitening cosmetics on the market achieve whitening effect by inhibiting,blocking or affecting the production and transportation of melanin.Therefore,the determination of the melanin content of the skin is a commonly used method to evaluate the whitening efficacy of cosmetics[3].

1.1 Skin melanin and heme tester

Mexameter MX18 is a widely used instrument for measuring melanin content in the skin.During the test,the spring in the probe can keep the pressure constant,so as to ensure the repeatability and accuracy of the experimental data.Mexameter MX18 is based on the principle of narrow-wave spectral absorption,and uses the receiver to measure the reflection of incident light on the skin to determine the content of melanin and heme in the skin.The position of the transmitter and receiver ensures that only diffuse and scattered light can be measured,and since the amount of emitted light is certain,skin absorbance can be measured to reflect skin melanin and heme content[4].Among them,the melanin content is characterized by the MI value,and the heme content is characterized by the EI value.The lower the measured value,the lower the melanin and heme content,that is,the whiter the skin.The main formula is:

Infrared:infrared light,Green:green light,Red:red light,Reflection:corresponding to the reflection of each light.

The instrument is easy to operate,but it can not fully reflect the multi-dimensional change of color,so it is more scientific to combine with other test methods.

1.2 Skin sensitivity tester-tissue viability imager TiVi 700

TiVi700 is mainly based on the principle of polarization spectroscopy.After the linearly polarized light emitted by the white illuminator reaches the skin surface,most of it is directly reflected by the skin surface and then is blocked by the detector's filter.Only part of it can reach the deep layer of the skin and be absorbed by red blood cells.The polarized light absorbed by the red blood cells can enter the detector through the filter.Among the polarized light reaching the detector,the green light component is weakened due to the absorption of red blood cells in the blood vessel,and the red component is hardly changed,while the absorption of green light and red light by other tissues around the red blood cells is basically the same.In this way,the deep vascular microcirculation of the skin can be obtained.

The TiVi700 system can be equipped with pigmentation analysis software,which can analyze the whitening effect of cosmetics by measuring the pigment content of the subjects areas.In addition,TiVi700 can also measure and analyze skin pores,wrinkles,damage degree and other related indicators.It has advanced image processing function,small size and easy to use;by adjusting the variable focal length lens,the observation range can be adjusted without touching the skin,avoiding discomfort of the subject and avoiding contamination.

2 Skin color measurement

Whitening cosmetics mainly reduce the pigmentation in the skin by inhibiting the production and transportation of melanin,and ultimately reduce the skin color[5].Therefore,the whitening effect of the product can also be verified by analyzing the change of skin color[6].

Previously,visual and photographic methods were commonly used to measure skin color changes.Since these two methods were greatly affected by lighting sources and other external factors,the chromaticity system(CIE,L*a*b*chromaticity system)stipulated by the International Lighting Commission was generally used to measure skin color changes at home and abroad.The skin color can be quantified by this system,so as to directly reflect the change of skin color in the form of numerical value[7].SinceL*a*b* can show the multidimensional changes of skin color,it can quantify skin color more scientifically and accurately,making the results more reliable.

2.1 Skin color test probe colorimeter CL400

At present,Skin-colorimeter CL400 is mainly used to measure the color of skin.There are eight LED white light sources uniformly distributed in the probe,and the light emitted is scattered in all directions inside the probe.Some light passes through the skin and some light is scattered through the skin.Only the reflected light of the skin is received by the XYZ sensor inside the probe,so that the color of the skin can be tested by the XYZ three-primary color method.The special design inside the probe ensures that the probe is only slightly pressed on the skin surfac with constant pressure during the test,without affecting the microcirculation of the skin,resulting in changes in skin color.Finally,skin color is evaluated comprehensively by mathematical calculation based on CIE XYZ system.The output result is expressed by theL*a*b* color space system.The main formula is :

L* =116(Y/Yn)1/3-16

a* = 500[(X /Xn)1/3-( Y /Yn)1/3]

b* = 200[(Y/Yn)1/3－( Z /Zn)1/3]

ITA°=[arctan(L*-50)/b*]×180 /π

L,a,b—The coordinate values of the three dimensional cartesian coordinate system,X,Y,Z—Tristimulus value of XYZ color system;Xn,Yn,Zn—Tristimulus value of the complete diffuse surface;ITA°—Individual type angle.

L* represents the black and white degree of skin.The larger the value is,the brighter the skin color is.a* represents the red-green degree,+ais the red direction,and ?a is the green direction ;b* represents the range from yellow to blue,+brepresents yellow,?brepresents blue[8,9].The greater the value of ITA°,the brighter the skin.

2.2 Tristimulus colorimeter chromameter CR-400

Tristimulus colorimeter chromameter CR-400(Minolta CameraCo,Japan)is a hand-held portable instrument recommended by CIE for color measurement.It is usually used in textile,paint and food industries,as well as dermatology and other skin-related research.This colorimeter mainly uses a 8 mm diameter pulsed xenon lamp to illuminate the surface of the skin,and then detects the ultraviolet-visible reflected light on the skin surface structure to measure and analyze the tri-stimulus value colorimeter.Its principle is to receive color stimulation through the measurement instrument to simulate the naked eye,and the output result is represented byL*a*b* color space system[10].ΔEabrepresents the comprehensive evaluation ofL*,a*andb*,also known as triple stimulus colorimetric method,which can accurately reflect the position change of skin color in color space.This method can comprehensively reflect the three-dimensional change of skin color,so it is one of the appropriate indicators for evaluating skin color.The formula is:

ΔEab=SQRT(ΔL*2+Δa*2+Δb*2)

ΔEabis the distance between two color points in color space;ΔL* is the difference of coordinate L;Δa* is the difference of coordinate a;Δb* is the difference of coordinate b.

3 Image analysis

Image analysis was previously mainly used in clinical skin treatment.Nowadays,its application in cosmetics evaluation is becoming more and more extensive.The image analysis method is mainly to collect images by using the microscope photography technology,and then through the computer processing system to determine the multiple parameters of the skin,so as to further analyze the condition of the skin.At present,the image collection instruments commonly used in the evaluation of cosmetics whitening efficacy mainly include:VISIA facial imaging,dermoscopy.

3.1 VISIA facial imaging

VISIA-CR skin analyzer is an instrument that can quantitatively analyze multiple parameters of human skin.Using multi-spectral imaging technology,the human face is photographed in all directions through four light sources.Polarizing photography can use a polarizing mirror to adjust the light entering the camera,so as to ensure that the image is not affected by the specular reflection of the skin surface,so that the photo is real and effective.Ⅰn addition,the virtual image overlap technology can also make the photos taken each time have good repeatability.

After the photos were collected,the relevant image processing software was used for image analysis.Firstly,the red spots and pigmentation in the skin were scanned,and the color index was quantified by gray processing technology.Finally,the measured value was converted toL*a*b*to determine the change of skin color[11].Therefore,VISIA-CR can not only judge the whitening effect of cosmetics by the change ofL*a*b*value,but also evaluate the whitening effect in many aspects by combining the visual images of the face before and after use[12].

3.2 Dermoscopy

Dermoscopy uses special medium to reduce reflected light and increase the light transmittance of the skin to achieve optical magnification,so that the contours and colors of various tissue structures below the skin surface that are invisible to the naked eye can be observed in vitro[13,14].Dermatoscopy can collect photos of the skin,and then process the collected photos through professional image software,analyze the color change of the skin,and then evaluate the whitening effect of cosmetics.The advantage of dermoscopy is that it can analyze the degree of local color uniformity,thereby reducing experimental errors.

Image analysis makes the results more vivid,intuitive and visual through image display ;images can be captured without touching the skin to prevent adverse effects of probe instruments on testing.In addition,the image analysis method can also independently select the region for analysis according to the actual demand,and the probe test is only limited to some areas of the probe induction.However,using image analysis to evaluate the improvement effect of whitening,there are still problems in operation and control,which are greatly affected by the photography conditions,and the application of many functions is relatively limited.

4 Conclusion

Due to the different test principles of the probe instrument and the image analysis method,the values obtained by the test results are generally different,but the trend of the final results must be the same.Visual evaluation method,instrument detection method and image analysis method,each method has its own advantages and disadvantages.Therefore,the efficacy evaluation can be carried out by combining various methods to make up for their shortcomings,makeing the evaluation method more scientific,the efficacy evaluation system more perfect,and also provide a scientific basis for consumers to choose products.