CC BY
12DOI 10.24412/cl-37136-2023-1-113-117
INVESTIGATION OF HAIR OPTICAL PROPERTIES FOR ITS LASER COLORING
VLADISLAV ERMOLAEV
Institute of laser technologies, ITMO University, Russia v_m_ermolaev@mail. ru
ABSTRACT
In order to successfully solve optical problems related to the effect of light on hair, it is necessary to know their optical parameters, such as refractive index, absorption and scattering coefficients (extinction), as well as anisotropy factor [1]. Unfortunately, data on optical properties of hair, unlike other biotissues, are rather scarce and fragmentary [2]. Therefore, obtaining the most detailed spectra of optical parameters of hair is an actual task of biophotonics.
The information about the optical properties of hair can be used for the creation of cosmetological devices for laser hair coloring. Laser hair coloring is a new approach to hair coloring. The idea is that exposure to radiation under certain parameters is able to change the color of the hair without damaging its integrity. Earlier it was shown that broad-spectrum light sources can cause noticeable yellowing and discoloration of hair [3,4]. Research into the possibility of using monochromatic radiation for hair coloring is the next step towards the creation of a new laser hair coloring technology.
Melanin pigments are responsible for hair color. Melanins come in two varieties: brown and black eumelanins and yellowish and reddish pheomelanins. Combinations of these pigments in different concentrations produce all sorts of natural hair tones. It is the change in concentrations of melanins in the hair when exposed to radiation that contributes to the change in hair color [5]. It is important to keep in mind that the initial differences in chemical composition between hairs of different colors create differences in their interaction with radiation.
The aim of the study was to obtain continuous spectra of the extinction coefficient of human hair of different colors for their laser coloring. The goal was achieved in two ways: by means of computer simulation and by experimental measurements.
To calculate the extinction spectra of hair of different colors, an optical model of hair was created in TracePro program (Lambda Research Corporation, USA; version 7.0.1). The model consisted of a three-layer parallelepipedal structure with dimensions of 2000x2000x60 microns: the middle layer, the cortex, 50 microns thick was enclosed between identical 5 microns thick cuticle layers (Fig. 1). The number and thicknesses of the layers corresponded to those of the real hair, except for the medulla, the inner hair layer, which was not included in the model because of its negligible effect on hair properties [6]. In addition, the cylindrical shape of the hair was not considered in the model, because it would introduce noticeable losses due to reflection of radiation from the surface, which are not of interest for the measurement of the desired coefficient.
Cuticle Cortex
E
E
tN
(a) (b)
Figure 1: (a) Hair model scheme (not in scale); (b) optical simulation scheme. In optical modeling it was assumed that the cuticle consists of pure keratin, and the cortex consists of keratin, water, eumelanin, and pheomelanin. Concentrations of cortex components were calculated according to literature data for five hair colors: dark, white, gray, brown, and chestnut [7]. The required optical coefficients of cortex components were taken from the literature [8-13]; the total cortex optical coefficients were estimated as the sums of the products of cortex component coefficients by the corresponding concentrations. In the TracePro program, the hair model was irradiated with a 1 W monochromatic circular radiation source with a diameter of 1 mm and zero divergence. The wavelength of the source was varied in the range from 330 to 2240 nm in increments of 20 nm. At each wavelength, 100,000 rays were calculated by the Monte Carlo method and the power of the transmitted radiation was recorded. The ratio of the incident radiation power to the transmitted radiation power gave the permittivity of the hair, which when divided by the total thickness of the model gave the extinction coefficient. Thus, the spectra of extinction of hair of five colors were obtained (Fig. 2).
Figure 2 shows that the character of hair spectra of different colors is similar, there are differences in the values of extinction coefficients. The maxima in the range from 330 to 400 nm and six peaks (1200, 1520, 1740, 1940, 2060 and 2180 nm) are clearly visible. The large values of the extinction coefficient in the ultraviolet (UV) range are associated with the absorption of melanins, the peaks at 1200 and 1740 nm give the stretching vibrations of CH molecules, the peak at 1520 nm gives the stretching vibrations of OH molecules, the peak at 1940 nm gives the stretching vibrations of OH molecules and the scissor vibrations of H2O molecules, the peaks at 2060 and 2180 nm give amides [5,8,14]. Fluctuations of OH and H2O molecules occur in water, and fluctuations of CH molecules and absorption by amides in keratin, which is part of the hair.
k, cm-1 -Chestnut White -Dark -Grev -Brown
700 650 600 550 500 450 400 350 300 250 200 150 100 50 0
300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 A, nm
Figure 2: Calculated extinction spectra of hair offive colors from 330 to 2240 nm.
To apply the data obtained according to the purpose of the study, it is important to understand what effects occur in the hair under the action of radiation of different wavelengths, primarily in terms of changes in their color. A number of works are devoted to the study of photodegradation of hair under the influence of radiation of the optical spectrum: UV, visible and infrared (IR). Effects of UV and visible light are studied in more detail due to the fact that melanins, responsible for hair color, are well absorbed by this radiation [5]. We know that UV and visible light can cause yellowing of blond, red, dark and brown hair, as well as reducing the yellowness of gray hair [3]. Yellowing in this case refers to the positive color coordinate b in the CIELAB system. It was found that UV radiation is capable of destroying the structure of hair, including through the destruction of disulfide bonds. In general, it can be argued that UV and visible radiation, with the former to a greater extent, contribute to the photodegradation of melanin, i.e. change its reflection spectrum, which determines the coloration of the entire hair [15]. As for the effects of infrared radiation, it has been shown that it can cause yellowing of gray hair, as well as its bleaching, similar to what is achieved by washing hair with hydrogen peroxide. However, to change the coloration of hair, infrared light requires a higher power density and a longer exposure time than UV or visible light [4]. Thus, the most preferred sources for laser hair coloring appear to be low-power UV sources capable of changing hair color without causing significant damage.
The initial step in the experimental determination of hair extinction spectra was to evaluate the colors of four hair samples. For this purpose, the CIELAB color system is used, as it more accurately shows how the human eye perceives colors. In this system, color is given by three coordinates: L - the lightness, a - the coordinate between red and green, and b - the coordinate between yellow and blue. There are different ways of measuring color coordinates. In this work, the color assessment was carried out by means of a scanner and a computer. Using a scanner allows you to evenly illuminate the samples and ensure identical image registration conditions. High-resolution photos of four hair samples were fixed with a scanner and averaged color coordinates of each sample were obtained on the computer in Adobe Photoshop (Adobe Systems, USA; version number 24.0.1). According to these coordinates, the names of hair colors closest to the experimental data were selected according to the palettes of hair dyes of the largest manufacturers (Fig. 3)
Appearance
Coordinates L, a, b 14, 2, -4 76, 6, 24
Hair color
Hair color
name blond
Figure 3: Results of hair samples color evaluation.
A UV/VIS T90 spectrophotometer (PG Instruments, United Kingdom) was used to measure hair spectra. Samples were prepared from blond hair. Hair was cut with scissors into fragments no longer than 1 cm in length and ground using a mechanical mill (manufacturer, country) to obtain a fine powder. Next, a piece of transparent scotch tape with a width of at least 1 cm and a length of at least 2 cm was attached to the inside of the mill cover with an adhesive side inside the mill. The thickness of the adhesive tape was 20±5 ^m. The mill was then run for about 10 seconds more, and the hair powder was deposited on the piece of scotch tape in the center of the inside of the lid. The piece of tape was then carefully removed from the cover and cleaned of excess powder and coarse hairs with a metal blade. Then, a piece of the same scotch without powder was placed on this piece of tape on the side containing particles of hair powder, and in this way a 90±5 microns thick sample for the experiments was obtained. Thus, in the sample, the hair powder was enclosed between the two pieces of scotch tape. Before the measurements of the hair spectrum, the baseline was measured: samples similar to those described earlier, but without hair powder, were placed in both channels of the instrument. Correction of the baseline made it possible to exclude the influence of light interaction with the tape on the
desired result. The measurements were performed in the fast mode with the spectral step equal to 1 nm. A total of 20 spectral measurements were made and the average extinction spectrum was calculated from them (Fig. 4). k, cm-1
900 850 800 750 700 650 600 550 500
400
300
180 220 260 300 340 380 420 460 500 540 580 620 660 700 740 780 820 860 900 A, nm
Figure 4: Measured extinction spectrum of light ash blond hair from 190 to 900 nm. Thus, as a result of the study, the spectra of extinction of hair of five colors in the spectral range from 330 to 2240 nm have been obtained using computer optical modeling, and the characteristics of the extinction peaks have been given. Data on the effects occurring in the hair under the influence of radiation of different wavelengths were systematized. Using a scanner and a computer, the color of four real samples of human hair using the CIELAB system was evaluated. Using a spectrophotometer, experiments were carried out to measure the spectrum of light hair extinction. Comparison of the calculated and experimental data showed the similarity of the characters of the extinction spectra with differences in the values of the extinction coefficient. This could be due to the difference between the concentrations of melanin in the model and in the experimental samples, which influenced the results, especially in the UV range, where melanin strongly absorbs radiation. In addition, the shredded hair structure in the samples may have somewhat different scattering characteristics, which could also affect the final values of the extinction coefficient. REFERENCES
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