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2025-02-11
Skin color changes can reflect the integrity of the skin barrier, the sensitivity of the skin, and the skin's response to drugs, beauty, and skin care products. Therefore, quantitative analysis of skin color changes is of great significance in dermatology and beauty and skin care work. This article introduces the application of colorimeter in skin color measurement.
The color of an object depends on the spectral composition of the light source and the proportion of each wavelength of light reflected from the surface of the object. For example, in daylight, if an object reflects light in the 480-560 nm band and relatively absorbs light of other wavelengths, the surface of the object will be green: white light interacts with the skin and is converted into colored light through reflection and absorption, making the skin appear different colors.
When light hits the surface of the skin, it is either absorbed by the pigment or reflected by the stratum corneum. About 4% to 8% of the light is reflected by the stratum corneum: the uneven absorption of light by pigments and transparent keratin particles makes the optical properties of the skin very complicated. In addition to the transparent keratin particles in the epidermis, the main reflective substances in the skin are the collagen fiber bundles in the dermis. The difference in the composition of the upper part of the stratum corneum is also an important factor in determining the reflective properties of the skin surface: the smooth, water-rich stratum corneum reflects regularly to form a bright luster, while the dry, scaly stratum corneum reflects light in a non-specular way, making the skin gray. If there is air in the stratum corneum, the surface of the scales will have a white luster.
The traditional skin color measurement mainly adopts subjective measurement methods, that is, comparing the color to be measured with the color reference is the basis of color evaluation. At present, the Munsell color system is widely used internationally as a method for classifying and calibrating surface colors: a three-dimensional space model similar to a sphere is used to fully represent the three basic characteristics of various surface colors, namely hue, lightness and saturation. Each part of the three-dimensional model represents a specific color and is given a certain number, among which the hue is divided into 100 levels, and the saturation and lightness are divided into 10 levels. The color atlas contains standard color samples made of paper, which can be used for color comparison. Many studies have shown that if different skin colors are evaluated at the same time, the human eye can distinguish colors as accurately as any instrument, but quantitative and (or) repeatable evaluation can only be carried out by instruments.
At present, many instruments that objectively and quantitatively analyze skin color have entered the market, such as tristimulus colorimeter, narrow-spectrum simple reflectance spectrophotometer, scanning reflectance spectrophotometer and digital imaging system. Among the above-mentioned measuring instruments, except for the digital imaging system, the basic principle of other instruments is to use a photometer to measure the reflectance of the skin to each wavelength of light. The visible light is gradually increased in units of 10nm wavelength, irradiated on the color surface, and then the reflectance is measured point by point to obtain the spectrophotometric curve of the measured color surface: the measured value can also be converted into other color space system values, such as CIE XYZ, CIE L*a*b.
The tristimulus colorimeter directly measures the surface of the skin to obtain a visual response proportional to the tristimulus values X, Y, and Z of the color. The X, Y, and Z values of the measured color are obtained through conversion, and these values can also be converted into color parameters in other color spaces. The Minolta CR 200 is commonly used in clinical practice. It uses a pulsed xenon lamp as the light source and detects the skin surface reflectance in the 450, 560, and 600nm bands.
Using the red, green and blue primary colors specified by the International Commission on Illumination (CIE), a color mixing and matching test was conducted on 317 subjects with normal vision. The number of red (R), green (G) and blue (B) primary colors required to match different colors is called the spectral tristimulus values, CIE-RCB spectral tristimulus values.
Since the CIE-RGB calibration of spectral color produces negative values, it is inconvenient to calculate and difficult to understand, so the CIE XYZ system is used. This system is based on the RGB model and uses mathematical methods to select three ideal primary colors to replace the actual three primary colors, thereby changing the color tristimulus values and chromaticity coordinates in the CIE-RGB model into positive values.
In order to obtain a uniform color system, the CIE XYZ system is converted into the CIE L*a*b* color space by mathematical methods. This space consists of a brightness (L) and two color (a, b) axes. Brightness is a scale that represents grayscale, and its value is between 0-100; 0 represents black and 100 represents white. a* is the color saturation from red to green, and its range is +60 to -60. Positive values represent changes in red intensity. b* represents the color saturation from yellow to blue, and its range is +60 to -60. Positive values represent changes in yellow intensity.
1. Measurement site
The skin color of different parts of the body is different, which mainly reflects the degree of skin darkening, vascularization, hematocrit and blood volume, which affect the melanin index and erythema index, as well as the changes in L* and a*.
2. Body Posture
When a healthy person's limbs droop, the body reflexively adjusts the tension of peripheral blood vessels. This postural vasoconstriction response reduces the blood flow and blood flow velocity of the microcirculation, while vertically raising the limbs can reduce the hydrostatic pressure of the capillaries and drain some blood. Therefore, changes in limb posture can change the color of the skin, especially the erythema index, melanin index, L* and a* values.
3. Measure temperature
The ambient temperature affects the temperature of the local skin and body. Temperature changes can change the degree of vasoconstriction. Increased ambient temperature causes vasodilation and erythema, while cold causes cyanosis or pale skin. These changes can affect the color of the skin.
4.Measure the pressure of the probe
Depending on the size and weight of the instrument, errors caused by it should be avoided when measuring human tissue. Some test probes are large and may affect the accurate positioning of the test site. The pressure exerted by the test probe on the skin compresses the skin blood vessels and may also cause measurement errors.
