A few important definitions first:
1. Optical transmittance is the ratio of the light intensity I(d) transmitted through a sample of thickness dto the incident intensity I0, T = I(d)/I0; it is a dimensionless quantity; from Eq. (3.1) it follows that I(d)/I0 = exp(-/ttd);
2. Transmittance spectra show the dependence of transmittance on the wavelength: T(l);
3. Reflectance (reflection coefficient) is the ratio of the light intensity reflected from a sample Ir to the incident intensity I0, R = I/I0; it is a dimensional quantity;
4. Reflectance spectra show the dependence of reflectance on the wavelength:
R(i);
5. Reflectance depends on the angle of view of a detector, when as a detector an integrating sphere is used diffusion reflectance Rd is measured.
For skin it is not easy to provide transmittance measurements in vivo, thus such measurements is used more often in in vitro studies for the prediction of skin optical parameters. Two types of transmittance are typically introduced—the collimated transmittance that is based on collection of light transmitted only in the direction of the incident beam, Tc, which can be measured by a distant detector with a small aperture, and the total transmittance that is based on a collection of all light transmitted (scattered) within the hemisphere in the forward direction, Tt, which can be measured by a detector with an integrating sphere. Both quantities can be measured for the definite wavelength or for a whole spectrum of interest, typically in the visible/NIR range.
The reflectance spectroscopy is much more suitable for skin measurements. A few different methods are available; each of them solving a particular problem in skin appearance or pathology monitoring and, thus, has its own algorithm of operation and corresponding hardware. One of them is a spatially-resolved reflectance technique (SRR)—a technique that uses two or more fibers to illuminate skin and collect the back-reflected light; the positions of the illuminating and light-collecting fibers can be fixed or scanned along the skin surface perpendicularly or having some angle to the surface. Usually, a grating spectrograph at the output of receiving fiber (fibers) in combination with an optical multichannel analyzer (cooled CCD or photodiode array) as a detector is used for such measurements.
Due to the lower thickness of the epidermis compared with the dermis, scattering in the epidermis is of less importance than dermal scattering. Dermal tissue is practically entirely responsible for the majority of light scattering that takes place in the skin, and also determines the diffuse pattern of light distribution within the skin and the formation of the back – scattered diffuse reflectance. Scattering is generally stronger in the UV spectral range, but the strong absorption of epidermal melanin and dermal blood is an important factor responsible for the reduction of the back-scattered light and the generation of the skin reflectance spectrum [6]. Thus, absorption and scattering determine the amount of light emerging from the skin surface, which is closely related to the diffuse reflectance Rd.
Representative diffuse reflectance spectra of human skin are shown in Fig. 3.9. In the UV spectral range (<300 nm) the reflectance Rd is generally very small due to strong epidermal absorption, which reduces the amount of backscattered light to the level comparable
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Figure 3.9 Typical diffuse reflectance spectra of white (Caucasian) and black (African) human skin [6]. |
with Fresnel’s reflectance. The penetration depth of optical radiation within the epidermis does not exceed a few cell layers, and epidermal chromophores have a small effect on the diffuse reflectance spectrum.
In the UVA spectral range (315-400 nm) the skin reflectance exceeds Fresnel’s reflectance, which indicates an increase in the back-scattered radiation. The penetration depth of optical radiation increases up to hundreds of micrometers, and epidermal chromophores affect the shape of the reflectance spectrum.
In the visible spectral range (400-800 nm) the penetration depth is between 0.5-2.5 mm (see Fig. 3.8). In this case, both absorption and scattering play a dominant role in the formation of the diffuse reflectance spectrum. The fraction of back scattered light increases due to multiple scattering within the skin. The value of Rd is between 15% and 70%, and the reflectance spectrum has a sharp minimum in the spectral range 415-430 nm, due to hemoglobin absorption in the dermis. The reflectance spectrum has the characteristic dips in the spectral range 540-580 nm which are due to the Q-absorption bands of hemoglobin. Additional weak minima in the reflectance may be noted due to absorption of carotene (480 nm) and bilirubin (460 nm).
I n the spectral range 600-1500 nm, absorption is even lower, scattering dominates absorption, and the penetration depth is of 3.5 mm. Light within the skin is entirely diffuse, thus the diffuse reflectance is increased up to 35-70%. In the near IR spectral range, the skin reflectance increases up until 800-900 nm, and then decreases due to water absorption bands.
In the study of the perception of skin color, the chromaticity coordinates are used. One of the first mathematically defined color spaces was the CIE XYZ color space (also known as CIE 1931 color space), created by the International Commission on Illumination (Commission Internationale de l’Eclairage—CIE) in 1931. The human eye has receptors for short (S), middle (M), and long (L) wavelengths, also known as blue, green, and red receptors; this means that one, in principle, needs three parameters to describe a color sensation. A specific method for associating three numbers (or tristimulus values) with each color is
called a color space: the CIE XYZ color space is one of many such spaces; however, the CIE XYZ color space is special, because it is based on direct measurements of the human eye, and serves as the basis from which many other color spaces are defined; the CIE1964 standard observer is based on the mean 10-degree color matching functions.