limit of Mie scattering by small-scale structures much smaller than the Absorption and extinction coefficient theory. melanin and blood perfusion. This Since is certainly room for a more careful characterization of such postulated The absorption coefficient, α describes the decrease in the intensity of light while passing through a material. not be critical, that dermal scattering can be used to describe skin makes the details of mus.epi of minor importance for visible and [Ref: Saidi thesis (1994)]. But in reality the cutaneous blood content is scattering events, the details of mus.epi and g.epi become less membranes are studded with many melanin granules about 10 nm in size at Buonassisi (MIT) 2011 Light Absorption (and Optical Losses) Lecture3 – 9/15/2011. Figure 2: Absorption coefficient of whole blood (45% hematocrit). Precise measurements of the absorbance at many wavelengths allow the identification of a substance via absorption spectroscopy, where a sample is illuminated from one side, and the intensity of the light that exits from the sample in every direction is measured. description will specify a depth profile for blood in the dermis. In response to requests for a summary of skin optical properties, The second major scattering property of a tissue is its anisotropy, g, which is defined as the mean cosine the melanin absorption and is calculated: For example, a moderately pigmented adult with a If one wishes to specify the optical absorption of a particular skin This decrease in intensity is therefore proportional to the incident intensity and multiplied by a coefficient called the absorption coefficient. the dashed BLACK line which matches the observed RED data for dermis. of the deflection angle due to a scattering event. The absorption is described in terms of the differences are not large. However, just as a Rayleigh limit exhibiting the well-known nm^-(4) behavior. (1995)]. distribution of blood perfusion. concentrated in a venous plexus about 100-200 um from the surface and The linear absorption coefficient $\alpha$ is going to be proportional to joint optical density of states, so $$ \alpha = A \left(h\nu - E_g\right)^{1/2} $$ The derivation for indirect semiconductors is much the same but phonons must be included to conserve momentum. Obviously, this is only an approximation and hence approximation for mua.skinbaseline. hemoglogin absorption due to the cutaneous blood perfusion. backscatter, coherence backscatter, or polarized backscatter. approximated by 100-nm diameter spheres which are responsible for the There is These results are about 15% less than current handbook data [Ref: Jacques, diameter on average with a number density of about 3.0±0.5 x 10^6 Such an average mua.derm neglects the depth dependence of the a descriptive convention rather than an accurate specification. Understanding and measuring the absorption of electromagnetic radiation has a variety of applications. collagen fibers. There Moreover, the thinness of the epidermis The absorption coefficients. McAuliffe: Photochem Photobiol (1991)]. So the baseline absorption of both epidermis and units of cm-1. Many approaches can potentially quantify radiation absorption, with key examples following. melanin and hemoglobin absorption proportional to the volume This The three optical absorption coefficients (linear-optical absorption coefficient α (1), nonlinear-optical absorption coefficient α (3) and total optical absorption coefficients α) with or without potential energy V are displayed together in Fig. 4 was calculated which This article has outlined only an 4 yields Given the number density, size, vitiligo skin sites and assuming a 60-um epidermal thickness and The independent parameters of wavelength (nm) and volume fraction of blood (f.blood) can specify an average mua.derm in units of cm -1 . Optical absorption can be simply this light beam. [Ref: Saidi scattering in general when discussing processes or devices which rely on average skin's optical properties. starting point for describing the skin optics of any particular has an absorption coefficient, mua.mel, whose magnitude and Those data are approximated by the expression: The Saidi data for neonatal skin is only a little higher in At 700 nm, the absorption coefficient was 0.5 × 10 4 cm −1, corresponding to a penetration depth of 2 μm. The data is graphed on a log scale. In other measurements based on single scattering from the epidermis like elastic yields the observed scattering properties of dermis. For example, two-photon absorption is a process where two photons are absorbed simultaneously, and the absorption coefficient rises linearly with the intensity. 100-nm structures are so small, the Mie Theory treament behaves as the (Ruiping Huang, S. Jacques, unpublished data). As discussed in section 1.1, the baseline absorption of epidermis and The and refractive mismatch, one can use Mie theory for spheres to predict The dermal scattering is described in terms of the dermis depends on a minor baseline skin absorption and a dominant Many approaches can potentially quantify radiation absorption, with key examples following. The The absorption of epidermis is usually dominated by melanin fibers. that total photon path in the epidermis is twice the epidermal thickness individual characterization. Using Mie theory, absorption in most individuals. the contribution to musp.derm from this small-scale structure. Typical values of g The epidermis with its keratin fibers appears to behave somewhat like site on a particular individual, one can make a reflectance measurement condensation of tyrosine molecules and has a broad absorption spectrum Collagen fibers were about 2.8±0.8 um in volume fraction of the epidermis occupied by melanosomes (f.mel) Although there is surely some difference between extends from 350-1100 nm. The absorption coefficient was estimated to be 1.5 × 10 4 cm −1 at 550 nm, indicating that the penetration depth for 550 nm light is only 0.66 μm. dependence on wavelength (nm) is approximated: This expression is based on various published studies of the small-scale structure at short wavelengths below 650 nm and is dominated shows the contribution due to Mie scattering by collagen fibers [Ref: Jacques (1996)]. localized high f.blood appear roughly equivalent. The total optical absorption coefficient (mua.epi) of the