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: […]

For skin, in vivo measurements of optical properties are possible only in the geometry of backscattering. The spatially resolved reflectance R(rsd) is defined as the power of the back – scattered light per unit of area detected by a receiver at the surface of the skin at a distance rsd from the source, rsd is […]

Soft tissue is composed of closely packed groups of cells entrapped in a network of fibers through which water percolates. At a microscopic scale, the tissue components have no pronounced boundaries. They appear to merge into a continuous structure with spatial variations in the refractive index. It has been shown that the tissue components that […]

Absorption is only one way by which light can interact with the skin. Absorption of the UV and visible light in skin is due to electronic excitation of aromatic or conjugated unsat­urated chromophores. A chromophore is a chemical that absorbs light with a characteristic spectral pattern. There are many kinds of chromophores in the skin, […]

3.1.3 Light Absorption and Scattering Absorbing medium is the medium that absorbs light at certain wavelengths or wave­length bands. The absorption band is a range of wavelengths for which a medium absorbs more strongly than at adjacent wavelengths. The process of light absorption is the transfor­mation of light (radiant) energy to some other form of […]

Individual photons from laser or noncoherent light sources are identical, and a tissue does not know how photon was born. The choice between laser and noncoherent light sources is strictly dependent on the particular application. In therapeutic applications, a laser is almost just one choice for short pulse (<0.1 ms), small spot size or microbeam […]

The success of light treatment is largely dependent on the light-delivery system used. Generally, a light guide, which delivers light energy to a target to illuminate it, consists of an assembly of optical fibers that are bundled but not ordered. A fiber is an optical waveguide that uses the phenomenon of total internal reflection for […]

Laser is acronym for light amplification by the stimulated emission ofradiation. Laser is a device that generates a beam of light that is collimated, monochromatic, and coherent. Laser radiation is characterized by its wavelength, power, and pulse – or continuous wave­mode of generation. Any CW lasers can work in the pulse mode by a switch-on […]

An LED is a semiconductor device that emits light when the forward-directed current passes the p-n junction. LEDs are the light sources with a wide range of selected wave­lengths from UV to IR. A typical wavelength bandwidth is 20-30 nm for LEDs working in the visible range. An LED’s light power ranges from a few […]