382

3 R. M. Tyrrell, UVA (320380 nm) radiation as an oxidant stress. In H. Sies (ed.), Oxfdutive Stress, Vol. 2, Academic Press, New York, in the press.

Coherent and non-coherent light sources

Giuseppe Roberti Dipatiimento di Scienze Fisiche, UniversitZl di Napoli, Napoli (Ita&)

The possibility of generating spatially and temporally coherent light beams is primarily based on the “stimulated emission”; a process in which the interaction of an electromagnetic wave with an atom lying in an excited level produces a downward transition and, hence, electromagnetic radiation with the same frequency, phase and direction as the incoming one. The ampli6cation of the incoming wave can occur only if the upper level of the considered transition is more populated than the lower one. This condition of “population inversion” cannot be achieved at thermal equilibrium at any temperature and, in lasers, is obtained by suitable pumping mechanisms (optical, electrical or chemical). The number of energy levels involved in the pumping schemes must be greater than two and is normally three or, better, four. To complete the scheme of a laser, a resonant cavity that supplies the positive feedback and transforms an amplifier into an oscillator is needed. A plane parallel wave will bounce back and forth into the resonant cavity and will be amplified on each passage. The laser threshold condition occurs when the flux gain per pass in the laser material balances the flux losses.

A laser can be operated in continuous-wave mode or pulsed mode. In the latter case the light bursts can be obtained by modulating either the pumping mechanism (e.g. electric pulses or flashlamps) or the feedback (e.g. rotating mirrors or saturable absorbers) (Q-switched mode) or by forcing longitudinal modes to oscillate phase-locked (mode- locking). The laser classiScation is based on both the active material (solid, liquid and gas) and the pumping mechanism. The properties of a laser beam are its low divergence and high monochromaticity which are consequences of the spatial and temporal coherence, respectively, and the brightness that is greater than that of the conventional sources by several orders of magnitude. The most commonly used lamps are incandescent filament lamps, fluorescent lamps and discharge lamps.

The incandescent filament lamps consist of a tungsten Lament contained in a glass or quartz bulb Illled with rare gas. Their spectrum is approximated, over the visible range, by a grey body spectrum, i.e. a continuous spectrum proportional to black body spectrum. The fluorescent lamps are mercury arc lamps which emit most of their radiation in the UV line at 254 nm. This radiation is absorbed by the lamp bulb coated with fluorescent phosphorus, which in turn emits continuous spectrum covering both UV and visible. The discharge lamps are a gas-tilled bulb containing two electrodes supplied by direct or alternate current. The applied voltage accelerates the electrons and ions thus producing anelastic collisions leading to the excitation of atoms and successive radiative decay. Increasing the value of the voltage the spectrum of the emitted radiation varies from a line spectrum, to a continuous spectrum and finally to a superposition of both. Usually the discharge lamps contain sodium, xenon, mercury or metal halide.

1 J. F. Rabek,ExperimentalMethods inPhotochemistry andPhotoph~sics, Wiley, Chichester, 1982.

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2 0. Svelto, Principles of Lasers, Plenum, New York, 1982. 3 L. Levi, Applied Optics: A Guide to Optical System Design, Wiley, Chichester, 1970.

Principles of light propagation in living tissues

Lam 0. Svaasand

University of lhm.dht?im @b-way)

The detailed calculation of light propagation in highly scattering and inhomogeneous media (e.g. most tissues) is very complex. However, several approximate analytical and numerical mathematical models can be used [ 1, 21. Among these is the so-called optical diffusion theory which is valid when the light is scattered to an almost isotropic distribution. The photons are then transported in a manner very similar to ordinary diffusion and the net flux of photons is anticipated to flow from regions with high optical fluence rates to regions with lower fluence rates. Thus, this flow occurs by a diffusion process very similar to the case of heat flow from high temperature regions to surrounding locations with lower temperatures.

In the one-dimensional case, such as the case of broad beam irradiation, the optical fluence rate can be approximated by [3]

where x is the distance from the exposed surface. The optical fluence rate, which is defined as the quantity of light irradiated onto an infinitesimally small sphere divided by the cross-sectional area of that sphere, is given by cp. Initially this equation looks identical to the well-known Beer’s law for optical propagation in non-scattering media. There are, however, important differences. The optical penetration depth S, which expresses the distance corresponding to a decay in the fluence rate by a factor of l/e=O.37, is now dependent on the scattering properties of the tissue as well as on the absorption coefficient. The reason is that the scattering process introduces a zig-zag formed photon path and thereby enhances the effective length travelled by the photons per unit depth from the surface. Thus, although the scattering process is a loss-less phenomenon, it increases the attenuation by increasing the probability for absorption.

In a highly scattering medium the incident beam will be scattered into a diffuse, isotropic light distribution at the surface layer. The large back-scattering will enhance the optical fluence rate in the surface layer. When the surface of a highly scattering tissue is irradiated with a non-collimated isotropic irradiation the surface fluence rate becomes

where yd and I,, are, respectively, the diffuse reflection coefficient and the power density of the incident beam. The diffuse reflection coefficient for red and near-IR irradiation is, in the case of non-pigmented tissues, typically in the range y,=O.2 to 0.5. The fluence rate in the surface layer can therefore be up to two or three times larger than the incident irradiation.

In conclusion, the scattering process will enhance the fluence rate in the surface layer well above the surface irradiation. But it will also enhance the attenuation and thereby decrease the optical penetration depth. Typical values for the optical penetration depth in several human tissues are [3]