Nonlaser Light Sources

Nonlaser light sources emit radiation in all directions as a result of the spontaneous emission of photons by thermally excited solids (filament lamps) or electronically excited atoms, ions, or molecules (fluorescent lamps, etc.). The emission accompanies the spontaneous return of the excited species to the ground state and occurs randomly, Le. the radiation is not coherent, in a laser, the atoms, ions, or molecules are first pumped to an excited state and then stimulated to emit photons by collision of a photon of the same energy. This is called stimulated emission. In order to use it, it is first necessary to create a condition in the amplifying medium, called popidatlon inversion, in which the majority of the relevant entitles are excited. Random emission firom one entity can then trigger coherent emission firom the others that it passes. In this way amplification is achieved. 

Light Sources and Delivery. In PIV, lasers are used only as a source of bright illumination and are not a requirement. Flash lamps and other white light sources can also be used. Some facilities prefer these nonlaser light sources because of safety issues. However, white light cannot be collimated as well as coherent laser light, and their use in PIV is not widespread. 

Incoherent (nonlaser) and laser light sources have been used for PDT. The wavelengths chosen must include those within the action spectrum of protoporphyrin and ideally those that permit maximum skin penetration. Light sources in use emit energy predominantly in the blue portion (maximum porphyrin absorption) or the red portion (better tissue penetration) of the visible spectrum. Nonhypertrophic actinic keratoses and superficial basal cell carcinomas and Bowen s disease seem to respond best to PDT. Topical ALA products for PDT approved by the FDA include LEVUIAN KERASTICK, BLU-U blue light, and METVIX. 

For surface irradiation (including accessible body cavities such as the mouth or intra-operatively), lamps or light emitting diodes (LEDs) provide high power density and total power at lower cost. Since these sourees are not monochromatic, they are less efficient in photoactivation (typically by a factor of about 2 for source bandwidths of 25-30 run) and this must be taken into aeeount when switching between laser and nonlaser treatments. Figure 6 illustrates different elasses of irradiation techniques in PDT. 

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