Light source incoherent

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

Conventional, incoherent light sources suitable for industrial-scale photochemistry and the reactors exploiting them have been reviewed in depth (2). Subsequent improvements in traditional light sources have been incremental. 

Due to diffraction effects of micron-sized mirrors in a regular array, commonly used techniques for surface characterization based on interferometry are inefficient. To overcome the diffraction effects we have developed a novel surface characterization method with an incoherent light source, based on the Foucault s knife-edge test (Zamkotsian and Dohlen, 1999). Since Leon Foucault introduced the knife-edge test in the last century (Foucault, 1859), it has been widely used for testing optical surfaces (see Ch. 3). The test offers a simple way of obtaining easily understandable, qualitative information of the surface shape. 

The computation of far-field radiation from a collection of incoherently radiating dipoles is in general quite a complicated problem. To calculate the angular dependence of the far-field intensity, the volume distribution of excited states must first be obtained, which, as we have seen, depends on the volume distribution of the absorbers and the electromagnetic field which stimulates them. The fields in turn depend on the frequency and linewidth of the exciting light source. Then the emission problem for the excited-state distribution (both spatial and frequency) must be solved including reorientation and depolarization effects. 

In summary, we have experimentally demonstrated laser control of a branching photochemical reactions using quantum interference phenomena. In addition we have overcome two major experimental obstacles to the general implementation of optical control of reactions (a) we have achieved control using incoherently related light sources, and (b) we have affected control in a bulk, thermally equilibrated, system. 

Figure 2. Cotton s experimental apparatus for measuring absorption coefficients for left-and right-circularly polarized light. p1 incoherent source Lj, L2, focusing lenses Nj, N2, Nicol polarizing prisms M2, limiting apertures Fj, F2, quarter-wave Fresnel rhombs S, sample cell p2, location for light detection (visual). From ref. [2].

Replacement of the conventional incoherent light source with a laser has allowed CD measurements to be made on extremely small samples, such as those encountered in capillary electrophoresis. In addition, the unique spatial properties of the laser are utilized to advantage in thermal lens spectroscopy. TL detection of CD, either via a single beam, or a differentially configured arrangement, has demonstrated significant improvements in the measurement SNR. Application to HPLC detection, or time-resolved studies, are currently under investigation. 

This article reviews direct and indirect (e.g., afterimage, flash blindness) light hazards from common incoherent light sources. For direct hazards specific to lasers and other specialized coherent sources, the reader is referred to organizations such as the Laser Institute of America and the International Electrotechnical Commission. 

In hollow-optical fibers for laser delivery, usually only some low order modes are excited because of small divergence angle of incident laser beam. In contrast, many high order modes are excited in hollow-optical fibers for spectroscopic applications because usually an incoherent light source like an arc lamp is used as the light source. Therefore, ray optic theory that can handle a wide divergence beam is used for evaluation of optical properties in this section. 

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