Quasi-monochromatic source

Now let us assume that a monochromatic source of flux is placed in the plane of the entrance slit so that there is no constant phase relationship between the fields at any two given points in the slit. This, in itself, is a contradiction, because a perfect source monochromaticity implies both spatial and temporal coherence. By definition of coherence, a constant phase relationship would result. To eliminate the possibility of such a relationship, we must require the source spectrum to have finite breadth. Let us modify the assumption accordingly but specify the source spectrum breadth narrow enough so that its spatial extent when dispersed is negligible compared with the breadth of the slits, diffraction pattern, and so on. Whenever time integrals are required to obtain observable signals from superimposed fields, we evaluate them over time periods that are long compared with the reciprocal of the frequency difference between the fields. We shall call the assumed source a quasi-monochromatic source. 

To understand spatial coherence, consider two points Pi and Pi on a diffracting object illuminated by radiation from an extended quasi-monochromatic source, as in Figure 1.14. If P] and Pi are so close to each other that the difference Ad = SPi — SPi between the paths from all points S on the source is small compared with the mean wavelength X, then it is... 

In the hrst case, the degree of self coherence depends on the spectral characteristics of the source. The coherence time Tc represents the time scale over which a held remains correlated this hme is inversely proportional to the spectral bandwidth Au) of the detected light. A more quantitative dehnition of quasi-monochromatic conditions is based on the coherence time all relevant delays within the interferometer should be much shorter than the coherence length CTc. A practical way to measure temporal coherence is to use a Michel-son interferometer. As we shall see, in the second case the spatial coherence depends on the apparent extent of a source. 

If two or more quasi-monochromatic beams propagating in the same direction are superposed incoherently, that is to say, there is no fixed relationship among the phases of the separate beams, the total irradiance is merely the sum of the individual beam irradiances. Because the definition of the Stokes parameters involves only irradiances, it follows that the Stokes parameters of a collection of incoherent sources are additive. 

In most cases, low-pressure excimer UV sources show a dominant narrow-band (1-3 nm) transition. They can be considered to be quasi-monochromatic. 

Excimer lamps are quasi-monochromatic light sources available in UV wavelengths. The light is produced by silent electrical discharge through gas in the gap between two concentric quartz tubes. Electronically activated molecules are produced in the gas phase and decompose within nanoseconds to produce photons of high selectivity. This process is similar to the process in excimer lasers. 

For the determination of the spectral sensitivity complicated and expensive laboratory equipment are required in conjunction with experienced personnel. Such a facility should be capable in providing calibrated monochromatic radiation of sufficient power to be sensed by the broadband detector in question. A typical system should be comprised by a double monochromator coupled with a stable, high-power source (e.g. a 1000-Watt Xenon lamp). A spectrally calibrated detector should also be available to measure the quasi-monochromatic radiation emitted by this system. Such facilities are rarely available and only a few laboratories worldwide are able to cany out such characterizations. 

This observation may be explained as follows. A wave emitted from a point source with the spectral width Aco can be regarded as a superposition of many quasi-monochromatic components with frequencies within the interval Act). The superposition results in wave trains of finite length A c = =... 

The arguments above may be summarized as follows the coherence surface 5c, i.e., that maximum area Ac that can be coherently illuminated at a distance r from an extended quasi-monochromatic light source with the area As emitting at a wavelength A is determined by... 

These PISs can initiate the photopolymerization of synthetic, as well as renewable, monomers/oligomers. A careful adaptation of the PISs to novel light sources avoiding the harmful mercury lamps emitting UVA and UVB or even UVC rays has been possible, and today, polychromatic visible light irradiation devices (Xe, Hg-Xe and doped lamps), quasi-monochromatic devices (LED arrangements) or monochromatic devices (laser diode arrays) can be safely used. 

An experimental technique that is useful for structure studies of biological macromolecules and other cry stals with large unit cells uses neither the broad, white , spectrum characteristic of Laue methods nor a sharp, monochromatic spectrum, but rather a spectral band with AA-/A, 20%. Because of its relation to the Laue method, this technique is called quasi-Laue. It was believed for many years that the Laue method was not useful for structure studies because reflections of different orders would be superposed on the same point of a film or an image plate. It was realized recently, however, that, if there is a definite minimum wavelength in the spectral band, more than 80% of all reflections would contain only a single order. Quasi-Laue methods are now used with both neutrons and x-rays, particularly x-rays from synchrotron sources, which give an intense, white spectrum. 

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