Quasi-monochromatic light

In the derivations above of the Stokes parameters we began with monochromatic light and then extended our results to the more general case of quasi-monochromatic light. However, the operational definition of the Stokes parameters in terms of a set of elementary experiments involving a detector and various polarizers, as opposed to the formal mathematical definitions (2.80) and (2.84), is independent of any assumed properties of the beam. Unless otherwise stated, we shall assume that all beams of interest are quasi-monochromatic, which includes as a special case monochromatic light. 

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. 

The additivity of the Stokes parameters allows us to generalize the principle of optical equivalence (Section 7) to quasi-monochromatic light as follows it is impossible by means of a traditional optical instrument to distinguish between various ineoherent mixtures of quasi-monochromatie beams that form a beam with the same Stokes parameters (/, Q, U, V). For example, there is only one kind of unpolarized light, although it can be eomposed of quasi-monoehromatic beams in an infinite variety of optically indistinguishable ways. 

If a quasi-monochromatic light wave I (co) with a statistically fluctuating intensity distribution (7.60) falls onto the photocathode, the probability of detecting n photoelectrons within the time interval dr is not described by (7.58) but by the Bose-Einstein distribution... 

Fig. 11. Influence of the peristrophic rotation on the reconstructed image obtained with (a) conventional quasi-monochromatic light whose central wavelength is 532 nm and whose spectral width is 1 nm, and (b) polychromatic light whose central wavelength is 815 nm and whose spectral width is 40 nm. <5i cz is the peristrophic rotation angle.

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... 

Nonmonochromatic Waves (1.16) Diffraction theory is readily expandable to non-monochromatic light. A formulation of the Kirchhoff-Fresnel integral which applies to quasi-monochromatic conditions involves the superposition of retarded field amplitudes. 

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. 

The wavelength of the emitted light depends on the choice of atoms A and B. Besides the principle of excimer formation, the technical parameters for the lamp and discharge are responsible for the quasi-monochromatic character of the emitted spectrum. The most important commercial excimers are formed by electronic excitation of molecules of rare gases (He2, Ne2, Ar2, Kr2, Xe2),... 

We will see later, however, that this identity may not hold for a quasi-monochromatic beam of light. 

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. 

---