Multiplex disadvantage

In a theoretical study of the advantages to be gained from nondispersive systems, Chester and Winefordner (28) have studied frequency-modulated sources in nondispersive atomic fluorescence, and have demonstrated a multiplex disadvantage for nondispersive atomic fluorescence systems. These studies have indicated that the limiting noise is flame background noise. 

Michelson interferometers can be combined with Rayleigh line filters (or subtractive double monochromators) in order to prevent the consequences of the multiplex disadvantage (Secs. 3.1.6, 3.3.6 Fig. 3.4-1). 

The presence of a multiplex disadvantage accounts for the enormous success of small multichannel visible spectrometers based on array detectors, as compared to low resolution visible FT spectrometers. In any case, for a particular application a careful analysis of the noise is indispensable for an optimum instrument. 

Multiplex disadvantage, which occurs because signal is being detected at all wavelengths simultaneously so shot noise from the entire spectrum is spread into every spectral element... 

Due to the multiplex disadvantage, many FT-Raman spectrometers use filters, which make it impossible to observe the Rayleigh line. Thus, any shift in the laser line leads to shifts in the observed wavenumber positions of the sample under investigation. Although an interactive correction using a comparison of the peak positions of Stokes and anti-Stokes, spectral features can be used to identify the current laser wavelength. 

For recording the weak Raman spectrum excited by the Nd YAG laser line at 1064 nm, interferometers are successfully used. They have to be combined with very powerful Rayleigh filters. The quantum noise of every strong line is distributed over the whole interferogram. By Fourier transformation it is distributed as white noise over the whole spectrum. To avoid this multiplex disadvantage all strong lines have to be removed from the spectrum to be analysed. 

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