Visibility interferometer technique

The fundamental components of a Michelson interferometer are depicted schematically in Fig. 1. One possible misconception regarding the use of a Michelson interferometer at visible wavelengths is that it is difficult to obtain a sufficiently precise movement for the scanning mirror,, to provide useful resolution in the absorption spectrum (Fourier transformed inter-ferogram). This is definitely not the case. The actual reason that FT techniques are not commonplace in visible spectroscopy is that the usual visible spectrophotometer with a photomultiplier tube for a detector is source-noise-limited rather than detector-noise-limited so that FT methodology does not improve (and actually degrades) the quality of the measured spectrum. 

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain specttum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the specttum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectmm the instmment used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

Comparison of the Visibility/Interferometer Techniqiie with the MgO Collection Technique. During one experiment, using kerosene as the primary fiuid, a MgO-coated microscope slide was used to collect droplets 40 in. from the nozzle. The impressions formed by the droplets in the MgO were then viewed and sized using an optical microscope. During the same experiment and at the same location, drop sizes were measured using the visibility technique. One hundred drops were analyzed using each technique, and comparisons were drawn. 

Noble gases are intrinsically difficult to detect by spectroscopy. For example, solar photospheric spectra, which form the basis for solar abundance values of most elements, do not contain lines from noble gases (except for He, but this line cannot be used for abundance determinations). Yet, ultraviolet spectroscopy is the only or the major source of information on noble gas abundances in the atmospheres of Mercury and comets. In the Extreme Ultraviolet (EUV), photon energies exceed bond energies of molecules and the first ionization potential of all elements except F, He, and Ne, so that only these elements are visible in this part of the spectrum (Krasnopolsky et al. 1997). Other techniques can be used to determine the abundance of He where this element is a major constituent. Studies of solar oscillations (helioseismology) allow a precise determination of the He abundance in the solar interior, and the interferometer on the Galileo probe yielded a precise value for the refractive index and hence the He abundance in the upper atmosphere of Jupiter (see respective sections of this chapter). 

---