INTERFEROMETRIC METHODS 



Haber-Lowe, (Zeiss, Mgf.), some of C. 

 Bams designs, our early model (Jobin-Yvon, 

 Mfg.), and the newer orthoptic microscope 

 interferometer (Aminco, Mfg.). The second 

 type is realized in interferometers of Jamin, 

 Michelson, Sagnac,the Zeiss-Opton interfer- 

 ence microscope (1950), Lotmar, and a few 

 others of fundamentally identical design 

 used in microscopic interferometry. Thus, 

 this first group includes both "narrow beam" 

 instruments (the first type) and "broad 

 beam" devices (the second type). 



Instruments with superimposed beams. 

 These include the Newton ring devices, the 

 Fabry-Perot interferometers and its nu- 

 merous variants, and the Dyson microscope 

 interferometer. Instruments of this type are 

 quite useful in metrology and for physical 

 optics investigation, but are ill-suited to the 

 study of transparent media. 



Another classification can be arranged on 

 the basis of the kind of information gained 

 by the use of the interferometer. It is a 

 rational classification based on the structure 

 of the device used to produce the two re- 

 quired coherent light beams, and such con- 

 siderations become of prime importance in 

 evaluating original designs. One thus finds 

 three classes: (a) instruments based on 

 "phase splitting", (b) instruments based on 

 "amplitude splitting", and (c) instrmiients 

 based on "polarization separation". 



This new classification is useful in general- 

 izing the field of applicability of interferome- 

 tric methods without limitation to any 

 spectral region. Indeed, the principles 

 previously smnmarized apply to practically 

 all electromagnetic vibrations, from electron- 

 waves and x-rays, to the visible and the 

 infrared radiations and up to the longest 

 radio waves. 



Instruments based on ^^ phase splitting." 

 These are represented by the father of all — 

 the Young interferometer — and the one best 

 known commercially — the Rayleigh in- 

 strument. The first produces a spherical 

 wave-front; the second, a cylindrical front. 



Instruments of this type correspond to 

 group (a) in the first classification. They are 

 all of the "narrow beam" type, and this 

 common appellation well describes their 

 practical limitations. The real advantages of 

 the phase splitting technique lie in the 

 simplicity of design, the flexible require- 

 ments for the optical parts involved, and the 

 relative independence of the performance 

 from wavelength limitations, at least within 

 the range of radiations within which the de- 

 vice producing the coherent pencils is prac- 

 tically realizable (UV to the middle of the 

 IR range). These conditions are discussed in 

 another article. It will be seen that the 

 critical width and separation of the aper- 

 tures used are also related to the wavelengths 

 (in air) of the radiations utilized. 



Instruments of the amplitude splitting type. 

 Such instruments generally make use of semi- 

 reflecting surfaces. Semireflecting beam 

 splitters are effective only within certain 

 narrow wavelength limits. No such really 

 good device exists for the electron waves, 

 the x-ray range or the extreme ultraviolet. 

 In the visible range, silver, aluminum, 

 chrome, titanium dioxide and "Inconel" 

 castings are quite satisfactory. 



In the infrared, the absorptive properties 

 of the layer becomes prohibitive, although 

 quite satisfactory mirrors may be made by 

 evaporating selenium, tellurium, and even 

 almninum. Even under the best conditions 

 the over-all efficiency is not very good, since 

 the energy is always distributed between 

 three fractions: reflected {R), transmitted 

 (T), and absorbed {A). The latter may be- 

 come preponderant at these wavelengths for 

 which the first two fractions are equal to one 

 another. This condition is found, for a X of a 

 few microns, by resolving Maxwell's equa- 

 tions, for the properties involved are inde- 

 pendent of X and the only variable is the 

 surface electrical conductivity of the coating. 

 For instance, when T equals R, the total A 

 may represent 80 % of the incident flux. 



Still another classification can be based on 



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