ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 



145 



Interference Filter. The action of an interference filter is based on the 

 cancellation of coherent light waves when they are added together in 

 phase opposition. In its action, each ray of the light beam is divided 

 into a large number of weaker rays, with regular shifts in phase between 

 adjacent rays, by the use of multiple reflections between lightly silvered 

 surfaces appropriately spaced by dielectric, these surfaces allowing a 

 slight transmission of energy at each contact (Fig. 4-10). When these 

 rays are recombined by a lens, the resultant intensity at the focus of the 

 lens will depend on the phase difference between adjacent rays, being 

 maximal when all rays are in phase, and minimal when adjacent rays are 

 exactly out of phase. If a stop is then placed about the focus of the lens, 



Fig. 4-10. Figure illustrating the splitting and multiple reflection of light rays origi- 

 nating from Pi by the silvered surfaces Ei and £"2, to provide a monochromatic image 

 of Pi at P-i. {Reproduction from. Fundamentals of Optics, 2d ed., by F. A. Jenkins and 

 H. E. White, McGraw-Hill Book Company, Inc., 1950.) 



only those wave lengths for which adjacent rays are exactly or nearly in 

 phase will be transmitted in appreciable intensity. By controlling the 

 thickness of dielectric between the reflecting surfaces, the variation of 

 phase difference between adjacent rays with wave length can be controlled 

 and thus the wave length or wave lengths of maximum transmission 

 selected. The simple interference filter can be regarded essentially as a 

 crude Fabry-Perot etalon (Jenkins and White^ 1950, Chap. 14). 



Thus a simple interference filter consists of two lightly silvered reflect- 

 ing surfaces, spaced by an appropriate thickness of dielectric (frequently 

 magnesium fluoride). The spectral selectivity of such a filter is depend- 

 ent extrinsically on the angular aperture of the radiation with which it is 

 employed (increasing with decreasing aperture), and intrinsically on the 

 reflectivity of the reflecting surfaces and the number of wave length paths 

 in the dielectric spacer (Mooney, 1946; Hadley and Dennison, 1947, 

 1948). Under favorable optical conditions, such filters can provide a 

 peak transmission of about 35 per cent with a band width of about 100 A 

 (at half-maximum transmission) when peaked for various wave lengths 

 in the visible region. The transmission of interference filters does not, 

 however, drop to zero outside the transmission band (or bands) but to a 

 minimum of about 1 per cent. The wave length of peak transmission 



