Multi-filament vapour source and spherical holder for interference filters

VACUUM

Classified Abstracts

I --

General

Science

and

Engineering

t

Abstraet, ~o. and References

I

Ceatd.

design in place of the single film to which it is equivalent. If a large n u m b e r of identical symmetrical units of this t y p e are combined to form a periodically stratified m e d i u m t h e n in certain regions of wavelengths the reflectance of the m e d i u m increases tending to 100% as the n u m b e r of periods tends to infinity. This region of the spectral response corresponds to the stop b a n d s of electrical filters. Similarly the other spectral regions are the pass b a n d s corresponding to a high transmittance. E a c h t h i n film combination corresponds in its electrical analogue to a symmetrical filter section h a v i n g a certain propagation c o n s t a n t a n d characteristic admittance. To m a t c h the filter to the surrounding m e d i u m corresponds to m a t c h i n g the generator and load in the electrical case. This is done by applying suitable m a t c h i n g sections to either side of the periodic m e d i u m and similarly m a t c h i n g sections have to be inserted at the junction i n the case of two different a d j o i n i n g periodic structures. These m a t c h i n g sections m a y consist of single anti-reflection coatings b u t in practice a compromise is often necessary because such coatings only have an optimal thickness of one-quarter wavelength at one wavelength. These principles are illustrated in the design of a band-pass filter using two periodic film combinations of 4 periods each and having suitable m a t c h i n g sections. The calculated reflectance curve for this design is shown. As another example the design of an anti-reflection coating is also discussed. Sommaire : Considdrations th~oriques sur la construction de filtres optiques, conduisant en m~me temps h des informations utiles concernant les recouvrements anti-r6fi~chissants.

Article bv L. I. Epstein J. Opt. 8oc. A met. 42, Nov. 195~, 806-810.

Multi-Filament Vapour Source and Spherical Holder for Interference Filters See Abstract No. : 9/III

24/x

Interference Filters in Optics United Kingdom. Interference filters consist of a n u m b e r of thin t r a n s p a r e n t films of different refractive index deposited on top of each other and usually supported by glass. The light entering the filter is subjected to multiple reflections between the surfaces of the individual films. Due to double transit of t h e light across the films and phase changes on the reflecting surfaces phase retardations occur which determine the transmission factor of the filter. The phase retardation depends on the ratio of film thickness to wavelength. Absorption plays no p a r t in the working m e c h a n i s m of the filter. Where absorption can be tolerated the Fabry-Perot type of filter is used which consists of semi-transparent metal films separated b y t r a n s p a r e n t dielectric films. This filter has a higher reflective power t h a n the all-dielectric filter. A (measured) transmission curve and tabulated d a t a of a typical Fabry-Perot filter consisting of a sandwich made of a film of a t r a n s p a r e n t dielectric between two semi-transparent silver films are given and reproduced below.

25/I

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m=4

m:lk

ms2

A

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W~Lt,,,CT.

M

2

3

4

5

Wavelength of centre of band

hm

8,645/~

5,794A

4,3934

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Half-width of band

W

Order of band

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Peak transmission of band

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Minimum transmission between bands . . . . . . . . . . . T.,..

J a n u a r y , 1953

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75h

624

774

177h

14.5%

30.2%

33.5%

24.3 %

0.03%

0,15%

Vacuum Vol. I I I No. I

1.2%

67