200 



RADIATION BIOLOGY 



be controlled with extreme care, since its thickness determines the wave 

 length of constructive interference and maximum transmission. 



The Fabry-Perot filters have several transmission bands representing 

 the various orders of interference (Fig. 3-21). If the first-order band 

 occurs in the red, a second order will appear in the blue, and a third in 

 the ultraviolet. The higher orders are readily removed by long-wave- 

 pass dyed-gelatin or glass filters. The transmittance of the background 



,*^ 



SEMITRANSPARENT SILVER FILMS 



TRANSPARENT SPACER FILM 

 (ONE-HALF WAVELENGTH THICK) 



4 



30 



20 



I 10 



300 



700 



750 800 



350 400 450 500 550 600 650 



WAVE LENGTH, m^ 



Fig. 3-21. Simplified diagram of a transmission interference filter of the Fabry-Perot 

 type (above) and a transmission spectrum of a second-order 546-m|U filter (below). 

 The third- and fourth-order bands in the near ultraviolet must be removed with a 

 yellow glass or gelatin filter. (From Turner, 1950.) 



is determined by the thickness of the metal films — the thinner the film, 

 the higher the peak transmission and also the background. For back- 

 ground transmittances of the order of a few tenths of 1 per cent, the 

 peak transmittance is usually between 10 and 40 per cent. The reso- 

 lution of interference filters is specified in terms of a nominal spectral 

 band width measured at half the peak transmittance. For many appli- 

 cations the effective width is several times this value. Commercial filters 

 have nominal band widths varying from 5 to 30 m^u. By placing matched 

 filters in tandem, the band width and background transmittance decrease 

 more rapidly than the peak transmittance. By tilting the filter the wave 

 length of maximum transmittance can be shifted over a range of several 

 millimicrons. As a general rule, the incident beam must be colfimated 

 to within 10°-15° on each side of the normal to avoid excessive widening 



