194 RADIATION BIOLOGY 



imposed between the two incompatible factors of radiant power and spec- 

 tral resolution. High irradiances over large areas are attainable with 

 simple solution and gelatin or glass-filter systems in conjunction with 

 both concentrated and distributed sources, but the spectral resolution is 

 usually limited to hundreds of millimicrons for the visible and near ultra- 

 violet, except at the few wave lengths where such sources as the mercury 

 arc have intense and well-isolated lines. With single interference filters, 

 one is limited to much lower values of total flux owing to the small size 

 of these filters and the collimation requirements, but the spectral reso- 

 lution is 5-50 m/x. With the spectrograph o: monochromator, the source 

 reciuirements become still more stringent, and a smaller proportion of the 

 source power can be used, but the spectral resolution can be extended 

 well below 1 m/u. 



FILTERS 



Radiant-energy filters may be divided into two general classes: the 

 selective-absorption filters and the optical filters. The selective-absorp- 

 tion filters consist mainly of organic dyes and inorganic ions in solution 

 in water or glasses, and since they require no collimation of the incident 

 flux, both concentrated and distributed sources may be used with high 

 efficiency. In general, the organic dyes have sharper absorption bands 

 than aqueous solutions of inorganic salts of most of the heavy metals 

 except the rare earths. The absorption bands of the inorganic ions 

 become still less sharp when incorporated in glass. The absorption bands 

 of dyes and inorganic ions are principally due to electronic resonance 

 phenomena in which the absorbed energy is eventually degraded to heat 

 within the filter, and means must be provided for dissipating that energy. 

 In the optical filter, selective interference or scattering causes the desired 

 region to be passed by reflection or transmission, and the unwanted 

 regions are deviated from the main beam by transmission, reflection, or 

 scattering. Although the optical filters have the sharper and more versa- 

 tile transmission bands of the two classes of filters, collimated optical 

 systems involving concentrated sources are required, and there is more 

 background transmission or "leakage" throughout the whole spectrum. 



Insufficient attention has been given to the general or background 

 radiant flux transmitted in the nonpass region. Published data seldom 

 give an adecjuate picture of the magnitude of this background. Direct 

 photometric measurements usually become uncertain at transmittance 

 values much below 1 per cent, although a transmittance of 10~^ per cent 

 may be significant when, for example, one is working at the limit of an 

 action spectrum curve. 



Selective-absorption Filters. Selective-absorption filters for the isolation 

 of narrow spectral regions have been described in various publications 

 (Corning Glass Works; Davis and Gibson, 1931, 1934; Eastman Kodak 



