LIGHT SOURCES AND DETECTORS 



even between those manufactured in the same batch, their sensitivities 

 usually lie between the limits shown below : 



Type of cathode 

 Type A (S4) 

 Type B (S8) 

 Type S (SI) 



Sensitivity in /nAjlm 



30 to 100 

 10 to 60 

 10 to 60 



One other photocathode worthy of mention is type S3. It is a silver, 

 rubidium-oxide, rubidium cathode, with a maximum response near 420 m^. 



120 



"S 80 



c 

 (b 

 I/) 



n 



JO 

 0; 



q: 



4000 6000 8000 10,000 



Wavelength 



12.000 



A 



Figure 28.25 Comparison of the sensitivities of the three types of cathode 

 to tungsten light of C.T. 2,600°K 



in a lime glass envelope. It has a high sensitivity through the visible spectrum 

 and it approximates to the spectral sensitivity of the eye. This photocathode 

 is sometimes preferred for photometric measurements of atypical light 

 sources, although it is more usual today to use type B cathodes with suitable 

 correction filters. The filters are chosen such that the overall spectral response 

 approximates to that of the eye. 



Vacuum versus gas filled photocells 



The simple photoemissive cell can be obtained with either a vacuum or a 

 gas filling. It is important to study the different properties which these two 

 types of filling impart to the cell in order to select the correct type for a 

 particular application. 



In some photocell applications a hnear relationship is required between 

 light intensity and photocell current over a wide range of illumination. In 

 such cases a well-designed vacuum photocell is ideal. However, the detection 

 of low light intensities with a vacuum cell is technically difficult due to the 

 very small currents involved which require to be measured accurately. If a 

 linear response is required at low illumination intensities the photomultiplier 

 type cell, which is described later, is more satisfactory. 



For the detection of moderately low light intensities the gas filled photocell 

 has some advantages, although there is not a linear relationship between 

 light intensity and photocell current. The greater sensitivity of the gas filled 

 cell is due to the collision of the electrons emitted from the cathode with the 

 gas molecules in the cell. This results in ionization of the gas and the 



358 



