270 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



with the level of illumination, as shown in Fig. (5-1)32. The transfer 

 function (output current versus illumination) deviates from linearity 

 (1) at very high levels of illumination, (2) if only a part of the sur- 

 face is illuminated, and (3) if the load resistance is not small com- 

 pared to the internal resistance of the cell, as shown in Fig. (5-1)32. 



£ 300 



a> 



200 



100 



o 







Fig. (5-1)31. Open-circuit output voltage as a function 

 of the incident light flux for a photogalvanic cell. 



O 

 [ 



D 



10.0 



Lumens 



20.0 



Fig. (5-1)32. Output current as a function of the incident 

 light flux for a photogalvanic cell; parameter load resistance. 



The variation of the characteristic with the load resistance may be 

 considered an advantage; it enables the user to construct instru- 

 ments with either linear calibration curves or curves that approach 

 a logarithmic function and cover several orders of magnitude of 

 light flux without switching the range of the meter (exposure 

 meters). 



Linearity of the transfer characteristic can be obtained by compensation 

 methods devised by N. R. Campbell and M. K. Freeth, J. Sci. Instr., 11, 125 

 (1934); by R. A. Houstoun and A. F. Howatson, Phil. Mag., 36, 279 (1945); 

 by L. A. Wood, Rev. Sci. Instr.. 7, 157 (1936); or by the feedback circuit 

 described by E. S. Rittner, Rev. Sci. Instr., 18, 36 (1947). 



