ULTHAVIOLET SPECTROSCOPIC TECHNIQUE 135 



emulsion and thereby greatly reduces the plate sensitivity. For work at 

 shorter wave lengths, very thin emulsions heavily laden with silver halide 

 grains may be used, such as the Ilford "Q" plates or the Eastman Kodak 

 SWR film (Schoen and Hodge, 1950). As an alternative, the surface of 

 the film may be coated with material that will fluoresce under the short- 

 wave-length radiation, so that the exposure is actually produced by the 

 fluorescent radiation (Harrison, 1925b). Such thin-emulsion or fluores- 

 cent-coated plates may be used far into the vacuum ultraviolet (Harrison 

 and Leighton, 1930). By the use of a fluorescent coating with constant 

 quantum yield of fluorescence, independent of exciting wave length, 

 problems of heterochromatic photometry may be greatly simplified. 

 Such coatings also eliminate the variation of contrast with wave length 

 (Harrison and Leighton, 1931). For photomicrography, however, the 

 use of fluorescent coatings generally leads to some loss of plate resolution. 



PHOTOELECTRIC DETECTORS 



Photoelectric detectors useful in the ultraviolet are of two general 

 types: the photovoltaic or barrier-layer cell, and the photoemissive 

 detector. The photovoltaic cells, which do not require an external power 

 source, are convenient and useful in instances where relatively large 

 amounts of radiant energy are available. The photoemissive detectors 

 require more elaborate accessory equipment but are far more sensitive 

 and are effective in a wide variety of applications. 



Photovoltaic Detectors. Upon illumination of a photovoltaic cell, a 

 potential difference appears across a semiconductor (usually iron sele- 

 nide), which potential can be used to drive a current through an external 

 circuit (Lange, 1938; Zworykin and Ramberg, 1949, Chap. 11). Elec- 

 trically, the photovoltaic cell acts as a source of current which is shunted 

 by an internal resistance and capacitance. The shunting internal resist- 

 ance is not constant, but decreases with increasing illumination and with 

 increasing current flow. Although the photocurrent generated within the 

 semiconductor is, at moderate light levels, linearly dependent on light 

 intensity, because of the internal resistance and its variation with light 

 intensity, the external current is a linear function of light intensity only 

 if very low external resistance is employed (Wood, 1934). As a conse- 

 quence, the output of a barrier-layer cell as a function of fight intensity, 

 with various external resistances, is as shown in Fig. 4-3. 



Electronic circuits have been developed to permit the use of larger 

 external resistance, if desired for purposes of amplification, without 

 introducing appreciable nonlinearity. Such circuits (Rittner, 1947) 

 employ negative feedback to effectively reduce the apparent resistance 

 external to the photocell. 



The internal capacitance of the photovoltaic cell also acts to shunt the 

 external resistance if an oscillatory photocurrent is produced by a modu- 



