THE BARRIER-LAYER PHOTOELECTRIC CELL 155 



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This filter transmits 60 per cent at 4500 A and 70 at 4250 A, so that 

 the brightness match is made in terms of the blue transmitted light. 



The number of grams of hemoglobin per 100 cc of blood may be read 

 directly off a scale to 0.5 gram and estimated to 0.1 gram. The filter 

 should transmit those wavelengths corresponding as nearly as possible 

 to the absorption maximum of the solution. 



Photoelectric Colorimeters 



In the design of modern colorimeters the aim is to embody rigorous 

 physical principles and to avoid all empirical procedures. The photo- 

 electric method of measuring light intensities can be used to give quanti- 

 tative results under the requirements laid down by Beer's law. 



Since Beer's law requires that the log 7\ be proportional to the thick- 

 ness d and the concentration C, for a constant incident intensity, it 

 follows that for a fixed thickness of solution the 



log 7\ ~ C 



There are two ways of approaching the design of a colorimeter which 

 uses a photoelectric cell and a galvanometer as an indicator of the trans- 

 missivity. The deflections may be observed on a logarithmic scale 

 with the inevitable crowding of the engraved divisions at large-scale 

 deflection, or else the electrical instrument may be a logarithmic device 

 with its deflections observed on an equally spaced or linear scale. 



The Barrier-Layer Photoelectric Cell 



The barrier-layer photoelectric cell is of the photoemissive type 

 requiring no vacuum-tube amplification. In the trade these types are 

 called " photronic cells." A photronic cell consists of a metal support- 

 ing disk upon which has been deposited a layer of photosensitive material. 

 Upon this is placed a special kind of metallic grid acting at once as 

 electrode and as collector for the current set up by the electrons freed 

 from the light-sensitive material. The light reaches this material 

 through the grid itself. 



Figure IV-15 shows the construction of a Weston Photronic Cell, and 

 Fig. IV-16 indicates the relative sensitivity to various wavelengths of 

 light as compared with the visibility curve of the eye. Under full sun- 

 light such a cell may deliver as much as 10 milliamperes current. The 

 response is linear, i.e., 100 ft-candles generates 100 times as much as 1 

 ft-candle when a current-indicating instrument having a resistance of 

 about 100 ohms or less is used to measure the current. 



A typical sensitivity curve of the average photronic cell indicates 



