230 RADIATION BIOLOGY 



The power sensitivity multiplied by the sensitive area in suitable units 

 gives the irradiance sensitivity. The power sensitivity in microamperes 

 per lumen multiplied by the area in square feet gives the irradiance sensi- 

 tivity in microamperes per foot-candle. If it is multiplied by the area in 

 square meters, the units become microamperes per lux. 



Every photoelectric cell produces a small but measurable signal when 

 it is receiving no direct radiant flux. This is known as the "dark current" 

 or "dark signal." This current has a random fluctuation with time and 

 establishes the ultimate minimum value of flux which can be measured 

 with d-c instruments. The component of rapid fluctuations in the dark 

 signal becomes the noise that determines the lowest limit of measurable 

 flux when a-c methods are used. 



The linearity of the response of photoelectric cells primarily depends 

 upon the cell type. The vacuum photoemissive cell, including the photo- 

 multiplier, produces a signal that is proportional to the intensity over 

 many orders of magnitude. The gas photoelectric cell and the photo- 

 voltaic cell are reasonably linear in response only with certain circuit 

 arrangements and are usually not employed where extremely linear 

 behavior is essential. 



The sensitivity of all photoelectric cells varies with time, previous 

 exposure to radiant flux, and temperature. The slow decrease in sensi- 

 tivity during irradiation is known as "fatigue" and sets a definite limit 

 to the precision with which intensity ratios can be determined in pho- 

 tometry. The fatigue characteristics must also be taken into consider- 

 ation whenever photoelectric qells are calibrated against standard thermal 

 detectors or standard sources and later used as calibrated detectors. 



The time constant of the photoemissive cells is so small that it seldom 

 becomes a consideration of consequence. Usually the time constant of 

 the input measuring circuit is the limiting factor. The photoconductive 

 and the photovoltaic cells, however, have relatively long time constants 

 which may limit modulation frequencies to certain ranges in the audio- 

 frequency spectrum. 



The electrical characteristics of a photocell determine the optimum 

 circuitry required. The principal considerations are the nature of the 

 signal and the internal impedance. The photoemissive cell has a very 

 high impedance, whereas the voltaic cell has a relatively low impedance. 

 The electrical characteristics of the external circuit also determine the 

 linearity of response of the photovoltaic cell. 



Photoemission Cell. The photoemission cell (Fig. 3-28) is available in 

 three general forms: (1) the vacuum photocell, (2) the gas photocell, and 

 (3) the secondary-emission photomultiplier. 



The emission of electrons by photosensitive surfaces is governed by 

 two principles: (1) the number of electrons emitted per unit of time is 

 directly proportional to the incident flux (number of quanta per second), 



