GENERATION, CONTROL, AND MEASUREMENT 229 



at 10 cps, SO that the vibrations of the flexible mirror produce an a-c 

 signal in the photocell circuit. The a-c signal is amplified and rectified 

 for operating a d-c recorder or meter. 



When properly adjusted, the Golay detector has a linear response over 

 a wide range of intensities. The noise equivalent power is about 10^^ w 

 (Table 3-16). When the detector is operating into a recorder with a 

 full-scale deflection time of 1 sec, 0.6 X 10"^° w or 10~^ w cm~- can be 

 detected. The time constant of the Golay detector element is less than 

 a millisecond but can be varied over a wide range. The sensitivity is 

 relatively uniform from the ultraviolet through the infrared to the micro- 

 wave region. 



PHOTOELECTRIC DETECTORS 



The theoretical principles of operation and application of all types of 

 photoelectric cells have been extensively covered by Zworykin and Ram- 

 berg (1949). Three types of photoelectric detectors are in general use: 

 the photoemissive, the photoconductive, and the photovoltaic cells. The 

 photoemissive cell consists of a photosensitive cathode from which elec- 

 trons are ejected as the result of photon absorption. The ejected photo- 

 electrons are collected by an anode. In the photoconductive cell, photon 

 absorption causes the displacement of electrons from semiconductor crys- 

 tal lattices, leaving "holes" into which other electrons maj^ migrate. 

 The resistance of such a cell is a function of the irradiance. The photo- 

 voltaic cell contains a semiconductor film sandwdched between two elec- 

 trodes and generates an emf when irradiated. It requires no external 

 power supply and can operate a microammeter directly. 



The selection of the proper photocell for any research application 

 involves consideration of seven primary characteristics: (1) relative spec- 

 tral sensitivity, (2) flux sensitivity, (3) dark current and noise, (4) line- 

 arity, (5) stability and fatigue, (6) time constant, and (7) electrical 

 characteristics. The relative spectral sensitivity is intrinsically deter- 

 mined by the nature of the photosensitive surface and modified by the 

 transmission characteristics of the window, if one is present. For each 

 type of surface there is a long-wave limit or spectral threshold beyond 

 which the quanta have insufficient energy to produce the necessary elec- 

 tron emission or transition. 



For those photocells with a high sensitivity in the visible, the flux 

 sensitivity is usually expressed as the microampere or microvolt per 

 lumen for an incandescent lamp of specified color temperature, which is 

 often 2870°K. The sensitivity of infrared cells is expressed as the micro- 

 volt or microampere per microwatt of radiant flux from a specified color 

 temperature, which is usually in the range 300°-600°K. The ultraviolet 

 cells are rated in microwatts at a specified wave length or on the basis of 

 the therapeutic unit of ultraviolet flux, the E-viton. 



