GENERATION, CONTROL, AND MEASUREMENT 247 



brated against a Planckian radiator in the vicinity of 1300°K, using the 

 Stefan-Boltzmann law for deriving total radiant intensity. With each 

 lamp is a certificate giving three values of current which will produce 

 corresponding values of irradiance in microwatts per square centimeter 

 at 2-m distance from the front face of the lamp bulb. The current and 

 voltage values range from 0.3 to 0.4 amp and from 75 to 110 v. These 

 lamps are especially useful for calibrating thermal detectors such as 

 thermocouples, bolometers, and pneumatic detectors. In Fig. 3-40 is 

 given a simple rectifier circuit for supplying direct current to a 50-w 

 standard lamp. 



In calibrating thermal detectors, several precautions must be observed. 

 If the detector has a window of glass, quartz, or fluorite, a correction 

 factor must be applied for window absorption. These correction factors 

 are given in the data accompanying the lamp. The detector should be 

 placed in the room with the calibration equipment for a sufficiently long 

 time to ensure complete thermal equilibration. If there is thermal dis- 

 equilibrium, an abnormally high drift will be observed, and frequently, 

 when the lamp is turned off, the instrument will not come back to zero. 

 If the zero reading is negative when the lamp is off, it implies that the 

 detector is "seeing" objects colder than itself and is therefore radiating 

 heat to its surroundings. If the zero deflection is positive, it is likely 

 that the surrounding objects are at a higher temperature than the detec- 

 tor. Warm objects, such as power supplies and the body of the operator, 

 must be kept out of the range of the detector. 



For the calibration of detectors in the vicinity of 300 m/z, the quartz 

 mercury arc can be used as a standard. When operated under properly 

 controlled conditions, the high-pressure mercury arc is a reproducible 

 source that depreciates at a predictable rate (Coblentz and Stair, 1936). 

 The mercury arc is preferable to the incandescent lamp as a standard of 

 ultraviolet radiant flux because of the ease with which the ultraviolet 

 lines may be isolated and the relatively high intensity in the ultraviolet 

 as compared with the red and infrared. 



Luminous Intensity. In order to obtain sufficient luminous intensity 

 for the calibration of foot-candle or lux meters, it is necessary to use a 

 lamp of high wattage and relatively high filament temperature. Because 

 high filament temperature is incompatible with stable characteristics, a 

 compromise has been effected in the form of a 500-w gas-filled projection 

 type of lamp operating at about 300 w (Teele, 1955). These lamps have 

 a luminous intensity in the vicinity of 500 candles. 



In the calibration of lux or foot-candle meters, it is necessary to meas- 

 ure the distance from the filament to the photocell (or test plate for visual 

 instruments) and to calculate the illuminance by the formula 



E = I/d\ (3-30) 



