244 RADIATION BIOLOGY 



merits of this type are capable of attaining almost theoretical sensitivi- 

 ties, as determined by Johnson-noise hmits. Vibrator types of contact 

 modulators with inverse feedback can be designed with input imped- 

 ances high enough for use with photoelectric cells. Their ultimate sensi- 

 tivity, however, cannot approach that of the vibrating-capacitor type of 

 electrometer. 



Kalmus and Striker (1948) have described a carrier-current system 

 based upon the modulation of the photoelectric current in photoemission 

 cells with an a-c magnetic field. This system offers many of the advan- 

 tages inherent in the radiant-flux-chopping systems. 



Data on the performance of several types of electrical measur- 

 ing instruments are compiled and compared in Table 3-18. For the 

 electronic instruments the period given is that for the output meter or 

 recorder. The sensitivities are based upon noise limitations for the 

 Liston-Becker amphfier and 0.5 per cent of full scale for the others. 



STANDARDS OF RADIANT ENERGY 



The calibration of sources and radiation instruments is based upon 

 secondary standards of radiant flux, wave length, absorption, and reflec- 

 tion. The standards of radiant flux are incandescent lamps cahbrated in 

 (1) absolute units of radiant intensity, (2) color temperature, and (3) 

 luminous intensity. Wave-length standards are gaseous discharge lamps 

 containing elements with distinctive and easily recognized Hue spectra, 

 and standard glass filters of the rare-earth elements whose absorption 

 bands are intense and narrow. The absorption standards are special 

 colored glasses and inorganic solutions whose transmissions have been 

 precisely determined by the U.S. National Bureau of Standards. The 

 reflection ""standards usually consist of magnesium oxide deposited on 

 metal surfaces, or soUd blocks of pure magnesium carbonate. 



STANDARD LAMPS 



The primary standard of radiant flux, luminous flux, and color temper- 

 ature is the complete, or Planckian, radiator, whose radiation character- 

 istics are a unique function of temperature. The Planckian standard 

 (black-body standard) is usually a furnace containing a ceramic tube, 

 the temperature of which is controlled by a pool of freezing noble metal 

 (Forsythe, 1937; de Groot, 1948). The furnace is so completely insu- 

 lated that the whole interior is maintained precisely at the temperature 

 of the freezing metal. The melting points of the noble metals give four 

 reference temperatures: 1336°K (gold), 1825°K (palladium), 2042°K 

 (platinum), and 2716°K (iridium). Tungsten, with a melting point of 

 3653°K, would make an ideal standard, but ceramic materials capable of 

 withstanding this temperature are not available (Stimson, 1949). The 

 primary standards are maintained by only a few of the leading stand- 



