252 RADIATION BIOLOGY 



photomultiplier tube. Such a calibrated system is suitable for deter- 

 mining the relative spectral energy distribution of sources. The spectral 

 energy distribution of the 2854°K color-temperature standard is given in 



Table 3-19. 



Absorption Standards. Four relatively permanent glass standards of 

 spectral transmittance and absorbance are available from the National 

 Bureau of Standards as cobalt blue, copper green, carbon yellow, and 

 selenium orange. Each filter standard is supplied with a certificate giv- 

 ing the transmittance and absorbance at various wave lengths. Typical 

 transmittance curves are given by Gibson (1949). 



For most applications, however, it is satisfactory to use solution stand- 

 ards that can be prepared in any laboratory. The composition of three 

 such solutions and their transmittancies in 10-mM steps are given by 

 Gibson (1949) and Haupt (1952). An abridged form of these data is 

 presented in Table 3-20 for solutions of copper sulfate for the red, cobalt 

 sulfate for the green and blue, and potassium chromate for the ultraviolet. 

 These solutions may be used for the calibration of the absorbancy (den- 

 sity) transmittance scales and as a secondary means of checking wave 

 length. If the instrument closely reproduces the values given in Table 

 3-20, it can generally be assumed that the wave-length calibration is satis- 

 factory. Transmittancy values are conveniently converted to absorb- 

 ancy by the use of the data in Table 3-21. 



VOLTAGE REGULATORS 



Random voltage fluctuations in power services often make necessary 

 the use of automatic voltage or current regulators, either as separate 

 devices placed ahead of the regulated equipment or incorporated as an 

 integral part of the instrument. When they are used as separate devices, 

 it is important to consider the basic limitations of the various types avail- 

 able. Regulators for use in the United States on 60-cps 115-v service 

 are usually 'designed for a supply variation of 95-115 v. Most of the 

 regulators fall into one of the following classes: (1) electromechanical 

 motor-controlled variable transformers, (2) magnetic resonant trans- 

 formers, and (3) those employing electronic or magnetic ampUfiers. 

 The electromechanical regulators frequently have a servomotor that 

 operates the contacts on a variable transformer. The reversible motor 

 is coupled to a voltage-sensing device and amplifier. This type is usu- 

 ally employed only in high-power applications. It introduces little har- 

 monic distortion but is relatively slow as compared with the other types. 



The magnetic-resonant-transformer type of regulator contains separate 

 primary and secondary windings and capacitors for producing resonance 

 to the power frequency. These regulators are rugged and require no 

 maintenance; the response time is less than 0.1 sec; and they are avail- 

 able in a wide range of output voltages. Since they operate on the 



