GENERATION, CONTROL, AND MEASUREMENT 251 



Wave Length. The primary standard of wave length to which all other 

 wave lengths and other physical-length measurements are referred is the 

 red line of cadmium, which has a value of 6438.4696 A. This value is 

 known with a precision of 1 part in 10,000,000. A more convenient 

 .standard proposed by Meggers and Westfall (1950) consists of an elec- 

 trode-less discharge lamp containing the single mercury isotope Hg^^*, 

 obtained from neutron bombardment of gold, Au^^^. This isotope pro- 

 duces the usual mercury spectrum entirely free of the isotope shifts and 

 hyperfine structure that characterize the mixture of isotopes present in 

 natural mercury. A low-pressure discharge tube containing the mercury 

 isotope is available from the National Bureau of Standards. The single 

 green line at 5460.7532 A may be isolated with filters. 



Secondary standards of wave length are obtained from line sources in 

 which the lines are sufficiently isolated for easy recognition (see Table 

 3-12). For the calibration of monochromators used for irradiation and 

 spectrophotometry, the most useful standards are the mercury, sodium, 

 cesium, and helium discharge tubes and flames to which lithium and 

 potassium salts have been added. In Table 3-12 are given the principal 

 lines of mercury, sodium, and helium. Discharge lamps for many other 

 elements are available, as discussed in Sect. 2 under Amalgam Arcs and 

 Alkali-metal Arcs. Cesium has two intense red lines at 852.1 and 894.4 

 m/x, which are especially useful since the mercury arc is deficient in the 

 red. The accuracy of the wave-length calibration of recording spectro- 

 photometers may be determined by making an absorption-spectrum curve 

 of a standard didymium glass filter. The absorption bands of the rare 

 earths are so sharp that the maxima make convenient wave-length refer- 

 ence points for recording spectrophotometers (Gibson, 1949; Mellon, 

 1950). 



Transmission and Spectral Energy Distribution. The transmission of 

 a monochromator may be determined by either of two methods: (1) by 

 passing monochromatic energy through the instrument and measuring 

 the ratios of energy passing through the entrance and exit slits, and (2) 

 by measuring the transmitted energy from a source of known spectral 

 energy distribution (Stair, 1951). With the second method the radiant 

 flux from a lamp of known color temperature and spectral energy dis- 

 tribution is passed through the monochromator, and the emergent energy 

 measured with a nonselective detector such as a thermocouple or bolome- 

 ter. It is important that no condensing optics that will introduce spectral 

 distortion be used with the color-temperature standard. A tungsten- 

 filament lamp of known color temperature, such as the 2854°K color- 

 temperature standard of the National Bureau of Standards, is well 

 adapted to these measurements in the spectral range of 320-2000 mAt. 



Stair (1951) has used the standard of color temperature to calibrate 

 the over-all spectral response of a quartz double monochromator and 



