GENERATION, CONTROL, AND MEASUREMENT 193 



silver-plated and polished surface smoked five times with burning mag- 

 nesium ribbon has a reflectance of over 97 per cent throughout the visible 

 and is still above 94 per cent at 250 m/z (Middleton and Sanders, 1951). 

 Aging and ultraviolet irradiation cause the surface gradually to lose its 

 ultraviolet reflectance and to become slightly yellow. Magnesium car- 

 bonate has nearly as high a reflectance as the oxide and is more stable 

 but must be deposited in a paint vehicle or used as solid blocks for a 

 white standard. Middleton and Sanders (1953) have described a barium 

 sulfate paint using methyl cellulose as a vehicle that is nearly as effective 

 as magnesium oxide for spheres and much more durable. 



BLACK COATINGS 



There are three classes of "black," or highly absorbing, coatings: 

 (1) those depending upon absorbing pigments such as carbon and the 

 black metallic oxides; (2) evaporated or sputtered metallic blacks; and 

 (3) black organic dyes. Carbon is unusual as an absorbing material in 

 that it has very high and uniform absorption throughout the ultraviolet, 

 visible, and near and middle infrared (Coblentz, 1912). The oxides of 

 iron and copper are not so uniformly black throughout the spectrum. 

 The base metals used in the fabrication of instruments, such as iron and 

 steel, and copper, brass, and bronze can be blackened with a tenacious 

 black film of oxides by the use of suitable alkaline oxidizing solutions. 



Small objects, as the targets of thermocouples and bolometer strips, 

 are blackened by the evaporation or sputtering of various metals and 

 certain sulfides in a vacuum (Strong, 1943). Extremely thin films can 

 be produced which absorb 95 per cent or more of the incident energy 

 throughout the ultraviolet, visible, and infrared. The high absorption 

 is due to extremely fine needle-shaped crystals so closely spaced that the 

 radiant energy is trapped by multiple internal reflections between adja- 

 cent crystals. Platinum black is produced when platinum is deposited 

 electrolytically in the presence of a trace of lead (Britton, 1951). 



By the proper choice of visible and ultraviolet absorbing dyes or stains, 

 fabrics and dyed films of gelatin and other plastics can be made com- 

 pletely absorbing in the visible and ultraviolet. However, organic dyes 

 transmit freely in the near infrared; some so-called "black" dyes begin 

 to transmit at 700 m^t. Such materials cannot be considered as opacjue 

 for any investigation involving photoprocesses whose action spectra extend 

 into the near infrared. 



SPECTRAL CONTROL OF RADI.\NT FLUX 



Modification of the spectral energy distribution of the source may 

 involve the use of a simple water cell to remove the infrared or, at the 

 other extreme, a double monochromator to obtain the ultimate in spectral 

 resolution and purity. Regardless of the method used, a compromise is 



