GENERATION, CONTROL, 



AND MEASUREMENT 



145 



In the determination of spectral absorption one is usually concerned 

 either with the total absorption of a sample in which all constituents are 

 summated in the result or with the absorption of one group of constituents 

 to the exclusion of others. An example of the first type is the measure- 

 ment of the transmission of a glass color filter. The second is typical of 

 the chemical spectrophotometry of solutions in which a comparison cell 

 of solvent cancels out cell and solvent absorption. For the two classes of 

 notations, Gibson has proposed different suffixes, -ance for the summated 



i;— ^ 



^lo 



(«) (b) 



Fig. 3-4. Diagram of the quantities involved in the measurement of transmission and 

 absorption. 



system treated as a single component and -ancy for the multicomponent 

 system. 



The following assumptions are made in precise spectrophotometry: 

 (1) the sample is homogeneous and isotropic, so as not to polarize, devi- 

 ate, or scatter the beam from a rectilinear path; (2) both surfaces are 

 smooth, parallel, and normal to the beam; (3) the beam divergence is 

 sufficiently small so that there is negligible variation in path length over 

 the beam cross section; (4) the radiant energy is unpolarized; and (5) 

 the energy is monochromatic. The maximum permissible spectral band 

 width as recjuired in item 5 is determined by the sharpness of the absorp- 

 tion bands of the sample. These factors are discussed in detail by Gibson 

 (1949), Mellon (1950), and Erode (1949). 



Single-component Systems. These terms relate to the transmission of 

 monochromatic radiant energy by homogeneous, isotropic, nonmetallic 

 media. They are applied to the complete sample of a single gas, liciuid, 

 or solid or to a complex system such as a multiple-layer filter. In the 

 case of liciuids and gases, absorption due to the vessel may be deducted 

 from the over-all determination by the use of a short comparison cell, 

 as diagramed in Fig. 3-4a. 



