GENERATION, CONTROL, AND MEASUREMENT 



191 



resistant to all organic solvents can be constructed without the use of 

 gaskets. 



Often it is necessary to diffuse a beam so as to get uniform flux dis- 

 tribution over a limited area. This can be accomplished with a variety 

 of diffusing materials such as tracing paper or sandblasted, ground, or 

 etched surfaces of quartz or glass or by the use of pattern-molded window 

 glass. Paper and ground glass produce much more general scattering 

 and more reflection loss than the molded glasses. Where extreme dif- 

 fusion of the beam is not required, the molded window glasses, with 

 patterned surfaces of closely spaced small lenses or tetrahedra, are very 

 effective and introduce relatively little energy loss in the visible region. 



REFLECTING MATERIALS 



The spectral reflectances of certain of the common metals and white 

 pigments are plotted in Fig. 3-17. Silver and amalgamated mercury 



ANASTASEs 



250 300 350 400 450 500 550 600 650 700 750 800 850 900 



WAVE LENGTH, m/A 



Fig. 3-17. Spectral reflection of white metals and paint pigments. 



were the earliest mirror metals, but evaporated aluminum, chromium, 

 and other metals have taken their place for critical applications. Silver 

 has the highest reflectance in the visible and is still used for second-surface 

 or back-silvered mirrors, but it tarnishes too rapidly for first-surface 

 mirrors and has a narrow region of low reflection at about 310 m/x. 



The front, or first-surface, mirror has the reflecting material evapo- 

 rated on the front of the glass and has replaced the more expensive 

 total-reflecting prism for many applications. Aluminum has excellent 

 reflecting properties (Taylor and Edwards, 1931) but is soft and easily 



