VISIBLE AND NEAR-VISIBLE RADIATION 149 



lamp. Such a standard lamp must be calibrated in terms of a black 

 body. 



MONOCHROMATIC MEASUREMENTS 



Three general types of problems present themselves for monochro- 

 matic analysis: 



A. The determination of the intensity distribution from sources as 

 a function of wave-length. 



B. The determination of the absorption characteristics of transparent 

 or partially transparent materials. 



C. The provision for monochromatic irradiation of materials. 

 Unquestionably the preferred method of attacking these problems 



involves the use of a monochromator, together with suitable detectors, 

 either black-body type or selective. In selecting a monochromator for 

 a given undertaking, one is concerned with: (a) Its dispersion and 

 resolution, determining the wave-length range isolated, (b) Its numer- 

 ical aperture, which determines the solid angle of radiation which the 

 instrument is able to transmit, (c) The percentage transmission of the 

 optical parts, (d) The extraneous or scattered light transmitted by 

 the instrument, (e) The slit characteristics. 



For our purpose, we may regard a monochromator (see Fig. 6(a)) simply 

 as an optical system which takes the radiation in a given solid angle 

 through a narrow opening (slit) Si and forms an image of that narrow 

 opening or slit upon a second slit S2 which permits the escape from the 

 instrument of a narrow wave-length range of known wave-length maxi- 

 mum and extent. Such instruments are commercially available in 

 numerical apertures varying from/12 to/4 and rarely to/2, the numerical 

 aperture signifying the ratio of the focal length of the collimator (first 

 lens) Li and camera (second lens) L2, assuming as is customary that the 

 two have the same focal length, to the diameter of the limiting aperture 

 (prism or lens size) P. Thus a spectrograph of numerical aperture /4 

 utilizes radiation from a given point on the slit having a solid angle of 

 Ke of a steradian. Since most sources emit radiation uniformly over 

 a fairly large solid angle, the light-gathering power of the instrument is 

 directly proportional to the sohd angle subtended by the limiting aper- 

 ture, or inversely proportional to the square of the numerical aperture. 

 Thus, an instrument of /4 would have a light gathering power nine times 

 as great as one of /12. 



Two characteristics determine the overall transmission of the optical 

 system: (a) The number of surfaces (since each normal surface reflects 

 approximately 4 to 8 per cent), and the angle of incidence on the sur- 

 faces (since the reflecting loss increases greatly with a greater angle of 

 incidence). (6) The absorption characteristics of the refracting materials 



