116 



COLORIMETRY-SPECTROPHOTOMETRY 



length end of the first order spectrum, necessitating special precautions 

 to eliminate stray light of undesirable colors. The grating, however, is 

 advantageous in producing linear dispersion, which simplifies the me- 

 chanical system used to control the wavelength setting of the instrument. 

 Some commercial instruments use prisms; others employ gratings; the 

 Gary Model 14 has one of each. 



A set of slits is an integral part of the optical system. A first slit passes 

 a beam of light from the lamp to the prism or grating and blocks out 

 light moving in other directions. A second slit isolates a band of light 

 from the dispersed spectrum. These slits may be adjustable in width. 



If the instrument is to be used only in the visible region, glass optical 

 parts are satisfactory. If the instrument is to be useful in the ultraviolet, 

 any elements (lenses, prism) which transmit energy are made of quartz. 



Radiation Detectors: The devices used to detect radiant energy are of 

 several types. Any of these responds more strongly in some regions of 

 the spectrum than in others. Commercial instruments commonly em- 

 ploy two or more detectors of different spectral sensitivity so that the 

 entire spectral range can be covered. The light-sensing device must be 

 able to produce an electrical signal adequate to drive the electrical meas- 

 uring system. Relatively insensitive detecting and measuring systems 

 are easy to construct and use and are adequate for some purposes. The 

 precision of measurement is limited, since the system will not be able to 

 measure small amounts of energy or small differences in energy. A more 

 sensitive, more precise combination may be desirable, but is usually ac- 

 companied by difficulties in operation, noise, delicacy of control, and more 

 elaborate, expensive circuitry. 



0) 



c 

 LlI 



Wavelength 



Fig. 9-3. Effect of slit width and of spectral band width upon energy 

 passing through exit sHt. 



