COLORIMETRY SPECTROPHOTOMETRY 1 1 5 



depend upon the source of radiant energy, the optical system for focus- 

 ing beams and for dispersion of the spectrum, the Hght-detecting devices, 

 and the electrical system. 



Energy Sources: The source of radiant energy must produce a con- 

 tinuous spectrum of illumination over the entire range of wavelengths 

 to be used. Ideally, the brightness of the source should be the same, 

 regardless of color. Since this ideal is impossible to achieve, the rest of 

 the spectrophotometer must compensate for differences in the output of 

 the energy source. The lamp must be bright enough to allow the isola- 

 tion of a narrow band of one color with enough energy to actuate the 

 light-detecting device even after all the reduction in intensity caused by 

 the various elements in the light path. The lamp must be operated from 

 a power supply which does not vary appreciably, so that the energy output 

 of the lamp does not change in time. Usually a tungsten filament lamp 

 is used for the visible region of the spectrum. The lamp is powered by 

 batteries or by electronically-regulated power supplies. A tungsten lamp 

 operating at a different temperature, or an electrically-heated block of 

 metal, may be used as a source of infrared radiation. A gas discharge 

 lamp, usually charged with hydrogen, is used for the ultraviolet region 

 of the spectrum. Hydrogen, when excited by an electric arc, emits a 

 fairly broad, continuous spectrum of ultraviolet radiation. The hydrogen 

 lamp must be encased in a material other than glass since ordinary glass 

 does not transmit much ultraviolet radiation. 



Monochromator: The optical system focuses a beam of radiant energy 

 from the source, disperses the spectrum of this energy, and focuses a 

 monochromatic beam through the sample to the light-detecting device. 

 A system of lenses can be used for focusing if the reduction in light in- 

 tensity by the lenses themselves is not objectionable. If the losses are 

 too great, a system of concave mirrors can be used to focus light beams. 

 The spectrum can be dispersed by a prism or by a diffraction grm:ing, 

 each of which has some advantages and some disadvantages. The prism 

 is efficient in that a high proportion of the energy entering the prism 

 is recovered in the single dispersed spectrum. Only one spectrum is 

 produced, but the dispersion is nonlinear; that is, the short wavelengths 

 are separated from each other more than the long wavelengths. The 

 prism is slighdy sensitive to temperature, but this is usually unimportant. 

 The diffraction grating produces spectra by the interference principle 

 (Chapter 8) and may operate by either transmission or reflection. Several 

 spectra appear, designated first order, second order, etc. The short wave- 

 length end of the second order spectrum may overlap the long wave- 



