PHOTOGRAPHIC LIGHT SOURCES 



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or neon lamp through a prism, it will be found that the light produced is analyzed into 

 its component parts but that the color distribution obtained is not at all continuous; 

 in fact, only a few colored lines will be observed. 



The spreading out of a complex light into a range of spectral colors is known as 

 analyzing the light, and the range of colors is known as the spectrum. When the range 

 of colors is continuous, as in the case of analyzed sunlight, the spectrum is continuous. 

 When only certain lines appear, as when gaseous-conduction light sources are analyzed, 

 the spectrum is discontinuous. The spectrum shows what colors (or light wave- 

 lengths) are present in the original light source. If the intensity of the energy at the 

 various wavelengths is measured, a spectroradiometric curve can be plotted which 

 shows in graphical form the relative or absolute intensity of the energy of which the 

 beam is composed, as a function of wavelength. Such spectroradiometric curves are 



200 



500 550 600 

 Wave length (m/i) 



Fig. 6. — Relative energy at various wavelengths for incandescent bodies at various 

 temperatures. The spectral-energy curve for daylight is shown by the dashed line, and 

 is closely approximated, over most of the visual region, by a body having a color tempera- 

 ture of 5500° K. 



of considerable importance, scientifically, in the investigation of the characteristics of 

 light sources. 



In a continuous spectrum all wavelengths within a given spectrum band are present 

 to some extent, and the relation between the wavelength and the energy of the light 

 source at that wavelength may be indicated by means of a continuous curve. Figure 

 6 shows the spectroradiometric curve for daylight at the earth's surface as well as for 

 tungsten lamps operated at various temperatures. 



In a discontinuous spectrum, energy may not be present for all wavelengths 

 within the limits of the spectrum band, lack of certain wavelength lines being made 

 apparent by a dark field. Such a spectrum is known as a "line spectrum," as com- 

 pared with the continuous spectrum of the sun or incandescent lamps, for instance. 

 In the curve of a line spectrum the presence of energy of a given wavelength is indi- 

 cated by a vertical line whose length represents the relative or absolute intensity of the 

 energy at this given wavelength. Figure 7 shows a line spectrum for a hot-cathode 

 neon lamp. 



Sometimes we may have a combination of the continuous as well as the line 

 spectra, as when light from a mercury-vapor lamp and an incandescent lamp are 

 analyzed simultaneously. Such a spectral curve unites the properties of both the 

 continuous and the discontinuous spectra. 



