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HANDBOOK OF PHOTOGRAPHY 



converges rapidly and the third term will seldom be required. Values of B2 for 

 various types of incandescent lamps are given in Table I. 



Table I. — Values op B for Various Incandescent Lamps 



Type of lamp 



Vacuum lamip. . 

 Gas-filled lamps 

 Gas-filled lamps 

 Gas-filled lamps 



Power rating, 

 watts 



15- 60 



40- 60 



60-150 



200-250 



Value of 



3.513 

 3.685 

 3.613 

 3.384 



1 . 5805 

 1.623 

 1.523 

 1.543 



. 5805 

 . 523 

 0.523 

 0.543 



13.5 

 13.5 

 13.5 

 13.1 



To illustrate the use of this equation, let us find the maximum permissible voltage 

 variation d for a variation of 1 per cent of the light output, i.e., b = 0.01, for a 100-watt 

 lamp. For lamps between 60 and 150 watts, B2 = 3.613, so that, substituting values 

 for b and B2 into Eq. (7), we obtain, for an increase in voltage, 



+d = 0.0027678 + 0.00001001 = 0.002778 = 0.2778 per cent 



Thus, in order to maintain the light output constant to within 1 per cent, the voltage 

 applied to the lamp must be maintained constant to within about ^^4 per cent. 



The plus signs in the above equations are to be used when d and b increase; the 

 minus signs are used when they decrease. We therefore get two answers for a given 

 voltage variation (depending upon whether the variation is an increase or a decrease). 

 The above example was considered to be an increase in both b and d. If it had been 

 considered a decrease, the result would have been —0.2758 per cent which, for prac- 

 tical purposes is the same thing. 



The best types of commercial voltmeters are accurate to within 0.25 per cent; 

 laboratory standards are available which are accurate to 0.1 per cent. Consequently, 

 for precise sensitometric work the most accurate voltmeters are required to maintain 

 the voltage constant, or recourse must be made to potentiometric methods of control. 



Filters for Incandescent Sources of Luminous Intensity. — Since none of the light 

 sources which are suitable for photographic sensitometry — from the standpoints of 

 convenience, reproducibility and stability of characteristics, or adequate intensity — 

 have spectral-energy-distribution curves approximating that of mean noon sunlight, 

 it is necessary to select a source of light on other grounds and then to modify its spec- 

 tral distribution through the medium of absorbing filters. 



An acetylene flame, used in conjunction with a Wratten No. 79 filter has been used 

 for some time as a suitable source of white light. Although the spectral distribution 

 of this combination departs appreciably from that of mean noon sunlight, this depar- 

 ture is frequently of little practical importance. In fact Jones states that "speed 

 values determined from this source-filter combination using materials differing widely 

 in spectral sensitivity agree very well with speed values determined by using actual 

 sunlight." 



A liquid filter has been developed by Davis and Gibson ^ which, when used with an 

 acetylene flame or incandescent electric lamp operated at 2360°K., produces a close 

 approximation to mean noon sunlight. The filters are stable and easily reproducible. 

 For use with an incandescent light source operating at 2360°K., the filter is made up of ■ 

 two solutions as follows: 



I Davis, R., and K. Gibson, Trans. Soc. Motion Picture Engrs., 12 (May, 1928). 



