PHOTOGRAPHIC MATERIALS 



141 



Others, with higher bromide content, may be used for either contact or projection 

 printing while the fastest ones are suitable chiefly for projection. 



In general, for any particular type of paper emulsion, the more contrasty grades 

 are slower than the softer grades. 



In connection with speed, the spectral sensitivity is important. The chloride 

 emulsions and the very slow chlorobromide emulsions are sensitive to the violet and 

 near ultraviolet but have very little sensitivity above 450 m^i. Thus they may be 

 handled safely under a bright-yellow safe light with a cut between 500 and 550 mM. 

 The faster chlorobromide emulsions have additional sensitivity in the blue, extending 



Chloride 

 papers 



Series 00 

 safeligh+ fil+er 



Chloro- bromi'de 

 Dopers 



Bromi'de 

 papers 



Series 

 Safeligh+ flfl+er 



Series OA 

 Safeligh+fi'l+er 



300 400 500 600 700 



Fig. 13. — Spectrograms for various paper emulsions, compared with the spectral trans- 

 missions of suitable safe-light filters. 



up to about 500 m^u, and the bromide emulsions are sensitive still further, up to about 

 520 m^i. Thus for safe handling these emulsions require safe lights which pass no 

 light at wavelengths shorter than about 550 m^u (Fig. 13). 



In all cases the maximum sensitivity is in the violet and near ultraviolet; conse- 

 quently the tungsten light which is nearly always used for printing is really very 

 inefficient, as only a very small percentage of the radiated energy is effective in print- 

 ing. Mercury-vapor lamps and other sources of ultraviolet radiation are much more 

 efficient so far as photographic action is concerned, but because of cost, inconvenience, 

 or insufficient intensity, they are seldom used. 



The so-called "high efficiency" tungsten lamps are sufficiently effective, and, since 

 they are convenient to handle and are comparatively inexpensive, they are generally 

 used for printing. However, it should be remembered that the proportion of light 



