316 HANDBOOK OF PHOTOGRAPHY 



Sometimes sharp-cutting monochromatic filters are employed to indicate by visual 

 examination the tone reproduction which will occur in black-and-white photography. 

 For this application, the filter must be sufficiently sharp so that colors in the original 

 subject are practically indistinguishable. The transmission region may lie anywhere 

 in the visible spectrum, although there is some advantage in putting it near the yellow, 

 where the eye is most sensitive. The Wratten No. 90 viewing filter is intended for 

 this use, and while it is possible to distinguish between reds and greens, these colors 

 are so dulled as to give sensibly a yellow visual image of varying luminosity. 



Safe Lights. — Optical requirements on safe lights are practically nil except that the 

 safe-light filter should transmit as much of the visible spectrum as possible, provided 

 the photographic sensitive material is not sensitive to radiations of these wavelengths. 

 In practice, ^he manufacturer of photographic materials usually specifies the type of 

 filter and gives some indication as to the intensity of illumination which can be used 

 with the various types of film and paper emulsions. 



Filters for Commercial Work. — Where commercial work is being done, the selection 

 of filters should be that which will provide maximum utility and flexibility with a 

 minimum investment. Practically all subjects encountered in commercial photog- 

 raphy can be treated adequately with modern photosensitive materials with less than 

 a dozen filters. Those filters which have been found to be most useful are the Wratten 

 (or equivalent) Kl, K2, XI, X2, F, G, E, A, B, and C5, or Nos. 6, 8, 11, 13, 29, 15, 

 23, 25, 58, and 47, respectively. For those who do not care to obtain a full set of 

 photographic filters, the A, G, and K2 filters will probably prove most satisfactory 

 for general work. 



The Kl filter is a light-yellow filter which gives some color correction with ortho- 

 chromatic materials without increasing the exposure time unduly. It is useful where 

 some color correction must be obtained with the least increase in exposure time. 

 The K2 is a stronger yellow filter and gives better correction but requires a longer 

 exposure than the Kl. Like the Kl, the K2 filter is intended for use with orthochro- 

 matic materials. 



The XI gives correction with panchromatic materials when used with daylight 

 whereas the X2 is suitable for panchromatic materials used with incandescent luminous 

 sources. They are not recommended for orthochromatic materials. 



The F is a strong red filter for contrast work, copying blueprints, screen-plate 

 analysis, or for haze cutting or infrared photography. 



The G filter is a strong yellow-orange filter with sharp cutoff for contrast work, 

 strong accentuation of clouds, telephotography, furniture, copying yellow faded 

 matter, and general landscape photography. 



The E filter is a general contrast red filter. 



The A, B, and C5 are used in color-reproduction work. The A is a red, the B a 

 green, and the C5 a blue-violet filter. 



The spectral-transmission characteristics of these and other filters are shown in 

 "Wratten Light Filters," published by the Eastman Kodak Co.. 



Filter Characteristics Summarized. — In this chapter we have discussed some of the 

 more important characteristics of filters and have mentioned some of the uses of vari- 

 ous kinds of filters. Considerably more might easily be written concerning the uses, 

 and particularly the artistic uses, of light filters in photography. But there are many 

 articles available in the literature on this subject, and, where artistic considerations arc 

 involved, it is impossible for a technical reference book to be of much value. 



A table of filter factors for various makes of emulsions and filters will be found in 

 Appendix D. While every effort has been made to make this list as accurate as pos- 

 sible, such a listing can be taken only as a guide. The results given in Appendix D 

 are to be used with proper judgment of the factors already enumerated in this chapter. 



