314 HANDBOOK OF PHOTOGRAPHY 



Two methods of three-color photography are possible, and both are in common use: 

 the additive process and the subtractive process. 



In both processes the purpose of the taking filters is to split up the light reflected 

 bj' the colored image into the three-component primary colors. If the final color 

 print or transparency^ is to be an accurate reproduction of the original colored image, 

 any filters suitable for making the partial-image negatives must fulfill certain require- 

 ments, both as regards optical requirements and spectral characteristics. 



Optically, the three separation filters which are to be used together for color- 

 separation photography must be accurately surfaced and polished and for the highest 

 qualitj' work must be matched as a set. This requirement is essential to insure that 

 the partial negatives are all exactly the same size, so that they will permit trans- 

 parencies or prints to be made which will register accurately when superimposed upon 

 one another in the final result. If precise color-separation work is to be undertaken, 

 it is probably desirable to purchase the separation filters as a set rather than to pur- 

 chase the filters separately and individually at various times. Unless experimental 

 work is in progress, it is unwise to use tricolor separation filters together which are 

 not recommended by the manufacturer to be used as a complete set. It is also 

 desirable that all three filters in a tricolor filter set be of the same manufacture unless 

 the user is sufficiently conversant with color-separation work as to be willing to 

 accept the risk and expense of mistaken judgment. 



The spectral-transmission curves for separation filters for three-color photography 

 should have low and uniform absorption in the transmission range, complete absorp- 

 tion outside the transmission range, and cutoff characteristics as sharp and abrupt 

 as possible. The spectral-transmission characteristics of the three filters should 

 overlap only slightly or not at all; the spectral characteristics of some filter sets recom- 

 mended for color-separation work show definite gaps in which certain wavelengths 

 in the visible spectrum are not transmitted by any of the three filters. 



Bj' making the absorption low in the transmission region, the exposure factor 

 for the filters will be reduced to a minimum. Uniform absorption in the transmission 

 region assures that no discrimination will be given to any of the spectral colors passed 

 by the filter. Tricolor filters should have abrupt cutoff characteristics to assure 

 that the color of the final result will be as nearly as possible independent of the abso- 

 lute exposure, although depending upon the relative exposure through the three tri- 

 color filters. The matter can be comprehended more clearly by reference to Fig. 7. 



Assume that a set of ideal tricolor taking filters have spectral-transmission charac- 

 teristics as shown in Fig. 7A. The blue filter will then pass wavelengths between 400 

 and 500 myu without absorption but will be completely absorbing for wavelengths 

 outside this region. Similarly, let the transmission range for the green filter be from 

 500 to 600 mM and that for the blue filter be from 600 to 700 m^. Then negatives 

 made through the use of these ideal taking filters will show the three primary colors 

 sharply differentiated, each primary color being passed by its corresponding taking 

 filter, and only by this filter. 



If instead of the theoreticallj^ ideal taking filters of Fig. 7A, we have a set of tri- 

 color filters as shown in Fig. 75, the situation is quite different. In the latter case 

 light from the colored image is not split into three well-defined primary colors. Because 

 of the gradual cutoff characteristics, anj' one filter may transmit more than one 

 primary color. As was true also in the case of Fig. lA, the relative exposures through 

 these three filters must be proportional to the relative exposure factors, of the three 

 filters. But because anj^ filter of Fig. IB transmits primary colors outside its own 

 primary color region, a proper color balance in the final result will be obtained only 

 for one definite exposure; for other exposures of the partial negatives, the fidelity of 

 color reproduction in the final result will be inaccurate even though the proper relative 



