LIGHT FILTERS 



313 



While these colors were easily distinguished in the original because of their difference 

 in predominant wavelength, the result in the final print is unsatisfactory because, 

 since we cannot reproduce in color, we must depend simply on shades of gray to 

 indicate proper tone values. To indicate the difference in color which we see in the 

 original, it is necessary to produce a contrast between these two colors in the black- 

 and-white print. When this is done, the accurate tone rendition of the original image, 

 as evaluated by the visibility curve of the human eye, is, of course, destroyed, but the 

 effect obtained is usuallj^ much more desirable, notwithstanding. 



To produce monochrome contrast between colors, it is necessary to photograph 

 the image so that one of the colors will produce a darker print (or lighter negative) 

 than the other. When it becomes necessary to make use of that form of distortion 

 known as increasing the color contrast, it is usually customary to reproduce the red 

 and orange colors lighter than the corresponding green and blue colors of the same 

 visibility. To accomplish this contrast in photography, we may make use of the 

 practical rule which states that to photograph a color in black and white so that the 

 print of it appears lighter than the original image, a filter should be used which is of 

 the same color as the image. Conversely, to reproduce a color in the final print 

 darker than it appears in the original, use a filter complementary to that color. The 

 filter should absorb light of the color to be rendered dark. 



The following table will give an idea of the colors of the filters which may be used 

 to render certain colors lighter or darker. For purposes of illustration Wratten 

 filters suitable for the purpose are also listed. 



Table I. — Filters for Producing Colors Lighter or Darker 



Color of subject 



Red 



Orange 



Yellow 



Yellow-green . . 



Green 



Blue-green .... 



Blue 



Purple 



Magenta; pink 



To render subject 

 lighter, use 



Color 



Red 



Orange 



Yellow 



Yellow-green 



Green 



Blue-green 



Blue 



Purple 



Magenta 



Examplei 



29, 25, 15 



15, 25 



15, 25 



11 



58, 15, 13 



47, 58 



47 



47 



29, 25 



To render subject 

 darker, use 



Color 



Blue green 



Blue 



Indigo blue 



Violet 



Red 



Red-orange 



Orange-yellow 



Green 



Green 



Example' 



47, 58 

 47 

 47 



35, 30 

 27, 25 

 29, 25 

 29, 25 

 58 

 58 



■ Numbers refer to Wratten filters. 



As examples of the use of contrast filters may be mentioned the use of a red filter 

 (Wratten No. 25 or No. 29) for photographing blue prints, which without such a filter 

 would give a gray and white result lacking in detail and contrast. The red Wratten 

 No. 25 filter also finds application in photographing furniture where it is desirable to 

 show the wood grain; the Wratten No. 15 filter is also useful in this application. 



Selective or Separation Filters. — In accordance with the theories of Wunsch, 

 Young, and Helmholtz and first demonstrated by Maxwell, three-color photograph}^ is 

 based upon the fact that a colored image may be constructed by superimposing in 

 register three partial (or separation) images, each of which is colored one of the three 

 primary colors, red, green, and blue. The partial images are made bj^ splitting the 

 color of the original image into three groups of primary colors by means of filters. 



