COLOR PHOTOGRAPHY 621 



which must not be underestimated if one wishes to avoid disappointments and waste 

 of time and material. 



Sensitive Material. — Panchromatic films and plates sensitive to all colors are 

 ordinarily used for obtaining color-separation negatives in connection with the three 

 primary-color-separation filters — red, green, and blue. 



It is possible to deviate from this rule by using an ordinary plate or film sensitive 

 to the blue end of the spectrum only for the blue-separation negative without blue 

 filter. This type of material approaches very closely the characteristic of the standard 

 blue filter. 



For the green-separation negative it is possible to use an orthochromatic emulsion 

 which is sensitive to blue and green in connection with a yellow filter or the standard 

 green-separation filter. 



For the red the usual panchromatic emulsion and red filter is always used, as an 

 emulsion sensitized for red only to be used with either a yellow or the standard red 

 filter is not easily obtainable. 



The combination of ordinary emulsion for the blue, orthochromatic emulsion for the 

 green, and panchromatic emulsion for the red can be obtained assembled as a unit 

 under the commercial name of Tripac. Put up in this form the ordinary and ortho- 

 chromatic emulsions are placed face to face, and the panchromatic emulsion is placed 

 behind. 



Fig. S. — Photometric wedge for emulsion tests. 



The ordinary or blue-sensitive emulsion carries a yellow dye which prevents the 

 blue light from penetrating into the followmg two emulsions. The orthochromatic 

 negatives carry on their backs a layer of gelatin containing a red dye acting as filter 

 for the back emulsion. 



The tripack can be exposed in any ordinary camera provided with a suitable holder 

 for maintaining the three emulsions in good contact. It has many disadvantages, 

 however. It is slow as compared with other arrangements. The green-separation 

 negative which prints magenta is slightly diffused owing to the light scattered by the 

 front emulsion, and the red negative or cyan printer, which should contribute most 

 to the definition, is decidedly diffused. The tripack represents solely a compromise. 

 Other combinations involving the use of tripack and bipack will be described in con- 

 nection with various types of color cameras. 



Negative Develop7nent. — The development of color-separation negatives should be 

 caref ulljr standardized and a developer which can be discarded immediately after use 

 is much preferred over others which allow repeated use. Very satisfactory types of 

 developers for this purpose are the three-solution pyro developer, the two-solution 

 pyro developer, and the three-solution metol-pyro developer. Eastman formulas 

 Nos. Dl, D21, and D7 are typical. 



A standard temperature for the developer should be decided upon and maintained 

 to assure uniform results. 



In order to determine the correct time of development, a photometric wedge (Fig. 

 8) and a densitometer (Fig. 9) are necessary. 



