COLOR PHOTOGRAPHY 629 



second mirror, also lightly coated, faces a very fast orthochromatic emulsion with a 

 yellow filter, and the major amount of the light in beam 3 faces a fast panchromatic 

 emulsion with the standard red filter. Other alternative variations are also possible, 

 but all these special arrangements seem to be hardly justified in view of extremely fast 

 panchromatic emulsions available today. ^ 



Whatever the type or combination of negative material used, a careful study of its 

 development characteristics must be made or obtained from the manufacturer in order 

 to standardize the various processing operations. 



Color-printing Methods for Prints on Paper. — Of the many color-printing processes 

 developed, only those of practical value and in actual use today will be considered. 

 Their description follows in the order of their practical importance. These processes 

 are 



Carbro 



Imbibition — Eastman wash-off relief and Pinatype 



Dyebro (Combination of Carbro and imbibition) 



Chemical toning- — Chromatone 



Gelatin relief with color pigment in suspension — Duxochrome 



Dye toning by the mordant process. 



The Carbro Process. Outline. — Bromide prints by contact or enlargement are 

 made from each color-separation negative. Carbon tissues containing soluble gelatin 

 with colored pigments in suspension are sensitized in suitable solutions and then 

 squeegeed onto the appropriate bromide. During the time of contact, the sensitizing 

 chemicals of the tissues react with the silver image of the bromide, and a partial 

 insolubilization of the gelatin of the tissue takes place in a manner proportional to the 

 quantity of the silver of the image of the bromide. After several minutes of contact, 

 the tissues are stripped from the bromides and transferred onto temporary supports. 

 After another brief lapse of time, the temporary supports with the adhering tissues are 

 placed in hot water in which the gelatin of the tissue, in still soluble condition, washes 

 away, leaving a colored image in relief adhering to the temporary support. The three 

 color images in relief are then finally transferred in register onto a final support. 



Measurement of the Negatives. — After the three color-separation negatives have 

 been exposed and developed, they must be measured to determine the correct printing 

 ratio. This is done by measurement of the densities of the various steps of the neutral- 

 gray wedge of each negative. The results of measurement should be entered in a 

 notebook in tabulated form somewhat as in Table II. 



The density of all the steps of the gray wedge need to be measured only when 

 checking the correctness of development. For the purpose of determining the printing 

 ratio it is only necessary to measure the densest step of the wedge in each negative.'* 

 Another important measurement, particularly in portraits, is the density of the fore- 

 head (F.H.). This measurement is used to compute the basic exposure for the blue 

 bromide. In addition an empirical factor based on the measurement of the forehead 

 density in the three negatives can be readily deducted, and this information will be 

 found useful at other times when for some reasons the gray wedge is omitted. The 

 measurements of the forehead are of value only in case of female studies. Great 



1 The effective speed to daylight of a panchromatic film in a double-mirror camera might range from 

 Weston 3 to about Weston 12. This last value can be realized only with extremely fast emulsions such 

 as Eastman Super-Panchro-Press with No. 25 filter for the red negative and Eastman Super-Ortho- 

 Press with the K3 and No. 47 filters for the green and blue negative respectively. 



- Theoretically this would be correct only for properly exposed negatives. Departure from true 



color balance, however, is mostly noticeable in the light tones of a finished print, and, therefore, it is 



. desirable to adjust the color balance with respect to the densest step of the wedge. In case of portraits 



it is best to compute the printing ratio from the step of the \fedge which most nearly matches the density 



of the forehead of the red filter negative. 



