PHOTOGRAPHIC MATERIALS 131 



Ordinary emulsions are used for photoengraving line and half-tone work, for 

 copying from various types of black-and-white originals, and. to a limited extent, for 

 certain types of commercial work and some outdoor subjects. 



Orthochromatic materials are widely used for general photography, particularly 

 for exposures by daylight. With materials having a high green sensitivity, the render- 

 ing of colored objects is appreciably better than with ordinary materials, though reds 

 are still rendered too dark and blues too light. Many workers, however, feel that 

 this disadvantage is offset by the convenience of being able to handle the ortho- 

 chromatic materials under red safe lights, which give considerable illumination in the 

 darkroom. 



Table III. — Resolving Power to White Light of Several Types of Emulsions 



Resolving 

 Material Power 



Eastman 50 Plates 3g 



Eastman 40 Plates 40 



Eastman 33 Plates 60 



Eastman Process Plates 80 



Eastman Spectroscopic Plates, Type 1 45 



Eastman Spectroscopic Plates, Type II 50 



Eastman Spectroscopic Plates, Type III 70 



Eastman Spectroscopic Plates, Type IV 85 



Eastman Spectroscopic Plates, Type 144 80 



Eastman Spectroscopic Plates, Type V 160 



The degree of orthochromatism, or the ratio of green to blue sensitivity^ varies 

 considerably among the many materials available. Thus the filter factors for the 

 various materials vary considerably, particularly with the heavier correction filters 

 and the contrast filters. 



Panchromatic materials are used in all cases where a reasonably true rendering 

 of the tone values of colored objects is desired. They are also very useful for exposures 

 by artificial light, which is rich in red and poor in blue light. Under such conditions 

 the red sensitivity gives the panchromatic materials a much higher effective speed 

 than either the orthochromatic or ordinary emulsions. Naturally, panchromatic 

 emulsions are necessary for natural color photography, at least for the red-filter 

 negative. , 



Panchromatic materials differ considerably among themselves in the relative 

 sensitivities to the blue, green, and red regions of the spectrum. While there are a 

 great many variations, panchromatic sensitizings can be grouped for convenience into 

 three types, for which typical spectrograms are shown in Fig. 6. 



Type A, or ordinary panchromatic, which was the only sensitizing available until 

 a few years ago, has a comparatively high sensitivity to the blue, with much lower 

 sensitivities to green and red. It is used in only a few materials at present. 



Type B, or orthopanchromatic, has a high green and blue sensitivity and only 

 slightly lower red sensitivity. It most nearly matches the color sensitivity of the e3'e, 

 though blues are still rendered slightly too light. With daylight this can be com- 

 pensated by the use of the light-yellow Wratten K2 filter, which holds back part of the 

 blue light. With artificial light the reds and blues both are rendered slightlj' too 

 light, and the correct rendering is obtained in combination with the light-green 

 Wratten XI filter. 



Type C, or hyperpanchromatic, has fairly high green sensitivity but even higher 

 red sensitivity. It is used principally when the highest possible speed is desired, 

 particularly with artificial light. Emulsions with this sensitizing render both blues 

 and reds somewhat too light and green and yellow-green too dark. However, a true 

 tone rendering can be obtained by use, with daylight, of the light-green Wratten XI 



