PHOTOGRAPHIC SENSITOMETRY 163 



Type II: Time-scale instruments 

 / constant, t variable 



A. Exposure intermittent 



1. Continuously varying exposure 



2. Stepped exposure 



B. Exposure nonintermittent 



1. Continuously varying exposure 



2. Stepped exposure 



Intensity-scale Exposure Devices. — Probably the first atteinpts at photographic 

 sensitometry were made with plates or tablets having numerous areas of varying 

 transmission (or density) which were placed in contact with the photographic material 

 under test and then exposed to a suitable light source for a known length of time. The 

 areas of varying density were numbered, and the faintest number which could be 

 read, after exposure and development, indicated the speed or sensitivity of the mate- 

 rial. A later modification of this general principle made use of tablets whose density 

 varied continuously, rather than in steps. 



Intensity-scale sensitometers have also been introduced which consist of a series 

 of tubes or cells of equal length, at one end of which is placed the photographic plate, 

 the other ends of the various tubes being closed by opaque plates containing apertures 

 of variable areas. If these apertures and tubes are properly illuminated, the exposure 

 of the film will be directly proportional to the area of the aperture. 



A method of obtaining directly the H and D curve of a photographic material using 

 a square neutral-gray wedge behind which the photographic material under test is 

 exposed was suggested in 1910 by R. Luther. The resultant negative is preferably 

 developed to a high contrast. After being rotated through 90° with respect to its 

 original position, the negative is placed in register with the wedge through which the 

 exposure was made, so that the lines of equal density on the negative are perpendicular 

 to the lines of equal density on the tablet wedge of continuously graded density. By 

 direct observation of this wedge-negative combination, the density-log E characteristic 

 may be seen. By making a print through the wedge and the negative, preferably on 

 high-contrast paper, a permanent record may be obtained. This method of determin- 

 ing the D-logio^ characteristics has advantages where a rapid means of testing is 

 desired. It is not likely, however, that sufficient precision can be obtained for stand- 

 ardized sensitometric work. 



It is generally agreed that an intensity-scale instrument represents the ideal form 

 of sensitometer since photographic materials are almost always exposed under variable 

 intensity conditions in practice. However, no really satisfactory sensitometer of the 

 intensity-scale type giving the required reproducibility, intensity, and precision has 

 been developed for commercial use. 



None of the intensity-scale instruments is completely satisfactory for sensitometric 

 work, primarily because such devices are inherently complicated and require the high- 

 est degree of mechanical and optical workmanship to obtain the necessary precision. 

 The wedges (either stepped or of continuously varying density) which are required are 

 difficult to produce accurately with nonselective absorption and proper density varia- 

 tion. They must be stable, reproducible, and permanent in their characteristics. 

 Since thej^ diffuse the light passing through them, the results obtained through their 

 use will correspond only to the conditions under which the graduated density tablets 

 were used. With the tube type of sensitometers, it is required that the illumination 

 level be as uniform as possible; moreover the intensity range is limited. 



Intensity-scale devices may be produced by: 



1. Varying the distance between the light source and the sensitive material for the 

 various exposures. 



