PHOTOGRAPHIC SENSITOMETBY 167 



If sensitoraetry is being carried out for the control of photographic processing, it is 

 desirable that the developer and the developing conditions be as nearly as possible 

 like those under which the processed materials are being developed. Not only should 

 the same type of developer be used, but the same developing solution should be used 

 at the same time that processing is accomplished. The developer becomes exhausted 

 upon use, and its effective composition changes. Consequently the action of the 

 developer after some use maj^ be considerably different from that when the developer 

 was freshly compounded. To insure that the developed sensitometric strips will 

 afford a precise index of the processing performance which it is desired to test, it is 

 essential that the sensitometric strips be developed along with the material under 

 test in such a way that both types of material receive identical processing. 



Technique of Development. — In its broad aspects, the technique of development for 

 sensitometry is no different from the development technique of other photographic 

 materials. However, the particular problems involved in sensitometry often make it 

 desirable to pay more than ordinary attention to such factors as (1) evenness of 

 development, (2) temperature of the developer, (3) agitation of developing solutions, 

 (4) prevention of bubbles, streaks, stains, etc. 



Through the use of thermostatic control, it is easily possible to control the tempera- 

 ture of the developing solution to ±0.1°C., and through more elaborate temperature- 

 control baths still further control can be effected. Temperature variations of 0.1 °C. 

 will not cause serious errors in sensitometric work due to the temperature coefficient of 

 developers. Where thermostatic control of the developing solutions is not possible or 

 practicable, the temperature of the developing bath may be maintained reasonably 

 constant by immersing the developing tank or tray in a large tank of water which is 

 thoroughly agitated. After the developing solution is in temperature equilibrium 

 with its surrounding bath, the thermal inertia of the water surrounding the developing 

 tank will assist in preventing sudden or appreciable temperature variations of the 

 developing bath. 



In any case it is desirable to use a comparatively large volume of developing solu- 

 tion so that the reaction products may be considerably diluted, thereby influencing the 

 effective concentration of the solution a minimum amount. The sensitometric strips 

 should be agitated in the solution so that fresh solution is constantly presented to their 

 surfaces. Rocking the trays or tanks has been found beneficial and produces fairly 

 uniform results. Brushing the surface of the strip with a camel's-hair brush is another 

 very effective means of removing exhausted developer from the surface of the strips 

 but is probably not so reproducible as rocking the trays, since different workers handle 

 the brush strokes differently. 



Density Determinations. — With the test strips exposed to a series of known expo- 

 sures and developed under specified and controlled conditions, it now remains to 

 evaluate in some appropriate manner the response of the photographic material (the 

 amount of silver deposit) to the stimulus (exposure). Visual inspection of the silver 

 deposit is unsatisfactory. For quantitative work, the magnitude of the silver deposit 

 corresponding to the various exposvires on the strip is determined optically by the 

 amount of light which the photographic material transmits or reflects. In the case of 

 negatives, the transmission of the material may be used; for printing papers, the 

 amount of reflected light determines the density. 



Fundamental Definitions Used in Sensitometry. — Before we can discuss the deter- 

 mination of density properly, it is essential to establish certain definitions. To do 

 this, let /o be the luminous flux incident upon the negative. It the luminous flux trans- 

 mitted through the negative, T the transmission of the negative, its opacity, and D 

 its density. Then, by definition. 



