APPENDIX 

 The Dimensional Stability of Photographic Films 



Abstract 



The dimensional stability of several photographic film bases to 

 relative humidity, temperature, processing, handling, and storage 

 is discussed. Permanent and temporary size changes are outlined. 

 Recommendations for the choice, processing, and storage of film 

 intended for photograxnmetric use are made. 



Sources of errors in stereo-photogrammetry 



In addition to optical factors, platform stability, camera tilt, etc. , 

 the inherent dimensional instability of flexible photographic film bases 

 may contribute to error in naeasurements made from aerial stereo photo- 

 graphs. In order to minimize error due to the last cause, care must be 

 taken in the choice of film, storage before and after exposure, processing, 

 and handling. 



Dimensional changes in photographic films may be classified under 

 two headings: temporary and permanent. There are two factors involved 

 in temporary changes: temperature and relative huinidity. 



Temperature effects 



The thermal coefficient of expansion of most common film bases (1944) 

 is approximately 5 xlO"^ inches per inch per degree F, or about 0.05 per- 

 cent per 10°F. Table I shows the effect of temperature, as well as relative 

 humidity and processing, on several film bases (Fordyce, Calhoun, and 

 Moyer, 1955). The expansion is generally 10 to 40 percent greater in the 

 widthwise than in the lengthwise direction. This is the result of the partial 

 orientation of the molecules in the base in the machine direction. It is evi- 

 dently easier, under these conditions, to increase the distance between them, 

 either by thermal agitation, or by the introduction of moisture, in a direction 

 perpendicular to this alignment. 



Humidity effects 



The humidity coefficient of linear expansion of common films varies 

 from a low of 1.0 x 10"^ for DuPont "Cronar" to about 10 x 10"^ inches 

 per inch per 1 percent relative humidity change, for standard cellulose 

 acetate. This effect is essentially linear between 20 and 70 percent relative 

 humidity, and somewhat greater below 20 percent and above 70 percent. 

 Photographic films exchange moisure with the air continually. The mois- 

 ture content of a film is determined almost solely by the relative humidity 

 of the air with which it is in equilibrium. 



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