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December, 1925
AMERICAN CINEM ATOGR APHER
Seven
Rack and Airbell Markings on Cinema Film
Causes and Effects of Different Timely Facts in Eastman Re
Types 0/ Marks Given q ' c search Laboratory Communica
Thorough Analysis. ' ' VCS tion Presented Herewith.
TOP OF RACK
Figure 4.
KEY TO FIGURES
BOTTOM OF RACK CFicA)
WETTED
f f,6. <r )
Fig. 1:
Typical Development Rack Marks on Motion Picture Film.
Fig. 2:
Streaks Caused by the Restraining Action of the Products of Development.
Fig. 3:
Diagram Illustrating the Probable Direction of the Convection Currents in a Motion Picture Developing Tank.
Fig. 4:
Film Developing Rack With Offset Spacing Pins.
When developing motion picture film by the rack and tank system it is very difficult to secure uniform development throughout the entire length of the film. Unless special precautions are taken, more development occurs at the top and bottom of the rack where the film passes over the end slats or bars than along the sides, so that bands of greater density occur at intervals corresponding with the height of the rack, which cause an objectionable flicker when the film is projected. These dark markings are termed "rack marks."
Another difficulty arises from the cling
ing of airbells to the film as the rack is immersed in the developer. These airbells prevent the access of developer to the film locally thus causing white spots.
Both the above defects can be overcome by correct manipulation, but their presence on much of the film shown in the present day theatre indicates a need for a better knowledge of the subject on the part of many laboratory workers.
It is the purpose of this article to explain the nature and cause of rack marks and air bell markings on motion picture film and to indicate methods for their prevention.