TESTS AND CONTROLS
Film as raw stock is an expensive product and after grading, cleaning, preparing and printing the time and effort we have invested in each roll of film is considerable. This is why it is important to ensure that we do not waste that investment by poor processing. We make sure that we produce a good result by running tests on the machine before processing work and by running tests with the work.
We must test both the physical and photographic properties of the machine and process on a regular basis. The frequency of testing will depend on the process, the stability of the process and the frequency of use of the machine.
Scratch tests
We test the physical properties of the machine by running a scratch test. We examine a length of exposed raw stock under incident light for scratches or other damage. If it is clear then we run it through the machine and re-examine it for damage. If it is clear then we can run the machine. If it has scratches then we have to find out where the scratches are occurring. We use exposed stock so that the film is black and the scratches easier to spot. The most common cause of scratches is squeegee lips because they are touching the film. Two strands of film running on the same bobbin will cause scratches as will the film touching any part of the structure of the machine. If the film jumps off a bobbin and runs over the supporting bar it will cause very severe scratches on the base side of the film.
When we are trying to trace the cause of a scratch, we must approach the problem systematically. We must start from the feed-on end and follow the film path through checking all the way. If we are still not successful then another scratch test can be run stopping the machine a number of times examining the film on the machine to find out where the scratch starts.
Sensitometric Wedges
Once we are satisfied that the machine is mechanically correct we have to run a Sensitometric wedge. The wedges are supplied by the control department with perhaps 10 or 20 in a tin. The tins should be kept in a refrigerator until they have to be opened. They should be allowed to reach room temperature before being opened to avoid condensation on the film surface that would cause marks and incorrect readings. There will be a fold or notch cut in the film to tell you where each wedge starts.