Process Control Strip Processing
Process control strips are made on a sensitometer in batches.
They can be stored for periods up to 3months in a freezer at less than -10C without significant changes in image. However it is usual to store freshly made control strips at room temperature, say up to 18C, for two days as the greatest changes in latent image occur within this period after exposure. This ensures that any changes that are to occur due to this latent image regression do so, and then the film can be stored without the likelihood of further major loss.
Strips should be removed from the freezer a few hours before use to allow them time to warm up. Removing a cold strip from it's packing usually results in condensation on the film surface.
Strips are usually stored in small batches, which correspond roughly to a days use.
Control strips are processed in between rolls of film and therefore they receive the substantially the same chemical processing as the film.
The frequency of processing strips depends on the laboratory standing orders, but should be at least every time the process is run and every hour or so when the processor is running continuously.
Once processed the strip needs to be evaluated.
Interpretation of a Process Control Strip
From the control strip a great deal can be interpreted. Some laboratories use absolute parameters, for example speed ratings, when the actual values of the Log E scale are known, especially if they are part of the Kodak Inter-laboratory Survey. In the case of camera negative materials absolute parameters are essential but an archive laboratory does not use camera negative film [or, at least, not for camera images] and need not work with absolute Log E values. The most usual procedure is to calculate the parameter and then display the numerical value on a record sheet in the form of a running plot or clothesline plot.
It is possible, if a plotting densitometer is available or only a small number of strips need to be evaluated to make a full plot of each process strip characteristic curve.
However, in practice most laboratories operate with one of two routine methods.
- Use an automatic plotting densitometer [like the X-rite] coupled to a computer programme that calculates absolute [or relative] parameters and plots them as a computer displayed running plot for each process.
- Use a manual densitometer, in which case absolute parameters are used occasionally, say once a week, to check the overall quality, and the daily routine control strips are monitored using the Fixed Step method.
