Photographic Quality Control
The Basic Principles
The most effective monitoring of the production procedure in photography is by sampling (in effect what the control strip does of a photographic process). 100% inspection methods as far as individual printed or processed frames are concerned is almost always impossible and often only relevant later in the sequence. Sample information on stages in the procedure can be plotted graphically so that management can be aware of any changes that occur, can avoid operating outside the control limits, and can instigate remedial action when necessary.
Monitoring of the various production stages in photography is always a sampling procedure, and is always devised to provide control decisions. At the end of the final stage of restoration there must come a quality check on the end product and in the case of fairly routine restoration where the image is being preserved because the film base is breaking down but the subject is not of major importance, then it is inevitable that only samples of the product are viewed, or the projection is visual and relatively cursory. 100% checking of the final end product becomes more relevant the more costly or significant, in cinema or historical terms, is the subject matter.
A Short History of Quality Control in Cinematography
In the 1960's the Eastman Kodak Co. in the USA was having problems with the quality it's photo finisher laboratory customers were generating when producing paper prints for the amateur market, and a special Quality Control Manual stated that "the quality standard should be such that the negatives, transparencies and prints produced will create good will, repeat orders, and the respect of customers. It should be flexible enough to allow for improvement whenever possible, but inflexible to the extent that no compromise with poor quality will be tolerated" This policy is one to which any motion picture laboratory aspires today and also one which can be appropriate inside an archive since there can be internal customers just as easily as external ones.
Shortly after Eastman Kodak introduced the Eastman Color motion picture films in the early 1950's the company started a unique control service to all the laboratories in the world that were to process Eastman Color and were their direct customers. The service, known today as the Eastman Color Inter-laboratory Survey, provides standard control strips to the laboratory, which it processes, keeps one as it's own reference and returns one to the local Kodak organisation running the scheme. The strip is read and analysed by Kodak and data returned to the laboratory showing how far the customer's process is from the Kodak standard [and giving the process an A, B or C rating]. This is the only "absolute" control standard provided by a manufacturer in this industry. The customers have an opportunity to check their own aim values from this scheme every few months. This system monitors the process only.
Certain pre-requisites are necessary for any control system to function satisfactorily.
- The personnel responsible for quality control decisions must have a good knowledge of the photographic process to interpret their findings and the resolution and seniority to see that these are acted upon.
- The production system itself must be set up correctly before the control system can operate efficiently.
- The materials used for control purposes must be beyond reproach.
- It must be understood that when a control reading is out of tolerance then the customer's or internal customer's work is similarly affected. If the work appears unaffected then two possibilities arise: a] that two of control factors are proving to be self-cancelling - this is an unstable and dangerous position to be in; or b] that the reference or aim point is wrong.
Quality control in the printing and processing of any photographic materials requires similar objectives to those used in the original manufacture of the products. Among these are achievement of correct speed, colour balance, contrast, freedom from physical defects etc., and provided that there is an appreciation of what the objectives are, the characteristics of a "correct" result defined, then quality control can become just another routine.
Remember what was said in the beginning of this section: Motion picture film
processing consists of mechanical, photographic, and chemical stages resulting
in an image that has to be evaluated.
The photographic control is done by means of densitometry and sensitometry.
Every numerical parameter must be related to the visual image and there is no
value in measuring film parameters that do not. To put this the other way round
it is essential to find numerical values that correspond to visual
characteristics of images so that these may be used to monitor the quality and
the consistency.
In a number of applications the image being handled could be a negative,
intermediate positive or a sound track, none of which have a relevant visual
character at this stage, but eventually the final visual [or audible] character
will become relevant further down the generations and the monitored parameters
of these intermediate materials have to relate to these final images or sounds.
A few of these variables at different photographic stages are:
Processing: The variables of machine speed,
temperatures, formulae, replenishment rates and formulae of replenishers,
contaminants, water supply, all effect speed, density, contrast, and colour of
the result.
Grading: The variables of viewing light sources,
screen and room conditions, analyser set, calibration and correlation, graders
vision.
Printing: The variables of lamp and lamp voltage
and current for picture and sound lamps, trim settings, filtration, and speed
of printing, modifications from flare, halation and reflections, alter density,
balance, colour balance, and contrast.
Film Stock: Different batches of film stock vary
and need testing to ensure consistency and acceptability and to enable settings
to be worked out for printing machines.
How to Control
Film measurements
& Sensitometry