The General Aims Of Quality Control
General Principles
Quality Control is usually twofold. Firstly to ensure that the product is of the highest quality and secondly to ensure that the quality is maintained consistently. Consistency is paramount in that procedures such as grading can only be satisfactory if the grader can be sure that a particular set of printing conditions will give identical results from day to day and year to year. Similarly a print made today should be able to be made with the same grading in the future and be visually the same.
Clay and Walley in "Performance and Profitability" provide the following definition of quality control:
"Quality control is a system for measuring and recording the variables that affect quality in a methodical manner, so that the values and trends can be compared with standards and thus act as a means of control."
Quality control, then, is a system that provides information to management - the actual control, however, is exercised by management and not by the system. The aim of the system should be to make available the right information at the right time so that any decisions taken may be more meaningful and certain. In effect the quality control procedures can be seen as occurring in three separate stages. The first, often called Monitoring, is measuring relevant useful information about the product or the process, Decision is the process of estimating the action, and Control is procedure of making the adjustment to the process.
Most manufacturing industries rely on 'statistical quality control'. Fault probability is linked to the number of component stages in manufacture and the reliability of each of those stages. If, for example, four stages (or components) are involved and each of them is only 80% reliable, then the reliability of the final product is only 41% reliable (0.8 to the power 4). Clearly, the reliability required is governed by the nature of the produce and its price. It is as futile to establish too high a standard, as it is undesirable to establish too low a standard. In practice the method used to monitor the production will be either by sampling or by 100% inspection. Where the latter is practised (as often it must be) by human skills, the effective inspection is likely to be less than 100%. Tests have shown that different inspectors will reject different items, and the same inspector will reject different items at different times of the day!
In the reality of film laboratory life, there is a sharp distinction made between what is called quality control, that is the procedures needed to monitor and thereby control the individual machines, chemical processes and photographic stages, and quality checking, which is carried out at the end to visually appraise the total success of the entire sequence of restoration. All monitoring is made more objective if the characteristics of the process can be converted into numbers, and these parameters, are widely used in quality control, but almost never in quality checking, mainly because of the subjective nature of vision. As a result of the quality control techniques in use, it is possible to plot graphs, which indicate the stability, drift or random fluctuation of any manufacturing process. Such graphs normally incorporate various limits for managerial guidance - these are usually called 'action limits' and 'control limits'. Sometimes a special lower limit is known as a 'warning limit'. Quality control should not be considered in isolation and the temptation to see it, as an end in itself must be resisted. Good quality control requires effective liaison between all other departments and control technicians must never lose sight of the fact that a laboratory exists to produce film, not to provide work for a Control Department.
Motion picture film processing consists of mechanical, photographic, and chemical stages resulting in an image that has to be evaluated. The chemical stages can be controlled by means of analysis, the photographic by means of densitometry and sensitometry. However the final result is a picture image or sound that has to be looked at or listened to, and that fact should never be forgotten.
