Radiography in Modern Industry - Kodak

3. Age of the developer replenisher.4. Replenishment rate of the developer.5. Age of the developer.6. Temperature of the developer at the time control strips are being processed.7. Comments on fixer and wash and their replenishment rates. (Although the condition ofthe fixer and the wash does not noticeably affect the variability of film densities, thecondition of each does have an effect on the physical quality of processed film.)Process Control ChartsTwo steps on the control strip are selected for measurement. One step should have a density of0.6 to 1.0; the other, 2.0 or higher. A specific area of the step is selected for measurement, andthat same area on each of the two steps is measured to obtain a high-density value and a lowdensityvalue. From the first day on, four values are obtained--the value of the high-density stepand the value of the low density step on the fresh control strip and the values of thecorresponding steps an the control strip exposed the preceding day--always in the same area oneach step.The upper and the lower control limits for the process density aim can be assigned arbitrarily onthe basis of acceptable tolerances in the process operation (2.0 ± 0.2, for example). Sometimes itis desirable to calculate more precise control limits, however, and a statistical method, such asthe standard deviation of density values with three sigma control limits, can be used to determinethe limits. If the standard deviation with three sigma control limits is used, 95 percent of all datacollected should be within the limits. (The method of calculating the standard deviation with threesigma control limits, which is given in most books on statistical quality control, has beendescribed in relatively simple terms by Mason -- Mason, R. D.: Statistical Techniques in Businessand Economics. Monograph in Irwin Series in Quantitative Analysis for Business. Published byRichard D. Irwin, Inc., Homewood, Illinois, 1970, pp. 116-123, 314-329.)When a new box of control film is introduced into the operation, control exposures are made onboth the old and the new stock for four days and a temporary process density aim is computed onthe basis of the average densities obtained during the four-day period. After 10 days, the processdensity aim, or the mean density, is recomputed on the 10-day average.The process density aim is reestablished whenever changes are made within the operation. Suchchanges as the introduction of a new control film and alterations to the exposing unit, forexample, make it necessary to reestablish the aim.Control limits for variables of the exposing unit and the film processing are wider than if eitherwere monitored individually.The densitometric data obtained from the control strips can be utilized in several ways. The tablebelow shows densitometric readings for a 10-day period. Some of the data in Figure 78 areplotted on one process control chart (See Figure 80) to show variations in exposure andprocessing; some are plotted on another process control chart (See Figure 81) to indicatechanges in contrast.Radiography in Modern Industry 123