From each processing run, you should plot the gamma   and         on   the   chart.   You   should   write other  data,  such  as  processing  time  (or  machine speed),  temperature,  rate  of  replenishment,  and  amount of   film   processed,   on   the   control   chart.   After connecting the dots, you will know what the processor is doing and the direction in which the process is going. Remember that the existing tolerance or control limits, once set, should not be left there indefinitely. You should continually strive to improve the degree of control and, to do so, reestablish closer control limits periodically. The  amount  of  information  you  can  plot  on control charts depends on several factors that may include, but are not limited to, the following: The product quality required The equipment available Personnel available and trained for QA duties You are not expected to monitor all the variables that are listed. Also you are not limited on what you can   monitor.   These   decisions   depend   on   the capabilities of each imaging facility, and they change periodically. Figure 2-13 shows the way a control chart might look. It is an example only and should not be used to establish control parameters in your imaging  facility. Before  control  charts  are  established,  you  must have  a  standard  or  starting  point  for  each  of  the variables you intend to measure. These standards are derived by sensitometric or chemical tests over a given   period   of   time   or,   in   the   case   of   color processing, are   provided   by   the   manufacturer. Generally  speaking,  when  these  tests  are  conducted within an imaging facility, they are to be performed by a PH with a background in photographic quality control  (NEC  8126)  or  by  a  PH  with  extensive on-the-job training in QA. These specialists analyze the data collected over the test period, apply statistical formulas, and arrive at workable standards or means and upper- and lower-control limits. Therefore, in the rest of the discussion, assume that these standards have already been established. A word of caution, however, the chart, plot, and curve illustrations that follow are presented as examples only. They should not be used as a basis for the QA program in any lab. PLOTTING GAMMA OR CI ON A  PROCESS-MONITORING CHART As explained previously, gamma and CI can be computed   from   the   information   plotted   on   a characteristic  curve.  Successive  values  are  then plotted on control charts. When gamma plots on a control chart beyond the control limits, several causes may be indicated, some of which include the following: The   developer   is   being   over-   or   under- replenished. The  film  was  over-  or  underdeveloped. The  processing  temperature  was  too  high or too low. PLOTTING  HIGH  DENSITY ON A PROCESS CONTROL  CHART A density step from a processed control strip is plotted as the high density (HD) on a control chart. The specific step number is determined in tests as discussed  previously. Once  this  step  has  been determined, it should be used for each reading or plot until a new standard or mean is determined. For the purpose of our example, we are plotting or measuring step 16 as high density. The following factors can cause the high density to be out of control: Variations  in  the  processing  temperature Variations  in  the  processing  time  or  machine speed Over- or underreplenishment PLOTTING  LOW  DENSITY  ON A  PROCESS  CONTROL  CHART As with high density, low-density (LD) readings should  also  be  taken  from  a  predetermined  density step of a control strip. 2-24