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Figure  2-16.Correction  factors  supplied  with  control  strips
Color Process Control Charts

Photography (Advanced) - Advanced manual for photography and other graphic techniques
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5.  Subtract  the  initial  reference  values  from  the averaged,  processed  control-strip  density  values. When a control parameter of high density minus low density is specified for the process, determine that also. The results of these subtractions are known as the   aim  values.   The   example   in   figure   2-17 summarizes steps 1 through 5 to show you how the aim- or center-line values are derived for an LD step. In the example below, LD red plots four lines below the center line, LD green plots one line above the center line, and LD blue plots one line below the center line. 6.  Adjustment  tolerances  may  be  applied  to  the aim values, as appropriate, to bring the control-strip values closer to the aim values. These adjustment tolerances  are  specified  in  each  process-monitoring manual. For example, the red low-density (LD) value for your processed control strip is 0.28. The adjusted reference  value  is  0.32.  The  process-monitoring manual  indicates  that  the  aim-value  adjustment tolerance  for  LD  is  ±.04.  You  can  adjust  the  red center-line value by subtracting 0.04 from the adjusted reference  values  (0.32  –  0.04  =  0.28).  The  adjusted aim value of 0.28 is your new center line or aim value on your control chart. NOTE: The adjustment tolerances change for each step and each type of process. You must consult the  appropriate  process-monitoring  manual  to  obtain these tolerances. 7.  Tolerance limits are defined and prescribed in each process-monitoring manual. When the average control values obtained in the preceding step fall within the tolerance limits, production work may be processed and the initial reference values may be used to determine future control values for that particular code of control strips. 8.  When  average  control  values  fall  outside tolerance limits, it is likely that there is a mistake in the following: reading the control strips, performing the  calculations,  setting  up  the  densitometer,  or starting up the process. If errors are not detected, there may be something wrong with the control strip. Improper storage or handling may be the problem. If possible, start over and use a new batch of control strips. Red Green Blue Initial reference strip readings of LD (averaged) 0.31 0.52 0.70 Correction  factors  of  LD +0.01 -0.01 Adjusted  reference  values 0.32 0.51 LD readings of five processed control strips (averaged) 0.28 0.21 Adjusted  reference  values  minus processed control-strip values +0.04 +0.01 0.00 0.70 0.69 -0.01 Figure  2-17.—Deriving  LD  aim  values  for  a  process-monitoring  chart. 2-33







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