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.

FiLyure 2-13.-Quality control chart.

The same factors that affect high density affect low density.

PLOTTING SPEED POINT ON A PROCESS CONTROL CHART

The speed point (SP) is a measure of the effective film speed or exposure index of a film. The speed point is determined by sensitometric tests. The speed point is established using a step on a sensi-strip with a density of 0.10 above gross fog for ground pictorial film. The speed point of aerial film is established by using the step on a sensi-strip that has a density of 0.30 above gross fog.

Once the speed-point step is determined, that step is read in successive sensi-strips and plotted on the control chart. Neither effective film speed nor the ISO for ground pictorial film should be confused with effective aerial film speed because they are not equivalent.