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Figure 2-13.—Quality control chart |
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
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