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Figure 2-13.—Quality control chart | Up
Photography (Advanced) - Advanced manual for photography and other graphic techniques | Next
Figure 2-14.—Control chart examples |
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.
PLOTTING GROSS FOG ON
A PROCESS CONTROL CHART
Gross fog (B+F) is read from a "clear" area of a
control strip; that is, an area that does not receive
exposure. All films have a gross fog density,
resulting from several factors that may include the
following:
The density of the film base
Chemical fog
Age fog
The development of unexposed silver halides
Inadequate fixation (film not cleared)
As stated earlier, the amount of information you
use to monitor or control your process depends on
several factors. However, when you choose to
monitor more than one processing variable, you
should construct the appropriate control chart or use
a piece of graph paper that can be posted near the
process. Figure 2-14 shows a typical family of
control charts for a process. A family of control
charts, such as this, will provide you with a wealth of
information about the process. Also, all the
information is in one place.
LIMIT LINES
The upper- and lower-limit lines on a control chart
are based on the assumption that the plotted points are
representative of a normal "population" or set of
circumstances of the process. The limit lines,
therefore, should include between them, all points
representing an unchanged or normal process. Limit
lines can never be placed in such a manner that all
data are included between them; there will always be
deviations. Samples from a black-and-white process,
for example, show a gamma average of 0.70. On a
subsequent test, a sensitometric strip was found to
have a gamma of 0.80. Obviously this process
appears to have changed or is changing. Should the
process be altered? The answer must consider the
factor of probability.
Two risks are involved in judging whether normal
limits are exceeded. One risk occurs when a certain
sampling appears outside one of the limit lines,
indicating that the process is out of control, but the
process is actually behaving normally and has not
changed. This situation is known as the alpha risk.
The reverse is also possible; it appears that the process
is normal when actually it has changed or is changing.
This is called a beta risk. These occurrences cannot
be eliminated, but they can be reduced to the point
where the probability of their happening is small.
One risk is usually more costly than the other, and the
limits are set accordingly. The limits are set far from
the mean when the alpha risk must be avoided. They
are set close to the mean when the beta risk must be
avoided.
It is standard practice in black-and-white
processing to place the limit lines at three times the
standard deviation above and below the mean, or ±3s.
The alpha risk is approximately 3 in 1,000 for limits
of ±3s. Before proceeding, it is necessary to define
the following two terms:
Population—all possible results (happenings)
in a certain process
Variability—the amount of departure of
measurements (parts of the population) from
the mean (average)
Variability may be expressed in the following
ways:
2-26
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