Carefully set up the x-ray exposure unit for the exposure technique. The cassette and the stepped wedge must be positioned identically each time control film is exposed.
Cut the exposed film into a minimum of twice the number of processors in the control system. If possible, cut more than the minimum, but do not make duplicate exposures and assume they are identical. Each exposure of control film must be considered a complete control.
Identify the strips as to date and exposing unit.
Place half or the exposed control strips in an airtight, light tight, and moisture proof wrapper and store them in the refrigerator. Remove the moisture proof package of control strips exposed in the same unit the previous day. When these latent-image control strips are at room temperature, they can be handled and processed.
Processing of Control Strips
Process the freshly exposed control strips and the strips exposed in the same unit the previous day in the corresponding processor (or processors).
If the strips are less than 21/2 inches wide, tape them to a leader. If a leader is not used, process a cleanup sheet before processing the control strips.
Always process a fresh control strip exposed in a designated unit with a control strip exposed in that same unit the previous day.
Record in the log the temperature of the developer at the time the strips are in the processor. Record in the log any other information pertinent to process control.
Densitometry
Check the accuracy and the precision of the densitometer each time before it is used to obtain the numerical density values from the control strips.
Determine the density value of each of the two steps selected for measuring density on the freshly exposed and processed control strip and the density of the two corresponding steps on the latent-image control strip. (Once these steps have been selected, the density of the same area on each of the steps is used every time control strips are measured.)
Record the densitometric readings and plot them on the process control charts.
Discussion
Densitometric data and process control charts for one exposing unit and one processor are presented as Figures 79 to 80. The process density aim (the mean density) and the upper and lower control limits on the charts (the table immediately above and the one below) were computed from the densitometric data for the 10-day period shown in Figure 79. As stated earlier, the control limits on both charts are wider than would be the case if either the exposing unit or the processor were monitored individually. Although two control charts are illustrated, they can be combined into one for convenience.
Exposure and Processing Variations
Variations in both exposure and processing are reflected on the process control chart reproduced as Figure 80. All the high-density readings are plotted, but only the readings from control strips exposed at the same time are connected. The lines represent the day-to-day repeatability of film processing, the difference between the point plots on a given day represents the repeatability of exposure.
Figure 81: Control chart for one exposing unit and one processor indicating changes in contrast. Plot the difference between the high-density reading and the low-density reading of the fresh-image control strips, and connect the plots. A decrease in density difference from the process density aim, or the mean density, indicates lower contrast; an increase indicates higher density.
Process Control Chart--Changes in Film Contrast
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Upper control limit
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1.30
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Process density
aim
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1.10
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Lower control limit
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0.90
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There will be a slight density loss of the latent image on the control strips processed one to three days after exposure even though they are refrigerated. However, this small difference in density is no cause for concern with the type of control chart shown in Figure 80. If the density of the refrigerated control strip is always slightly lower (0.03 to 0.04) than that of its corresponding fresh control strip, the process is repeating identically.
Contrast Variations
Variations in film contrast are reflected on the control chart reproduced as Figure 81. The difference between the high-density reading and the low-density reading of the fresh-image control strips is plotted on this chart. (Latent-image control strips serve equally well for determining density difference.) A decrease in density difference from the mean density, or the process density aim, indicates lower contrast, an increase in density difference indicates higher contrast.