Fixed Step Process Monitoring

This procedure requires that there exist a correctly processed strip that can be used as the reference strip.

The reference is either a strip supplied with a commercially available pack, or it can be a strip that corresponds to the aim for the absolute parameters the management believe is correct, or it can be a strip processed with an Inter-laboratory Survey Strip and it's correct values interpreted from the Survey report.

In the case of Eastman Colour this last method is the most used by commercial laboratories. Fixed step control simply requires that a number of steps on the control strip be measured in order to plot their variation from the reference on a running plot against time.

A typical set of steps could be:

1. A step corresponding to the Minimum Density. This is the density of the film where there has been no exposure. The density obtained will be the result of chemical fog from processing, exposure to light during manufacture and during use in the laboratory.
2. A step corresponding to or near the Speed point. For example close to a point 0.1 above D.Min, in the case of a negative film.
3. A step corresponding to or near the upper point used to calculate the contrast For example at a point corresponding to the highest exposure from an average 1.8 Log E subject brightness range.
4. A step corresponding to the Maximum Density. This is the highest density the film can produce and will depend on the amount of silver or dye in the film. In order to show a value that corresponds more closely to contrast, rather than just a single upper step, the value plotted on the running plot is the difference between the upper and the speed point, or Step 3 - Step 2 above.