A reliable and reproducible testing procedure for the development, product improvement and production control of pressure sensitive copy papers (PSP) is described.
The crucial aspect of the testing procedure (FM-test) is the alignment of many small individual areas so close to each other that a full tone area results from the contact between and partial overlapping of the individual areas. The full tone area surface copies of practically any size, characterized by a very uniform appearance of intensity, can be measured very simply by means of commercially available remission photometers or densitometers. Another important aspect of the FM test is very good reproducibility and adjustment of the writing force, which is the key to reliable conclusions concerning changes of quality and their causes.
Mathematical and statistical evaluations point out that the quality of PSP can be described by a “characteristic curve” (CC), showing the change of image intensity as a function of writing force. A three-parameter equation for the CC allows an easy calculation of several quality criteria characterising the PSP, without the need of performing a multitude of different tests. In an example, discussed in more detail, it is shown, which paper qualities affect the performance of a PSP and how quickly and reliably the correlations can be evaluated by the FM-test.