Making a step exposure
A step exposure is best made with a test positive containing at least 5 identical images featuring positive and negative fine lines and half-tones.
Masking film (miscellaneous)
The mask must block UV-A light (100 % opaque). Ascertain the number of steps for determining
the correct exposure time!
: The exposure time is assumed to be around 80 units. So: 20 units per step, such that step four lies approximate to the correct exposure time.
: Under- and over-exposed areas should become visible within the five steps afforded
by the exposure test film.
During development, it becomes apparent that stepped exposure times produce different degrees of coloring on the stencil.
The difference between steps is especially visible if the first three steps are underexposed.
There is no color difference between the remaining steps. We can therefore assume that step four is the minimum exposure time. At step four, the stencil should no longer be smudged on the squeegee side. This indicates a correct exposure.
On the other hand, when there is no color difference between the first and second steps (already hardened through), make another step exposure based on a shorter exposure time.
With diazo coatings, the color difference between individual steps is easily recognisable. The effect is more subtle with pure photopolymer coatings, although it is possible to judge exposure based on the presence of smudging: no smudging = hardened through.