Using the Alkemi Exposure Calculator - Part III
The Alkemi Exposure Calculator as a Printing Aid
These can be used to assess printed edge definition (straightness/clarity of the printed edge) and print resolution (fineness of detail achievable) The target is designed to allow the user to select the best angle to position film positives/ stencils in order to avoid 'sawtooth' and mesh interference. Resolution is checked by assessing the degree of 'filling-in' at the center of the target.
The below image shows the Alkemi Plus 7000 emulsion zoomed in 200x to show the detail of the edge definition near the center of the target. While screen printers do not need to see this level of detail to assess their exposure with the exposure calculator, we think it’s fun to see.
The 10% and 90% areas can be used to gauge the degree of highlight dot loss and flooding of shadow areas respectively.
The use of a square dot configuration means that the 'corners' of the dots just meet at 50%. Examination of a print will show at a glance whether there is 'dot loss' or 'dot gain' in the midtone.
Failure of the 'corner' to meet indicates 'dot loss'. This may be due to undercutting, caused by overexposure or poor light geometry, drying in of ink, or the choice of a low resolution photostencil
The merging of dots, or thickening of the join between dots, indicates 'dot gain'. This is due to flooding, which may be caused by over-thinning of ink, or by use of a photostencil with poor edge definition.
Before starting a print run with half-tones. it is always recommended that a test strip is printed, which contains a full range of tone values in the selected dot count. Positives with square dots are not recommended for fine half tone printing.