Screen E-Book (.PDF) - MacDermid Autotype

As the distance between the light source and the vacuum frame increases so less light hitsthe stencil. This means that the time to achieve full exposure will increase. The formulafor calculating this is:X = Y x (a)2/(b)2Where:X = New exposure timeY= Original exposure timea = New distanceb = Old distanceAt the same time, the light hitting the stencil becomes closer to 90°. This will result in lessundercutting and improved resolution. This is because light rays that are not parallel canexpose bits of stencil that should have been masked by the film. This is often referred toas light undercutting.From a practical point of view poor light geometry effects do not present a problem whenprocessing general work, i.e. lettering down to about 6 point and halftones of 65 lines (25cm) or coarser.It is only when trying to accurately reproduce very fine line halftones or images that attentionmust be paid to light geometry. Very high quality work uses a lens system on theexposure unit which collimates the light, ensuring that it all arrives at 90º.65