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Description of dyed mesh
By exposing a direct stencil to UV light, the illuminated areas become hardened. Light rays striking the white fibres of the mesh are reflected, and scatter under the black edges of a film.
Light is also conducted through the fibres themselves, leading to yet more under-cutting.
The results are unsharp printed edges, causing colour shifts in multicolour half-tone artwork. There is a reduction of the open printing areas, particularly in fine detail work. To keep these phenomena under control, it is necessary to work out exposure times leading to proper exposure.
Emulsions and films are sensitive in the UV range, from approx. 350 – 420 nanometres wavelength. To be effective, light scatter protection must absorb UV light over this wavelength range. The obvious choice in achieving this is to use the complementary colour, which by definition absorbs the desired wavelengths. Absorption tests show that the most effective absorber is a warm yellow colour.
When UV light falls on a yellow fiber it is largely absorbed; only those wavelengths of the spectrum are reflected that cannot cure the emulsion. The result is crisp print margins and reproduction of the finest details. Since no undercutting occurs, the exposure time can be selected such that the emulsion is thoroughly cured.
Normally, the exposure time for Sefar dyed fabric should be extended by 50 – 125%: this results in more robust stencils with longer lifetime – a step-exposure test to determine the optimal exposure time is, however, unavoidable.
The large upward exposure latitude reduces the risk of underexposure.