It is important that the black areas can block UV light, but toner that looks dense on vellum may actually be closer to translucent – so how do you know if your film is working properly? A simple test: hold the artwork up to a light source (sunny window, lamp, etc.). If you can see any light through the black areas, the artwork is not dense enough.
For example, hold a piece of black construction paper up to a light source. Light will not bleed through. This is the density your artwork should provide, but note that different types of artwork media may provide better results.
For a general comparison, artwork types are listed in order from highest density and resolution to lowest: Genuine Film, AccuArt Waterproof Inkjet Film, PFX Drafting Vellum, UVII Standard Vellum.
-Michael Sullivan, IKONICS Imaging
