It is important that the black areas can block ultra-violet light. Toner that looks dense on vellum may actually be closer to translucent in its ability to block UV light. 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 an example of high density, hold a piece of black construction paper up to a light source. No light will bleed through. This is the density your artwork should provide. 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.