Spot Color Transparency and Screens in Illustrator

Question

(Illustrator CS5)

I have a design that will be printed on a line printer. Possible are up to 7 colors, or a 4-color process with 3 line colors.

I'm currently sticking to PMS only (only 5 PMS colors used), and use screens to achieve certain effects. In the composition, I have a group of many different shapes, all stacked on top of each other, that are all of the same PMS color. Most of these shapes are opaque.

My question is about Illustrator's Transparency panel vs. the Color panel:

Illustrator shows the different opacity of each element in the group in the Transparency panel, while the percentage of the color in the Color panel is always at 100%. Does this make a difference when printing, or will the printer understand the transparency setting of a PMS color that's set to 100%, and convert it to screens in the printing process?

Answer 1

To achieve "transparency" effect Illustrator simply mixes color of given object with color of its background. For example: objects fill is set to Pantone 451. Object have opacity set to 50% and tint to 100%. It has white background. If mixing mode is "Normal" resulting color will be 50% of given Pantone. Exact recipe how resulting color is calculated is described as a math formula. Different formulas are known by specific names as "mixing modes".

Take a look at "Separation preview" in your Ill to see actual values of separate component colors on output.

When all your objects have "white artboard" as their background it doesn't make any difference if their color is made lighter with transparency setting or with tint control. It does however make a difference if their background isn't white. Similar things happen when "overprint" (most unfortunate name) is on.

Printer, in fact, won't understand transparency at all. If something will, it'll be RIP, but it'll also "simulate" transparency by telling printer to use in specific ares e.g. lighter colors. Exactly what Ill does before "sending" image to monitor.

Answer 2

The printer's RIP will understand it. To get a real feel for how things will process, turn on Overprint Preview. That's how it will print. If you have objects set to overprint that should knock out, or vice versa, you'll see the difference quite clearly.