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Color Separations and Film
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| We have seen that we can fool the eye into perceiving a wide range of continuous color, using only cyan, magenta, yellow, and black inks. But practically speaking, how does the printing press know how much of which ink should go down where? An individual painter working with a canvas controls how much paint goes down with their own hand on the brush. How does a graphic designer reach out across space to control the amount of ink that hits paper as it rolls through the press? With film.
Film used in printing does not look like the 35mm film from a camera. For starters, it's made to the actual size of the finished piece. Black emulsion is printed onto clear acetate: one sheet per color. |
Negative Film:
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Composite
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Cyan
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Magenta
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Yellow
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Black
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Positive Film:
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Composite
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Pantone 100 yellow
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Black
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| Film is manufactured using an extremely expensive, extremely high resolution printer called an "imagesetter." Typical imagesetters print at 2400 dpi or above. On a 2400 dpi machine, every square inch is divided up into a 2400x2400 grid, and every dot in that grid is set to either on or off - imagesetters are only capable of printing with black emulsion, not colored inks. Either there's a black dot laid down in a given area of 1/2400" x 1/2400", or the area is left blank and clear acetate shows through.
If a consistent 33% halftone dot is called for in a given area, then 33% of the printer dots will print and the rest will be left blank. Ultimately, that area of the film will be half covered with emulsion. The resulting printed piece will receive a 33% tint of whatever ink is assigned to that piece of film. [note to the astute pro: we're ignoring dot gain for the moment.] |
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This is a diagram of a 100 lpi cell, printed on a 2400 dpi imagesetter, with a 33% halftone dot. The halftone dot is set to "elliptical" shape, which is preferred for screen printing. |
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Positive and Negative Film
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| With positive film (used for on-disc screen printing), the emulsion on the film corresponds to the amount of ink laid down. If there's a question-mark-shaped blob of emulsion on the positive film assigned the color Pantone 285 blue, the final piece will have a Pantone 285 blue question mark of similar shape and size.
With negative film (used for the offset printing of the paper products), emulsion coats all of the acetate except where ink should go down. If there's a clear spot on the film for the color magenta in the shape of an exclamation point, the final piece will have a magenta exclamation point of like shape and size.
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Positive film,
labeled Pantone 285
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Printed result |
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Negative film,
labeled magenta
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Printed result |
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