9.
Color Separation.
When reproducing a full graphic, you must produce four separate printing
plates, a plate of each of the four basic colors. This is known as
color separation. The photoengraver filters the sensitized emulsions to
eliminate the three colors not desired on the negative. For example,
when reproducing an image with green in it, the photoengraver uses a
filter that allows the desired amounts of yellow and blue (the basic
color that make green) through to the negative and holds the amount of
red and black in the green area to a minimum. There are two methods
used for color separation: indirect and direct.
a. Indirect Method of Color Separation. When using this color
separation method, you produce continuous toneseparation negatives and
then produce contact positives from these negatives. You retouch the
contact positives and rephotograph them as halftones. You can use this
method for difficult copies having very dense areas that the finisher
would have problems opening if furnished as an ordinary set of prints.
b. Direct Method of Color Separation. You would use this method
when you make a halftone color separation negative directly from the
copy using filters. You then print the screen negatives on the metal
and the color etcher not only etches the picture into the metal but also
colorcorrects by etching, to create a reproduction of the image.
Color separation is a subtractive process that uses filters that
compliment the colors used for printing. When the photographer produces
the yellow printer negative, he use uses a blueviolet filter. When
producing the blue printer negative, he uses a red filter, and when
producing the red printer negative, he uses a green filter. When the
photographer produces the black printer negative, he uses a combination
of red, green, and blue filters or no filter.
c. Indicating Color Separations. As an illustrator, often you
assist the photographer or lithographer by making color separation
drawings they can refer to when making the printing plates. The
drawings range from using simple instructions when separating the
colors, to indepth drawings that do the job of separating the colors
the filters do automatically.
(1) Tissue color separation overlays. This is the easiest method
used to indicate color separation. You start this process by laying a
sheet of tracing paper over the graphic and blocking out each color area
with a crayon or pencil. The lithographer then uses the overlay when
shooting each printer.
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