Creating Duotone Maps
A duotone is the use of an additional color to increase the tonal
range of a grayscale image--a printing press can only produce around 50
shades of color per ink, even though your computer can generate a grayscale
reproduction of up to 256 levels of gray. Traditionally, duotones are printed
with a black ink and a gray ink--black reproducing the darker tones of
an image, and gray ink the mid and lighter tones. However, duotones are
more usually printed using a colored ink for highlight areas. This can
give a significant increase to the dynamic range of an image. Strictly
speaking, the use of two different colored inks--as distinct from two shades
of the same color ink--is called a "duplex halftone," but "duotone" is
now the common vernacular. An even greater tonal range can be achieved
using three inks ("tritone"), or even four inks ("quadtone").
Photoshop provides a powerful duotone feature that allows you to
define two (or even three or four) colors for an image without having to
create a separate channel--thus without increasing the file size of the
image. Photoshop's duotone feature is ideal for using Mountain High Maps
USA Relief for a print job in which only spot color is available. It is
also the best way to apply a process color to a map, or a specific "spot"
color such as one from the Pantone® Matching System, before changing
the mode to RGB color.
To convert a map image to duotone
-
Select "Duotone..." from the "Mode" menu. In the dialog box which opens,
select "Duotone" from the "Type" pop-up menu.
-
Specify your second color by clicking in the solid square for that
ink. Use the color picker or the Custom Colors option to choose your ink
color.
Click "OK" and then name the color in the panel alongside its
swatch.
-
If one of your duotone inks is black, the printed result may be too
dark. You can compensate for this by adjusting the duotone "curves." Click
on the curve box (the box with the diagonal line) to the left of the swatch.
The "Duotone Curve" dialog box appears.
-
Adjust the duotone curve either by clicking on the graph and dragging,
or by entering percentage values in the appropriate boxes. Or you can load
a previously saved curve--a set of duotone curves is supplied with your
Photoshop application.
|
MHMtip
SPOT COLOR AND DUOTONE MAPS
Use
the "Duotone" feature of Photoshop to apply spot colors to a grayscale
map. The Duotone feature is also useful if you wish to assign a precise
PANTONE color to a grayscale map, which can then be used for print output
either as it is (saved in EPS format) for spot color printing, or converted
to CMYK mode for four-color process printing. |
If you want to view the individual channels of a duotone, choose
"Multichannel" from the "Mode" menu. This enables you to view the individual
printing plates by clicking on the appropriate channel in the "Channels"
palette. To switch back to duotone mode, simply select "Undo" from the
"Edit" menu.
If you intend outputting your map from QuarkXPress, save it as an
EPS file with the "Binary" encoding option checked.
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