About duotones

Duotones are used to increase the tonal range of a grayscale image. Although a grayscale reproduction can display up to 256 levels of gray, a printing press can reproduce only about 50 levels of gray per ink. This means that a grayscale image printed with only black ink can look significantly coarser than the same image printed with two, three, or four inks, each individual ink reproducing up to 50 levels of gray.

Sometimes duotones are printed using a black ink and a gray ink--the black for shadows and the gray for midtones and highlights. More frequently, duotones are printed using a colored ink for the highlight color. This technique produces an image with a slight tint to it and significantly increases the image's dynamic range. Duotones are ideal for two-color print jobs with a spot color (such as a PANTONE Color) used for accent.

Because duotones use different color inks to reproduce different gray levels, they are treated in Photoshop as single-channel, 8-bit, grayscale images. In Duotone mode, you do not have direct access to the individual image channels (as in RGB, CMYK, and Lab modes). Instead, you manipulate the channels through the curves in the Duotone Options dialog box.

To convert an image to duotone:

1. Convert the image to grayscale by choosing Image > Mode > Grayscale. Only 8-bit grayscale images can be converted to duotones.
2. Choose Image > Mode > Duotone.
3. In the Duotone Options dialog box, select Preview to view the effects of the duotone settings on the image.
4. Select Monotone, Duotone, Tritone, or Quadtone for Type.
5. To specify ink colors, click the color box (the solid square) for an ink. Then use the color picker or the Custom Colors dialog box to select an ink. (See Using the Adobe Color Picker.)

Note: To produce fully saturated colors, make sure that inks are specified in descending order--darkest at the top, lightest at the bottom.

6. Click the curve box next to the color ink box and adjust the duotone curve for each ink color. (See Modifying the duotone curve.)
7. Set overprint colors, if necessary. (See Specifying overprint colors.)
8. Click OK.

To apply a duotone effect to only part of an image, convert the duotone image to Multichannel mode--this converts the duotone curves to spot channels. You can then erase part of the spot channel for areas that you want printed as standard grayscale. (See Adding spot colors (Photoshop).)