In Photoshop, duotone refers to monotones, tritones, and quadtones as well as duotones. Monotones are grayscale images printed with a single, non-black ink. Duotones, tritones, and quadtones are grayscale images printed with two, three, and four inks. In these images, colored inks, rather than different shades of gray, are used to reproduce tinted grays.
Duotones 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. For this reason, a grayscale image printed with only black ink can look significantly coarser than the same image printed with two, three, or four inks, each ink reproducing up to 50 levels of gray.
Sometimes duotones are printed using black ink and gray ink—the black for shadows and the gray for mid-tones and highlights. More frequently, duotones are printed using colored ink for the highlight color. This technique produces an image with a slight tint and significantly increases the dynamic range of the image. Duotones are ideal for two‑color print jobs with a spot 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 produce fully saturated colors, specify inks in descending order—darkest at the top, lightest at the bottom.
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.
In a duotone image, each ink has a separate curve that specifies how the color is distributed across the shadows and highlights. This curve maps each grayscale value in the original image to a specific ink percentage.
The default duotone curve, a straight diagonal line, indicates that the grayscale values in the original image map to an equal percentage of ink. At this setting, a 50% midtone pixel is rendered with a 50% tint of the ink, a 100% shadow is rendered in 100% color, and so on.
In the curve graph, the horizontal axis moves from highlights (at the left) to shadows (at the right). Ink density increases as you move up the vertical axis. You can specify up to 13 points on the curve. When you specify two values along the curve, Photoshop calculates intermediate values. As you adjust the curve, values are automatically entered in the percentage text boxes.
The value you enter in the text box indicates the percentage of the ink color used to represent the grayscale value in the original image. For example, if you enter 70 in the 100% text box, a 70% tint of that ink color is used to print the 100% shadows.
You can use the Info panel to display ink percentages when you’re working with duotone images. Set the readout mode to Actual Color to determine what ink percentages will be applied when the image is printed. These values reflect any changes you’ve entered in the Duotone Curve dialog box.
Overprint colors are two unscreened inks printed on top of each other. For example, when you print a cyan ink over a yellow ink, the resulting overprint color is green. The order in which inks are printed, as well as variations in the inks and paper, can significantly affect the final results.
To predict how colors will look when printed, use a printed sample of the overprinted inks and adjust your screen display accordingly. Keep in mind that this adjustment affects only how the overprint colors appear on-screen, not when printed. Before adjusting these colors, make sure to calibrate your monitor.
Use the Save button in the Duotone Options dialog box to save a set of duotone curves, ink settings, and overprint colors. Use the Load button to load a set of duotone curves, ink settings, and overprint colors. You can then apply these settings to other grayscale images.
Photoshop includes several sample sets of duotone, tritone, and quadtone curves. These sets include some commonly used curves and colors. Use these sets as starting points when you create your own combinations.
Because duotones are single-channel images, your adjustments to individual printing inks are displayed as part of the final composite image. In some cases, you may want to view the individual “printing plates” to see how the individual colors will separate when printed (as you can with CMYK images).
The image is converted to Multichannel mode, with each channel represented as a spot color channel. The contents of each spot channel accurately reflect the duotone settings, but the on‑screen composite preview may not be as accurate as the preview in Duotone mode.
If you make any changes to the image in Multichannel mode, you can’t revert to the original duotone state (unless you can access the duotone state in the History panel). To adjust the distribution of ink and view its effect on the individual printing plates, make the adjustments in the Duotone Curves dialog box before converting to Multichannel mode.
When creating duotones, keep in mind that both the order in which the inks are printed and the screen angles you use have a significant effect on the final output. (If needed, change the halftone screen angles on the printer’s RIP.)
You do not have to convert duotone images to CMYK to print separations—simply choose Separations from the Profile pop‑up menu in the Color Management section of the Print dialog box (for setting printer options). Converting to CMYK mode converts any custom colors to their CMYK equivalents.
To export a duotone image to a page-layout application, you must first save the image in EPS or PDF format. (However, if the image contains spot channels, convert it to Multichannel mode and save it in DCS 2.0 format.) Remember to name custom colors using the appropriate suffix so that the importing application can recognize them. Otherwise, the application may not print the colors correctly, or it may not print the image at all.