Creating color traps

With CMYK images, you can adjust the color trap. A trap is an overlap that prevents tiny gaps from appearing in the printed image, due to a slight misregistration on press. In most cases, your print shop will determine if trapping is needed and tell you what values to enter in the Trap dialog box.

A B

Misregistration with no trap, and misregistration with trap

Trapping is intended to correct the misalignment of solid colors. In general, you don’t need traps for continuous-tone images such as photographs. Excessive trapping may produce an outline effect. These problems may not be visible on-screen and might show up only in print. Adobe Photoshop uses standard rules for trapping:

•All colors spread under black.

•Lighter colors spread under darker colors.

•Yellow spreads under cyan, magenta, and black.

•Pure cyan and pure magenta spread under each other equally.

To create trap:

1Save a version of the file in RGB mode, in case you want to reconvert the image later. Then choose Image > Mode > CMYK Color to convert the image to CMYK mode.

2Choose Image > Trap.

3For Width, enter the trapping value provided by your print shop. Then select a unit of measurement, and click OK. Consult your print shop to determine how much misregistration to expect.

Printing duotones

Photoshop lets you create monotones, duotones, tritones, and quadtones. Monotones are grayscale images printed with a single, nonblack ink. Duotones, tritones, and quadtones are grayscale images printed with two, three, and four inks. In these types of images, colored inks are used to reproduce tinted grays rather than different colors. This section uses the term duotone to refer to duotones, monotones, tritones, and quadtones.

2 Click the curve box next to the ink color box.

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 prints with a 50% tint of the ink, a 100% shadow is printed in 100% color, and so on.

3 Adjust the duotone curve for each ink by dragging a point on the graph or by entering values for the different ink percentages.

•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, Adobe Photoshop calculates intermediate values. As you adjust the curve, values are automatically entered in the percentage text boxes.

•In the text box, the value you enter indicates the percentage of the ink color that will be 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 will be used to print the 100% shadow areas of the image. (See “Using the Curves dialog box (Photoshop)” on page 139.)

4Click Save in the Duotone Curve dialog box to save curves created with this dialog box.

5Click Load to load these curves or curves created in the Curves dialog box, including curves created using the Arbitrary Map option. (See “Saving and loading duotone settings” on page 482.)

You can use the Info palette to display ink percentages when you’re working with duotone images. Set the readout mode to Actual Color to see the ink percentages that will be applied when the image is printed. These values reflect any changes you’ve entered in the Duotone Curve dialog box.

Specifying overprint colors

Overprint colors are two unscreened inks printed on top of each other. For example, when a cyan ink prints over a yellow ink, the resulting overprint is a green color. The order in which inks are printed, as well as variations in the inks and paper, can significantly affect the final results.

To help you predict how colors will look when printed, use a printed sample of the overprinted inks to adjust your screen display. Just remember that this adjustment affects only how the overprint colors appear on-screen, not when printed. Before adjusting these colors, make sure that you have calibrated your monitor following the instructions in “Creating an ICC monitor profile” on page 117.

To adjust the display of overprint colors:

1Choose Image > Mode > Duotone.

2Click Overprint Colors. The Overprint Colors dialog box displays the combinations that will result when the inks are printed.

3Click the color swatch of the ink combination you want to adjust.

4Select the color you want in the color picker, and click OK.

5Repeat steps 3 and 4 until the overprint inks appear as you want them. Then click OK.

Saving and loading duotone settings

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.

The Adobe Photoshop application includes several sample sets of duotone, tritone, and quadtone curves. These sets include some of the more commonly used curves and colors and are useful as starting points for creating your own combinations.

Viewing individual printing plates

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).

To view the individual colors of a duotone image:

1 After specifying your ink colors, choose Image > Mode > Multichannel.

The image is converted to a multichannel image, 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.

Important: If you make any changes to the image in Multichannel mode, you will be unable to revert to the original duotone state (unless you can access the duotone state in the History palette). 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.

2Select the channel you want to examine in the Channels palette.

3Choose Edit > Undo Multichannel to revert to Duotone mode.

Printing duotones

When creating duotones, keep in mind that both the order in which the inks are printed and the screen angles you use dramatically affect the final output.

Click the Auto button in the Halftone Screens dialog box to set the optimal screen angles and frequencies. (See “Selecting halftone screen attributes” on page 475.) Make sure that you select Use Accurate Screens in the Auto Screens dialog box if you’re printing to a PostScript Level 2 (or higher) printer or an imagesetter equipped with an Emerald controller.

Note: The recommended screen angles and frequencies for quadtones are based on the assumption that channel 1 is the darkest ink and channel 4 is the lightest ink.

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. (See “Printing color separations” on page 483.) Converting to CMYK mode converts any custom colors to their CMYK equivalents.