Sometimes, a bitmap graphic (such as a logo or line art) needs to be converted into a CMYK file. When that happens, separation problems can happen. This technique exploits the power of Color Settings to make the conversion hassle-free.

Have you ever needed to convert a line art graphic�consisting of black and white�into CMYK? I do it sometimes when I get a CMYK logo from a client (JPEG, of course) and have to convert it into a two-spot color graphic. The biggest obstacle for a conversion like this is the fact that Photoshop usually converts black into a combination of cyan, magenta, yellow and black; you’ll see it if you look at the channels in the Channels palette after conversion. Black graphics (and even black type) becomes rich black that can cause problems if misregistration occurs when printing. If you need to convert to CMYK but the blacks must remain 100% black ink, you can do some cutting-and-pasting to get the right data into the right channel�or use the technique below.

Step 1: Before converting a grayscale graphic into CMYK, select Edit �> Color Setting… (Shift-Cmd-K/Shift-Ctrl-K) to access the Color Settings dialog box (see Figure 1).

Figure 1: The Color Settings dialog box.

Step 2: In the Working Spaces area of the Color Settings dialog box, click the drop-down menu for CMYK and select Custom CMYK…. The Custom CMYK dialog box appears (see Figure 2).

Figure 2: The Custom CMYK dialog box.

Step 3: In the Separation Options at the bottom of the Custom CMYK dialog box, you’ll find two types of separation�gray component replacement (GCR) and undercolor removal (UCR)�that dictate how Photoshop separates color into CMYK equivalents. UCR will build color using cyan, magenta and yellow but no black. GCR, on the other hand, uses black in varying degrees as dictated by the Black Generation drop-down menu below the GCR and UCR radio buttons. A setting of “None” creates the same CMYK combinations as UCR. As the settings increase�from “Light” to “Medium” and then “Heavy”�more and more black is introduced as cyan, magenta and yellow are removed. The highest setting, “Maximum”, will dictate Photoshop to use as much black as possible when converting to CMYK. It is this Maximum setting that makes this technique work; after specifying GCR and Maximum, click OK on both dialog boxes to return to the open grayscale graphic.

Step 4: Convert the graphic to CMYK with the usual Image �> Mode �> CMYK Color, then look in the Channels palette to see the cyan, magenta and yellow channels with no data and the black channel with the full logo. Congratulations�you’ve just generated a CMYK graphic that uses only black ink, very useful for logos and line art!