Ad hoc separation of colour channels

Nearly all print methods that deliver commercial results in full colour operate on a multi-level structure (e.g. separations, plates, or positives) of images in layers. This process method usually has four stages and works with specially matched process colours that can reproduce a wide range of colours very accurately and deliver vibrant results.

The best known method is the four-colour euro scale (CMYK) in conventional offset. CMYK is an acronym of the four colour layers that a picture consists of: cyan (C for cyan), magenta (M for magenta), yellow (Y for yellow) and black (K for key plate, black, the colour that is usually first in the paper run and that the registration marks of the other colours are keyed to 1). This processes standard provides a simple way of transmitting full-colour images from file to print during pre-printing and printing without needing to use special colour pigments while still getting colour results closest to the original. In Photoshop, an image in CMYK mode is automatically separated into the four colour separations in the channel window. In order to adequately edit a picture, it is highly recommended to modify the picture in RGB mode first and then convert it to CMYK via Edit › Assign Profile › CMYK. The colours may change slightly, which is why the image should be edited afterwards with settings like hue / saturation or a gradation curve to change the colour balance.

If the colour impression of the subject has been fixed, the image can be converted via  Image › Mode › Multichannel, which makes the picture available in different channels. When an image is used as a starting point in RGB mode, three channels (cyan, magenta and yellow) are needed. If a CMYK image is used as a starting position, there are four channels (cyan, magenta, yellow, black). Beware: After converting to a multichannel image, Photoshop can no longer display it in full colour and it may have a red cast. In the last step, final colour adjustments can be made by selecting the desired channel and applying the desired correction (e.g. gradation curve) via Image › Adjustments. If the image has been edited to satisfaction, it can be saved like this:

Save as multiple greyscale-TIFF files

In the channel menu, select Options Split channels to convert the image into four files, and save each of them as a TIFF file.

Save as a multichannel file for separations in InDesign

In order to retain the file as a multichannel file, many conventional file formats (e.g. Photoshop, TIFF or JPG) are ruled out. If the file is to be saved as a multichannel file, in order for the separation preview to be used in InDesign or to use separations in a PostScript file (see Chapter 4), the format should be Photoshop-DCS 2.0 so it can also be imported and viewed in Adobe InDesign. If saving the multichannel image as a Photoshop file is imperative, a grey channel should be added to the multichannel file (e.g. via Image Mode greyscale). Although the image now has an additional black channel, the spot colour channels of the desired RISO colours are still preserved.

  1. Jane Hosie-Bounar, New Perspectives on Adobe Photoshop CS6: Introductory (Florence: Cengage Learning, 2012), 77.