colour separation in photoshop

artwork to film When we have your final artwork the next stage is then to colour separate the image file. Each colour in the artwork file is separated by one of the graphic applications such as Adobe Photoshop. There are a number of ways of doing this. Which method is chosen depends on a number of factors including image tonal range, the print process, the ink type and fabrics.

Spot-colour images, which incorporate solid colours, work well for cartoons, type, logos and line art. A spot-colour image can be as simple as one colour or as complex as 10 or more and can include shading, gradations and detail. Spot-colour images (using one, to usually no more than six colours) comprise the bulk of screen printers work.

Four-colour process images are comprised of cyan, magenta, yellow and black (CMYK). When combined, these four colours can recreate most colours of the rainbow. Printers use four-colour process to reproduce paintings and photographs, but they generally work well only on light-coloured shirts because the inks are transparent. This means they don't show up on coloured garments.

Simulated process images have a photorealistic look but aren't printed with CMYK inks, making this method best for printing onto dark coloured t-shirts. The colour separations for simulated process colour are made up of halftone images of spot colours such as reds, greens, yellows, greys and blues. Good simulated process prints will produce sharp, photorealistic images with smooth gradations and bright colours.


running out the film positives

Once the file has been separated into each of the colours to be printed, a clear film positive is then printed out on an image setter for each of those print colours. These will be used in the next stage of the process in making the actual printing screens.

preparing the screens Firstly the correct mesh size of screen is chosen for each colour. Varying gauges of mesh are used. Finer meshes are used mainly for detailed artwork or halftone images. Screens with a larger mesh sizes will allow more ink through, and are used mainly for printing bolder images or for specialist inks that may block up the finer meshes. Next stage is to coat the screens with a photo sensitive emulsion that will carry the images. Once the screens have been coated carefully with a thin layer of this very light sensitive emulsion, they are placed in a dark drying cabinet for the emulsion to harden and dry.

exposing the screens When the screens are ready and prepared, we now place each of the film positives back to front on the front of each screen. (In a registered position) The screens are then placed in a high power UV light exposure unit. A vacuum then holds the screens and films very firmly in place while the UV light gives a timed exposure, (according to the mesh size). The exposure unit shines UV light on the screens, which exposes the photosensitive emulsion that is not being blocked by the images on the film positives. When exposed, the films can then be removed from the screens and put away (and kept for later use if required again). The screens are now ready to go on to the next stage.

washing out the screen image When the screens have been successfully exposed (and carefully checked) , the screens are placed in a wash-out booth where the screens are subject to a high pressure water spray. The areas of photosensitive emulsion that were blocked out by the film positives then dissolve and wash away with the water spray. The emulsion that was exposed to the UV light is now chemically bonded to the screen and does not wash out with water. So after spraying the screens, the only mesh openings on the screens visible, are of the negative images created by the film positives for each colour in the design.