CMYK colour control

ImageIt is amazing how technology can change responsibilities. If you think back five to ten years ago it was the job of the scanner operator to ensure your CMYK file was ready for print. But today the responsibility is in the hands of the content creators, the designers and the photographers.

Unfortunately, the problem with this shift in responsibility is that the content creators have not been trained in things like GCR, ink limits, tone value increase or how a Web press differs from a Sheetfed press.

These are areas that have been reserved for the prepress and printers that have years of experience in these areas. But if you are willing to learn how the process works you gain better control of your images and in the end have happier clients.

Creating a CMYK file is an easy step if you own Photoshop and know how to use a computer. But creating a CMYK separation that is appropriate for a specific printing condition requires more thought and an understanding of a few of the variables involved.

What are the Main variables in Print?
Paper Type: Coated or Uncoated.
Paper Colour. A yellow stock can have a huge impact on the colour reproduction. Think of paper colour as the fifth colour in printing.
Press and Press Age. Older presses tend to have a higher dot gain.
Press Type: Sheetfed vs. Web.

Guidelines to Controlling Colour on Press:
1) Ask questions. Talk to the printer or print broker. Try to learn as much as you can about the variables. Paper type, paper colour, press age, stochastic or traditional halftones, proofing options, dot gain amounts (tone value increase TVI), total ink limit recommendations (TIL) and what have they done in the past that worked well.

2) Monitor calibration:  If you plan to use your monitor to make colour adjustments you need to calibrate and profile it using a colorimeter and calibration software.

3) Soft-Proofing: Learn how to use Photoshop's Soft-Proofing options to see a more accurate representation of how your job will print when converted to CMYK and after conversion. (See figure below)

4) Use Photoshop's "Convert to Profile" option when converting from RGB to CMYK. Select the profile that best represents your target. If you are unsure the safest option is to use: USWebCoatedSWOP.icc.

5) Using Photoshop's colour adjustment tools such as levels and curves to optimize the image for maximum impact. Ensure that your blacks are dark and the whites are white, sounds simplistic but it is very important to maximize the dynamic range of your image to ensure maximum scalability.

One of the most important elements in a good colour separation is the ability to print with predictability and consistency throughout the print-run. When the grey balance of an image is made up of mostly Cyan/Magenta/Yellow, the colour on press will shift with the slightest variations in density and registration. Therefore it is very important to have a strong black separation that will stabilize the colour balance. Unfortunately, digital proofs and soft proofs (your monitor) may look fine, but the results on press will not be satisfactory.

Gray component replacement (GCR) and Undercolour removal (UCR) determine how much cyan, magenta and yellow are replaced with black when an image is converted to the CMYK colour space.

UCR reduces the amount of cyan, magenta, and yellow primarily in the shadow areas of an image and increases the black. UCR will affect only the neutral areas of the image—it has no affect on the colour areas of a printed reproduction. It helps alleviate potential printing problems associated with heavy ink coverage such as set off or blocking.

By contrast, GCR is more aggressive—it affects the neutral and colour areas throughout the entire image. GCR replaces the gray component of the trichromatic colours with black during colour separation. (A trichromatic colour is any colour that is made up of all three primary printing colours: cyan, magenta and yellow.) The gray component of the trichromatic colours is the level to which all three primaries are equally present. Applying GCR replaces the tertiary colour with black.

GCR improves colour consistency on press. GCR separations will produce more consistent, repeatable colour throughout a press run. The disadvantage of this, however, is the reduction in the ability to make colour changes on press.

Lastly, if you want to be sure your job is printed correctly go to the press approval. Work with the pressman or woman to achieve your desired result. Keep in mind that only slight adjustments can be made on press. This is not the time to make up for you not doing your homework.

Resources:
http://www.printtools.org/
http://www.idealliance.org/
http://www.swop.org/    
http://www.gracol.com/
http://www.colourmanagement.com•

Angus Pady is the president of Digital Solutions. Complete colour control from desktop to press.
T: 905-764-6003
Angus@ColourManagement.ca
www.colourmanagement.ca

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