If you're serious about creating your own documents, there are some things you need to know about full colour printing. Getting the file you have created to print predictably in full colour won't be possible unless you are familiar with the following:
- CMYK & RGB Colour Models.
- Why colour appears differently in different mediums.
- Limitations of full colour printing.
These topics are so complex that there is no way to completely delve into them in this short article. However, if you read on, you'll have a good basis to start adding to your knowledge in the world of full colour CMYK printing.
In attempting to understand RGB & CMYK colour models & challenges, lets follow the typical path of an image from the time it is photographed to the time it is printed as a part of a brochure:
- The image is photographed. If the image is photographed using traditional film negatives, the film will capture a very wide range (gamut) of the colours that are present in the original scene. The film will not, however, capture the entire colour gamut that is present.
- Prints are produced from the image. The photo lab will rely on its automatically calibrated equipment to produce a print from the negative. The operator will generally visually doublecheck the colour quality as well. Not having the original scene to refer to, and working with a more limited gamut, the print will not exactly match the colours of the original scene.
- The photo is scanned by a desktop publisher, print shop, or service bureau. When this happens, the scanner interface will do its best job to reproduce the photograph in RGB (red, green, blue) colours. RGB is the colour model that is used by monitors to display colours. The monitors have red, green & blue light sources in them that project light onto the monitor screen to replicate colours. Also, as RGB colour has a more limited gamut than photographic paper, the range of colours will once again be reduced.
- The image is printed as part of your brochure. Before this happens, the photo must be converted (either by the user or by the output device) to CMYK (cyan, magenta, yellow & black) colour. CMY is the exact opposite of RGB and is used with the addition of black so that deep, rich colours & blacks are possible when printing on a press or digital printer/copier. CMYK is the model that for technical reasons is always (with a few limited exceptions) used in full colour printing. Also (you guessed it), when converting from RGB colour to CMYK colour, the gamut is reduced even further. Before printing, it's up to the desktop publisher or designer to adjust the colours so they will be pleasing when printed.
In reviewing the above steps, there are a few things that become obvious:
- The image can be technically correct at each step but will differ from the original and will differ from the previous medium it was being used in.
- The issue in colour management with photos becomes one of creating an image that will be visually pleasing in the medium it is being used, as opposed to creating an image that is an exact match of the original (an impossible task).
In reviewing the above information, we can also see that it is an unrealistic goal to have your image look in print exactly as it does on your monitor. This is further complicated by the fact that the colours in an image will look slightly (sometimes vastly) different on each different monitor you view it on.
The question at this point in CMYK colour management becomes: "What is correct colour?" In the world of professional printing, correct colour is achieved when the final print matches the digital data as closely as possible. By using this standard, printers are able to print CMYK colours consistently and in a manner that can be physically measured. You've probably noticed the Cyan, Magenta, Yellow & Black calibration and density bars that appear outside of the crop marks on a printed piece. When a piece is being printed, these bars can be measured using a densitometer to confirm whether the colours in the job are being printed correctly. Also, don't necessarily use your ink jet print to tell you if your colours are balanced correctly. Ink jet printers are usually configured with printer drivers that oversaturate colours. By doing this they are able to appear reasonably close to the RGB colours displayed on screen by the most common office software in use. While this is great in an office setting, it is the kiss of death when printing your document in CMYK on an offset press. Companies such as Adobe have started offering software (Adobe PrintReady) that can be used with your ink jet printer. With this software running, your ink jet will print in a manner that emulates full colour printing the way it will appear on a press or digital printer.
If you want to have truly great looking images in your documents, you have two options:
- Pay to have a good service bureau do your scans.
- Do your homework. There are a number of good books (and info on the web) for learning colour correction and colour management. Unfortunately, there are no shortcuts in this area. A CMYK colour swatch book will give you a good idea as to how many combinations of CMYK look together. Also, there are many books available that will give you examples of what combinations of CMYK colours will create a variety of "memory" colours. "Memory" colours usually refer to such things as skin tones, common foods & natural features that generally appear the same whenever they are viewed and would look unnatural if they were subtly different in colour.
* Please note that you must have a good quality scanner and scan your images at an appropriate resolution to get professional results.
Luckily for everyone, there are proofing options that will significantly reduce the possibility of a final print job having unexpected colours. Many print shops run well calibrated colour laser printers. These devices can give you a relatively inexpensive full colour proof that will deliver a good approximation of what your job will look like on the press (or exactly what it will look like if you are using colour laser printing for your job). When you receive this proof, you have an opportunity to go back to your computer and correct glaring colour imbalances. Some colour correction is relatively simple. For example, if you have an image that is "yellowy," you can open it in Photoshop or Photopaint and reduce the amount of yellow in your CMYK values. If you have very subtle colour requirements, you are often best off to contract a professional in the area of colour correction to adjust it for you.
The final step after a colour laser proof is usually inkjet "plot" proofs. These types of proofs utilize the same "ripped" file that has been produced for your job and will illustrate any final colour problems. Changes are still relatively inexpensive at this stage. It's also good to keep in mind that your final print may look slightly different than your proofs, depending on such issues as what stock it is being printed on. Some stocks can have a slight blue or yellow tinge that will do the equivalent of adding a couple of percentage points of cyan or yellow to your final print. These changes are so subtle, however, that you would not usually even notice them.
While it may seem challenging to master the intricacies of printing images in CMYK, take heart. With a little bit of practice you will get a better and better understanding of the issues involved. Watch for next month's "Desktop Tip" when we will explore the world of "Spot Colour" printing and look into the "Pantone" colours and how they are used.