RGB or CMYK Images: How this is relevant to your publication.

Which type of image?

I'm often asked, 'Which type of image should I use in my publication?' There is a general rule of thumb which helps you decide.  If your book or publication is to be printed, either by a professional printer or using your own home printer, you should use CMYK images.  If, on the other hand you are publishing an ebook or publication to be viewed on display screens, you should use RGB images, as these will look better.

Let's take a look at the differences between the way our eyes detect colour, RGB and CMYK systems:

• Our eyes have special cells in the retina that detect red, green or blue light. Our brain interprets the signals from these cells to give an impression we call colour.
• RGB stands for Red Green Blue. This is the system used by screens and all display methods where colour is created by light directly, e.g. pixels on a screen backlit by LEDs on your E-book reader, TV, laptop or smartphone. This system is also used by most digital cameras and scanners.
• CMYK stands for Cyan Magenta Yellow and Black. This is the system used by printers where the colours are created by inks and reflected light. Printers print in cyan, magenta, yellow and black, as seen in your own inkjet printer cartridges. Printed colour photographs from photographers also use this principle.

Each system has a different ‘colour space’, or palette of colours that they can perceive or reproduce. Furthermore, the ‘make’ of equipment used can also have an effect on good colour reproduction. This needs to be taken into account when converting from one to another.

Consider the following:

• How the image is displayed on your particular screen/ make of equipment.  It will differ between different devices in our own homes and almost certainly differ from those of others.
• The software used to display images will also affect how the colours of an image are displayed.
• There are different algorithms used to convert an RGB image to a CMYK one.
• The program used to achieve the RGB to CMYK conversion will also have an impact.
• The software used by the printer’s equipment will affect how it interprets the CMYK image.
• The inks and the brand of machine will affect CMYK colour printing and reproduction.
• The type of paper and finish will also affect colour reproduction.

Converting from one type of image to another:

If you have a collection of RGB images that you would like to use for print, you can usually convert them to CMYK using most good image editing software or software that converts documents into PDFs.

Many commercial printers will accept RGB images and convert them to CMYK for you.  They will print them automatically using their own algorithms without bothering you with the complexities.

Taking all this into account, if you prefer to adjust the images yourself, you can optimise your screen display on one device, use a standard RGB to CMYK conversion algorithm available in whatever image processing software you have or wish to afford (or that the printer may specify) and then send your CMYK images to the printer.

Remember: The success of a colour conversion can only be seen when the printed colour proof comes back. as the printer will probably have different software and calibration systems.

Ultimately, the type of image you choose is determined by the type of publication being produced; whether it is to be printed or read on screen as an ebook.  If printed, the final result is only known upon seeing the printed proof from the printer. How much time and effort you want to put into worrying about the RGB to CMYK conversion is therefore down to personal choice.

For further advice contact Chris Thomas

chris@miltoncontact.com