Light vs Ink / RBG vs CYMK
Understanding the difference between light we see from our screens and the ink that printers use to create our photos, is an important piece of the puzzle towards creating great prints at home.
Visible light is part of the electromagnetic spectrum, with different wavelengths representing each of the colours we see.
The light sensitive areas of our eyes have specialist photoreceptors, with the two most well-known being ‘rods’ and ‘cones’. Rods are active in low light situations, whereas the cones require more light to be stimulated, but allow us to discern different colours. In low light, only the rods are stimulated, making it difficult to see colours in the dark.
There are three types of cones present, each being sensitive to a particular wavelength of light, namely Red, Green and Blue. The colour we see depends on how much each type of cone is stimulated. When we stimulate all thee to a sufficient level, we perceive the light as being white light.
Light is Additive
Red and Blue light combine to produce Magenta, Blue and Green produce Cyan, with Green and Red coming together to create Yellow. Combining Red, Green and Blue light results in white light.
Red, Green and Blue (RGB) coming together to form white light is called an ‘additive’ process.
With the Cones in our eyes being divided between Red, Green and Blue light, these are then the base colours upon which we can produce all the light we can see.
The screens on our computers and mobile devices use light to produce colour, with each pixel being made up of varying amounts of Red, Green and Blue light. With just these three primary colours we produce all the colours our monitors display.
RGB ‘Additive’ Light
Ink is Subtractive
When we look at pigments of ink, the way the colours combine is rather different to light. The colour an ink appears depends on what wavelengths of light are absorbed and which are reflected. An ink appears Red as it reflects Red light but absorbs Green and Blue light. A Blue ink appears Blue as it reflects Blue but absorbs Green and Red light.
Imagine then if we combined Blue, Red and Green ink together. Between them, they would absorb Red, Green and Blue light, resulting in a very dark colour approaching Black.
Ink then is a subtractive process, combining different colours together eventually results in black.
Unlike light, Red, Green and Blue would not be suitable primary colours to use as printer ink, with their combination leaving us with very few colours possible to produce.
When printing to paper, the primary colours we use are Cyan, Yellow and Magenta. Combining these allows us to create, Red, Green and Blue, as well as the other colours we require. Combining them all produces Black.
CYMK ‘Subtractive’ Ink
Whereas together the primary colours can combine to produce Black, a separate cartridge of Black ink is usually added alongside. This allows for more economical printing when Black is the only colour required, as well as removing the need to balance the exact amounts of the primary colours required for Black print. Black ink in printing is referred to as Key. Primary colours for printing therefore are CYMK. (Cyan, Yellow, Magenta and Key)
So why use K for Black ? I have seen a few explanations, but not using B to avoid confusion with Blue is the one I have seen most often.
Printers with more colours
A typical home printer is based upon CYMK, but some printers will come with additional colours. My Canon Pro 100S used a total of 8 inks. In addition to the standard Cyan, Yellow, Magenta and Black, it also has Photo-Cyan, Photo-Magenta, Grey and Light Grey inks. These additional inks allow a wider array of seamless colours to be produced.
Whereas the colour Red can be produced using CYMK, you cannot reproduce all the same Red hues you can see on a monitor using the light based RGB system. The addition of a separate Red ink to a printer can allow you to produce a wider colour range that better matches what you can see on your screen. Higher end printers can therefore contain Red and Blue ink for this reason.
Summary
We have discussed how and why Light is an additive process based upon RGB primary colours, and Ink is a subtractive process based upon CYMK. The difference between how we see light on our screens and how we see reflected light in our inks gives us a good foundation to move on to looking at why the image on our screens often looks rather different from the photos we print. This will be the subject of another article.
Recommended resources
If you want to learn more about this subject I would suggest the resources below.
Color Management and Quality Output by Tom P Ashe, is a really good book that covers the above in far more detail and I would highly recommend it.
Jose Rodriguez’s YouTube channel is an amazing resource for anyone thinking of getting into photo printing. There are many videos available covering topics from the basics of colour management through to advanced printing. Really worth taking a look at.