If you have a colour inkjet printer, you might have wondered how it can generate all those colours from just four ink cartridges- black, cyan, magenta and yellow. Actually, it’s pretty interesting- read on to find out more!

Fooling the Eye

To explain this, you first have to understand how the human eye detects colour. There are two types of light-detecting cell in the eye, “rods” and “cones”. The cones can detect colour (the rods don’t, but they work better in low light- this is why colours appear dull or nonexistent in near-dark conditions).

There are three types of cone cells which are sensitive to reddish, greenish and bluish light respectively. A red object mainly stimulates the red-sensitive cells, a green one stimulates the green-sensitive cells and a blue one?… You get the picture.

It gets more interesting when you consider that (say) a yellow object stimulates both the red and green-sensitive cones- that’s how the eye “knows” it’s yellow. However, you can also simulate “real” yellow by mixing red and green lights. Similarly, cyan-coloured objects stimulate both the green and blue cones, and magenta ones stimulate the red and the blue cones. So we can simulate those colours by mixing green and blue, and red and blue respectively. (You can see this in the top picture)

White light is a mixture of all colours, and can be simulated with a mixture of red, green and blue light (see the top picture again). Other colours can be simulated by altering the amount of red, green and blue (e.g. orange contains a bit more red than green and little or no blue- see right).

So far, so good- this is what’s known as additive colour, and it’s how (e.g.) a television generates colour.

Subtractive colour

However, when we’re printing, we start with a sheet of white paper that reflects all colours back at us- that’s why it appears white (see above!). So how do we get other colours from white? Simple- we block- or “subtract”- the ones we don’t want and let the others through. (Hence the name “subtractive” colour).

“Magenta” ink blocks the greenish colours and only lets the reddish and bluish ones get reflected back. And as we said above, red plus blue looks like magenta- bingo!

Cyan ink blocks reddish light- the greenish and bluish colours remaining appear cyan. And yellow ink blocks the bluish colours, leaving red and green which appear yellow (again, see above).

Placing magenta ink (green-blocking) on top of yellow ink (blue-blocking) means that only red light can pass- and that’s how we print “red”!

Placing all three on top of each other should give us black. (Well, that’s theory- we’ll come back to that!) See above right for the effects of overlaying different inkjet ink colours.

Put It All Together…

By altering the amount of tiny ink droplets squirted on the paper by your inkjet printer, we can get the illusion of paler or darker versions of these inks. Choosing the correct amount of magenta (green-blocking), yellow (blue-blocking) and cyan (red-blocking) inks let us generate most colours that the human eye can see.

So What About Black, “Photo” Inks and Others?

Black Ink

Black inkjet ink is required for three reasons. Firstly, due to the limitations of the inks, combining cyan, magenta and yellow never quite give a perfect black- it usually lacks “punch”. Secondly, using three ink colours is a waste of expensive ink. Third, all that ink on the paper tends to make it go soggy and “bleed”, particularly if it’s ordinary photocopier paper.

Cheap printers that only had yellow, magenta and cyan and generated the black by mixing were common in the late-1990s. You don’t see them much now- probably because the results were poor for the reasons given above.

Photo or “Light” Cyan and Magenta Inks

“Photo cyan” (or light cyan) and “Photo magenta” (or light magenta) are paler versions of the standard cyan and magenta inks. They don’t add any new colours, but they can improve the print quality.

If we only have (e.g.) ordinary cyan ink, then the tiny ink spots required to generate the illusion of (say) a pale sky will be sparse and far apart- this may be noticeable to the human eye as a “grainy” look. Having a lighter cyan available means that we can use more dots for the same pale sky and the “dithering” effect is less obvious. (You can see the difference in the picture to the right- the paler cyan makes the “spots” less obvious). Photo magenta has a similar advantage when printing light to mid skin tones.

Yellow dots are near-invisible on the page to start off with, so there isn’t really the same need for separate pale/photo-yellow inks- we’ve never seen them in normal use.

Other Ink Colours

As for other ink colours, we’ve seen inkjet cartridges containing red, green, blue, orange and other ink colours. These are less common than the ones above, but can appear more pure than colours formed by mixing the four standard types. This increases the range of displayable colours slightly.

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