The colors behind TVs and Printers

The colors behind TVs and Printers

Posted by nag.rajan on Wed, 2008-03-19 08:26 in

Some time ago, I wondered about this question...
Is there a reason why RGB (Red Green Blue) is used to create pictures in television and CMYK (Cyan Magenta Yellow and Key[black]) is used in printers? Can we use RGB in printers?
Corollary question - What color paint do we get if we mix red paint and green paint?

After some investigation, I compiled a detailed answer, which follows...
There are 2 kinds of objects that we percieve, differentiating by the method by which they manipulate light. There are sources and there are reflectors.

In a television, the CRT "gives out" red, green and blue light. The specturm of these lights adds up to form the final spectrum to make the final colour. Television is a combination of light sources.

On the other hand, Reflectors absorb particular wavelengths out of incident light and reflect the colour that we percieve the object to have.
So a red object absorbs green and blue light and reflects red. (note - this is different from emitting red light on its own). You could say that a red object is a low pass filter for light. Green and Blue objects are mid and high pass filters respectively.

To know more about how we see colour and how vision works,

Now, when we mix red and blue coloured paints, the red part will absorb green/blue and the blue part will absorb the red/green colours. Its like having filters in series connection. So all colours will be absorbed and what we will see is black paint (well, nearly black. We will be able to see some hint of dark brown as the spectrum of paints is broad and there will be parts where the spectrums overlap and we will see that colour in a dark form).

Red, Green and Blue are called primary colours. I am not sure why they are called so, but i think its because our eyes have sensors that have peak responses to these colours.
There also exist secondary colours, which are obtained by the sum of the spectrum of primary colours. They are yellow (RG), magenta (RB) and cyan (GB)

So here is a test question - Using reflective paints RGB, can you make yellow paint?

Secondary to primary light "reflectors" conversion

Red = Magenta + Yellow
Green = Yellow + Cyan
Blue = Cyan + Magenta
Black** = Magenta + Yellow + Cyan
White = Cannot be produced - Thats why the paper base should be white in colour.

This happens in Printers.
Its impossible to get secondary light reflectors from primary light reflectors.

** The black obtained by adding the secondaries is not perfectly black due to the overlap of spectrum (rather its dark and dirty brown), so for a deeper/richer black, pure black colour has to be used. This pure black is the Key in CMYKey.

Primary to secondary light "sources" conversion

Cyan = Green + Blue
Magenta = Red + Blue
Yellow = Red + Green
White = Red + Green + Blue
Black = Cannot be produced - hence the display screen background should be black.

This happens in television.
It is impossible to get primary light sources from secondary light sources.

Moral of the story
1. RGB is used with sources, CMYK is used with reflectors.
2. Hence RGB cannot be used in printers.
3. If you mix Red and Green paint, you will get a dark yellow color (brown like).
4. If we used CMYK in televisions, we will not be able to produce primary colors.

As a passing note - Filters can be placed in series as well as parallel configuration. In our case, for a printer using RGB (if we decide to make one), instead of mixing colors, we can place the colours side by side in a pattern of closely packed dots, taking care that different primary colours do not mix. This is very similar to dots in a television. In that case, we can simulate secondary reflectors using primary reflectors, provided that the eyes cant make out the difference. This, of course, will decrease the pixel density to 1/3 and epson wont be able to boast of 1440 dpi anymore.

Strictly speaking, Television uses a form of parallel reflectors (the screen is actually formed by closely packed dots). A better example of a combination of light sources is an image produced by a projector.