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Author Topic: Why do televisions use red, green and blue?  (Read 7459 times)

Rob Bartlett

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Why do televisions use red, green and blue?
« on: 23/08/2008 15:51:43 »
Rob Bartlett  asked the Naked Scientists:

Hi Chris,

I listen to your podcast every week - definitely one of my favorites!  I recently found the Ask The Naked Scientists podcast and love it...  Thanks to you and your crew for the energy that you put into it.

Anyway, I have a question that you all can probably answer:

If the primary colors are yellow, red, and blue, then why are the pixels in a television green, red, and blue?

Thanks!

Rob Bartlett
Kennebunk, Maine, USA

What do you think?


 

lyner

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Why do televisions use red, green and blue?
« Reply #1 on: 23/08/2008 17:30:01 »
The 'primary colours' are not red, yellow and blue. If, by 'primary' you mean the colours with which, by additive mixing, you can produce a large number of colours, then Red, Green and Blue phosphors give a big range of displayed colours. For subtractive mixing (as with pigments and filters) the 'primaries' which are used are Yellow, Magenta and Cyan.
See this topic, which deals with your question.
http://www.thenakedscientists.com/forum/index.php?topic=16368.0
 

Offline graham.d

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Why do televisions use red, green and blue?
« Reply #2 on: 23/08/2008 23:00:12 »
Red, Green and Blue are used in TVs and our eyes perceive a natural mix of colours because our eyes essentially also detect Red, Green and Blue, although the "cones" (the cells in the eyes that detect colour) have spectral response curves that overlap into each other. The "green" receptors probably peaks more in the yellow part of the spectrum, for example, and overlaps quite a bit with the red receptors' response curve. Because of this these receptors don't distinguish very well between the colours and our perception of colour is "fooled" by a mix of colours that stimulates the nerves with the right balance. So yellow will stimulate the red and the green receptors but in the same way the right mix of green and red will do exactly the same thing. This is why we perceive mixes of colours to be the same as a single colour of some intermediate wavelength. A different species (other than a human) may not perceive TV pictures as representative of reality.
 

lyner

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Why do televisions use red, green and blue?
« Reply #3 on: 26/08/2008 15:04:23 »
Quote
A different species (other than a human) may not perceive TV pictures as representative of reality.
Any other analysis system will get it 'wrong'. Try, for instance, to get a good (film) photograph of a colour tv picture; you never get a good match. That's because the film analysis is not good. The Analysis of an electronic camera is much much better because you can get much closer to the analysis of your eye.
As long as it's done properly (the appropriate amounts of each primary), you can use virtually any set of phosphors and get any chosen colour, reliably. The only proviso is that the target colour must lie within the triangle corresponding to the three primary phosphors (On the CIE chart, that is).
When they produce a 'better' phosphor, it usually means that it gives you a larger area of primary triangle and allows you to display more colours.
 

lyner

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Why do televisions use red, green and blue?
« Reply #4 on: 02/09/2008 09:21:53 »
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Using yellow in place of one of the primary additive colours would produce a hole in the colour spectrum produced by that device, because it would be unable to produce the entire spectrum using two primary colours and one secondary colour.

It would not produce a "hole in the spectrum", necessarily. What it would do is limit the number of displayable colours.
In fact, using primaries- even spectral R,G and B, still fails to produce other spectral colours because spectral colours lie on the outer curve of CIE chromaticity space and the primaries can only produce colours within the triangle of straight lines between them.
 

Offline Pumblechook

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Why do televisions use red, green and blue?
« Reply #5 on: 16/09/2008 21:20:42 »
Turn an old fashioned telly upside down and the colours will change.
 

lyner

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Why do televisions use red, green and blue?
« Reply #6 on: 16/09/2008 21:35:50 »
And the valves will fall out.
 

Offline Pumblechook

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Why do televisions use red, green and blue?
« Reply #7 on: 16/09/2008 22:05:27 »
I wasn't thinking quite that old.
 

lyner

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Why do televisions use red, green and blue?
« Reply #8 on: 16/09/2008 22:17:52 »
Convergence and purity used to be a real problem with old colour tubes. They had to be adjusted more or less in situ to get it right. The local magnetic field lines point downwards rather than parallel to the Earth's surface in the UK (angle of dip). turning the set upside down negates the vertical compensation - makes it twice as bad, in fact.
Modern tubes have better electron optics - due to computer power, I suspect, with better design. The old shadow mask tube was particularly susceptible to stray fields, too.
 

Offline Evie

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Why do televisions use red, green and blue?
« Reply #9 on: 16/09/2008 22:26:31 »
This reminds me of a science "oops!" I came across while in high school.

To set up the story, I was quite active in theater during my teenage years and when lighting a stage, you are taught that the "primary" colors of light are red, green, and blue. The overhead lights above the stage were rows of red, green, and blue and when all up full, or at the same levels, the light was white. Depending on the combination of levels used, you could wash the stage with almost any color you could think of, and we had a nifty little flip book telling us the percentages needed.

Where I lived, we had a children's science discovery museum, and one of the exhibits talked about the "primary" colors red, yellow, and blue. All of the exhibits in this museum incorporated some sort of hands-on element (to make it kid-friendly) and the way they decided to show color mixing was by using lights. I guess using paint or something like that would be too messy. Well, whoever came up with this idea must not have known that you needed green instead of yellow for the lights, and of course when you tried to move the little spotlights they had and get a range of colors, it didn't really work. I couldn't actually find a museum employee, so ended up writing down on a comment card what their problem was. I was incredibly dissapointed that a "science" museum had gotten something so wrong!  :)
 

Offline Pumblechook

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Why do televisions use red, green and blue?
« Reply #10 on: 16/09/2008 23:46:22 »
Yes, it is the vertical component of the magnetic field which pushes the beam sideways (horizontally) like a current carrying wire moves at right angles to magnetic lines of flux.  Most later tubes have  short vertical elongated phosphors with a red above a red etc so the beam has to move sideways to cause a colour shift so nothing to do with gravity.  Also it has to be the vertical component because it doesn't matter which way the set is orientated..screen facing north or whatever.  Turn the set on its side and then colous will shift depending on which way the set is pointing because it is now sensitive to the horizontal component of the magnetic field. 
 

Offline Make it Lady

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Why do televisions use red, green and blue?
« Reply #11 on: 17/09/2008 00:20:29 »
The printing industry use cyan, magenta and yellow. Why doesn't the TV use these as their 3 colours instead. It seems to work well to me.
 

Offline lightarrow

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Why do televisions use red, green and blue?
« Reply #12 on: 17/09/2008 14:56:01 »
The printing industry use cyan, magenta and yellow. Why doesn't the TV use these as their 3 colours instead. It seems to work well to me.

Those are the primary colours in subtractive synthesis (pigments on paper)
http://en.wikipedia.org/wiki/Subtractive_color

not in additive syntesis (lights on/from a screen)
http://en.wikipedia.org/wiki/Additive_color_system
 

lyner

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Why do televisions use red, green and blue?
« Reply #13 on: 18/09/2008 23:03:57 »
Cyan, Magenta and Yellow are, in fact, minus red, minus green and minus blue, in conventional colourimetric terms.
 

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Why do televisions use red, green and blue?
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