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Offline ironwoman

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Light and paint colours
« on: 28/01/2009 02:23:33 »
Why is it that you mix paint colours of red, blue and yellow to get all colours of the rainbow, but with light (say with the TV) its blue, green and red that are mixed to produce all colours?


 

Offline Chemistry4me

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Light and paint colours
« Reply #1 on: 28/01/2009 02:38:09 »
I am not sure what you mean ironwoman ??????
 

Offline ironwoman

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Light and paint colours
« Reply #2 on: 28/01/2009 06:35:04 »
I thought my question basically said what I meant, but obviously not, so will try to explain my thoughts a little better.

I learned in art (a subject I wasn't good at!) that you only need 3 tubes of paint to create colours - the primary colours red, blue and yellow.  They can be mixed in varying ratios to give orange, purple, green, brown or whatever.  But say you have 3 tubes of paint, say red, green and blue, you cannot get all colours.  For instance, you couldn't get orange by mixing red and green, you would get brown, and obviously you can't mix any of those to get yellow. I guess black and white paint tubes are also needed, but never mind, I need to keep this simple...

Yet, as I understand it is red, green and blue light that are mixed to create all colours (you know when you look really closely at the screen of a cathode ray TV you see lots of little dots in red, green and blue, but not yellow).

Why does one "medium" (say paint) have red, blue and yellow as primary colours and the other (say TV) have red, blue and green?  How does the TV make yellow?

Does that make more sense or I am I a lot further off track than I thought I was??
 

Offline Chemistry4me

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Light and paint colours
« Reply #3 on: 28/01/2009 07:04:27 »
Colour is not about what is on the paper, or on the screen, but what is in your eye.

Your eyes cannot see the full spectrum of colour, but only what approximates to red, green, and blue.  Your eyes cannot see magenta, but sees equal amounts of of red and blue, and assumes it is generated by a colour that is mid way between red and blue, and that colour is what we see as magenta.  When we create artificial colours, we try and fool the eyes (e.g. when you see magenta on a computer screen, you are really being fed equal amounts of red and blue, but the eye and brain assumes that is it actually seeing only a single colour that is midway between red and blue, so it assumes it is really seeing magenta on the computer screen, but it is being fooled into thinking that).

Paint (and printing) is different from screen colours, because what you see on the screen is created by emitted light, while paint and print relies on reflected colours.  This has a significance because on the screen, you can add colours by emitting more light of different colours, whereas when you are relying on reflected colour, you can only take away colour from what is already there.  This is why, when you add all 3 primary colour on a computer screen, you get white (and in order to get black, you must emit no light, and so no colours are added), whereas on the printed page, when you add all the colours you get black (because each colour you add, really just takes away more colour, until you are left with nothing being reflected at all).

The difference between reflected and emitted light also means that the colours regarded as primary colours for the different media are different.  On a computer screen (or TV), because we use emitted light, and create colour by addition of primary colour, the colours that are primary are those that closely relate to the colours the eyes see, i.e. approximately red, green, and blue.  For printed or painted colour, because we rely on reflected colour, and we create new colours by subtraction, the colours that are primary to the process are the complement of the colours our eyes see (i.e. most cheap computer printers will use cyan, yellow, and magenta, as their primary colours – expensive printers will also include inks, mostly lighter versions of the colours so that when the colours become mixed they do not become too dark).

In theory, the order in which the colours are applied should not matter, but in practice some inks, paints, dyes, etc., might impose restrictions that make the order significant (e.g. if a dye does not reflect a pure colour, or if there might be a chemical reaction between the dyes, etc.).  As for colours on a computer (or TV) screen, they are not actually overlaid anyway – all that is happening is that the colours are shown side by side, but so closely packed that the human eye at normal viewing distance cannot separately see the different blobs of red, green, and blue, light and so sees them and a single merged colour.  This is also what happens in half tone printing that is used in cheap newsprint.

http://www.thenakedscientists.com/forum/index.php?topic=6614.msg68662#msg68662

Also have a look here
 

Offline LeeE

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Light and paint colours
« Reply #4 on: 28/01/2009 11:58:31 »
There are two ways in which colours can be mixed; additive and subtractive, and their names give a clue to the principle behind them.  The additive model deals with emitted light, where different colours of light are added together, and the subtractive model deals with reflected light, where the light is filtered to limit the range of colours that're reflected.

With paint, or printed stuff, you're dealing with reflected light, so the subtractive model applies, but when you're working with emitted light, as in computer displays, TV screens or lasers etc, the additive model applies.
 

Offline yor_on

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Light and paint colours
« Reply #5 on: 28/01/2009 16:49:47 »
Nice.
 

lyner

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Light and paint colours
« Reply #6 on: 28/01/2009 23:03:08 »
Quote
Your eyes cannot see the full spectrum of colour, but only what approximates to red, green, and blue.  Your eyes cannot see magenta, but sees equal amounts of of red and blue, and assumes it is generated by a colour that is mid way between red and blue, and that colour is what we see as magenta.  When we create artificial colours, we try and fool the eyes (e.g. when you see magenta on a computer screen, you are really being fed equal amounts of red and blue, but the eye and brain assumes that is it actually seeing only a single colour that is midway between red and blue, so it assumes it is really seeing magenta on the computer screen, but it is being fooled into thinking that).
This is all a bit inside out. Colour is our brain's response to the light received. Colour doesn't 'exist' anywhere else. Our eyes receive a mixture of wavelengths. So called spectral colours are those with a spectrum which is grouped around one particular wavelength. Non - spectral colours are what we experience when the spectrum has more than one peak and we do not see it as being the same colour as that of any single wavelength.

The analysis of our colour vision is, strictly, not based on red green and blue - it is based on three very wideband analysis curves which may be centred around R,G and B but include, between them, all visible wavelengths.
The RGB bit refers to how we can produce the same colour sensation (with lights or TV phosphors) as any colour which falls within the colour space that our three 'primary' light sources encompass by using appropriate amounts of each.
There are many combinations of more than one wavelength which can produce the same subjective effect. Using 'good' red, green and blue phosphors will give the best range of displayable colours with just three sources.

Printing / colour film works by subtracting bands of wavelengths and has severe limitations. If you want a really vivid (saturated) colours by mixing pigments, then you need to subtract so much light  that the picture gets darker and darker. This is such a problem that, to produce good vivid colours in print requires specially made inks / dyes because you can't get them with combinations of just three primaries. Mixing more and more pigments just makes the result darker and more sludgy.
 

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Light and paint colours
« Reply #6 on: 28/01/2009 23:03:08 »

 

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