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Author Topic: am i looking at true yellow or a mix of red and green?  (Read 2053 times)

paul.fr

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not that it really matters, but how could i tell?


 

another_someone

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am i looking at true yellow or a mix of red and green?
« Reply #1 on: 24/11/2007 00:42:34 »
The human eye (assuming you are not colour blind) cannot distinguish between the two; but if you use a narrow band colour filter, then you could filter out the green light, and be left with red or pure yellow (alternatively, you could filter out the red, and be left with green or yellow; or filter out the yellow, leaving the mixed read and green to show as yellow, but the true yellow itself  would become black).

Note that this requires narrow band filters rather than wide band filters, so that you only effect the colour you desire - this is particularly true when you are filtering out the yellow, since a wide band filter could easily spill over to having some effect on the red and green.
« Last Edit: 24/11/2007 00:45:33 by another_someone »
 

Offline Soul Surfer

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am i looking at true yellow or a mix of red and green?
« Reply #2 on: 24/11/2007 08:27:13 »
Although the eye could tell the difference if the colours were matched correctly. A simple direct vision spectrometer pointed at the objeccts would quickly show the difference as long as the red yellow and green sources were reasonably pure spectrally themselves (narrow band). Most coloured pigments however have relatively broad band spectra ans so it would be more difficult but it would probably still be detectable.
 

Offline Bored chemist

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am i looking at true yellow or a mix of red and green?
« Reply #3 on: 24/11/2007 19:15:42 »
Look at a tomato.
If you are trying to see if its a lamp that emoits red and grren light or yellow light then put the tomato in the beam from the light, if you are trying to see if an object reflects red and green light or yellow light look at the tomato by the light reflected off the object.

If the tomato looks red then it's a mix of red and green lights, if it looks yellow then it's yellow light.
 

lyner

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am i looking at true yellow or a mix of red and green?
« Reply #4 on: 25/11/2007 00:26:05 »
The eye does the best it can with three analysis curves. You can fool it with funny lighting or with a colour TV screen, The eye is not a spectrometer - certainly not once the signals have got into the signal processing on the way to the brain. It sees 'yellow, when presented with red and green because the red and green sensors produce the same relative output levels as when you are presented with spectral yellow. (The sensors are broad band and have no way of 'knowing' which actual frequency is arriving.)
A tv camera can make the same 'mistake' because it, too, has a three colour analysis - similar to that of our eyes.

The 'colour' is a concept entirely in your head. Most people happen to agree, roughly about what to call a certain colour but, as it's not a skill needed to survive in the stone age, we don't do any better than that.
My daughter seldom agrees with me about colours. (Or about most things, actually!)
« Last Edit: 25/11/2007 17:00:48 by sophiecentaur »
 

The Naked Scientists Forum

am i looking at true yellow or a mix of red and green?
« Reply #4 on: 25/11/2007 00:26:05 »

 

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