## A question about light and color...

Study matter and its motion through spacetime...

### A question about light and color...

There's probably a really simple answer to this, but I haven't figured it out yet, so I thought I would just ask you awesome peoples.

Here's what I already know, so correct me if any of this is wrong: Color is to light what pitch is to sound, the wavelength of the light is what determines what color our brain will assign to the light we're seeing. With red being the longest wavelength of light that we can see, and violet being the shortest. (Red-orange-yellow-green-blue-violet.) And objects that don't emit their own light appear certain colors because they absorb all other wavelengths and only reflect the one that you see.

Now, since I'm an artist, I was thinking about this in terms of mixing paints one day, and was a little puzzled. See, if you mix red paint, which is reflecting a long wavelength, with the shorter-wavelengthed yellow, you get a color whose wavelength is in-between the two: orange. Likewise, if you mix yellow and blue, you get a color who's wavelength is in-between the two: green.

But if you mix red paint (longest wavelength we can see) with blue paint (very short wavelength) you don't get a wavelength between the two. You get violet, which has a shorter wavelength than blue. How does that happen?

paceetrate

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### Re: A question about light and color...

Let's look at what is going on in the eye first. You have three types of color receptors to detect red, green and blue light. When you see light from an object, the color receptors detect the amounts of red, green and blue light, and this information is put together to get a color.

When you mix colors, the spectra of those colors get superimposed on top of each other. This means that any red is still there, and any blue or green is still there.

So why does it seem that you can get violet by mixing red and blue? The answer is:

You don't.

You get purple. Purple is not the same as violet: They are different colors.Well, it is often called violet. Let's say instead that purple is not the same as spectral violet. Spectral violet actually cannot be made by mixing red, green or blue together - which will be apparent from what you said about its wavelength.
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Paul Almond

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### Re: A question about light and color...

Ah.... Which explains why you can get that wonderful shade of violet on a computer screen, but it never looks right when you print it (even with an awesome and perfectly calibrated printer.)

Thanks!

paceetrate

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### Re: A question about light and color...

There are more interesting oddities about colours seen in reflection compared to colours isolated as spectral lines.

For instance, in reflection, brown is a colour, but there is no brown spectral line. Also, mixing of spectral lines will give no brown hue (although white, which also does not exist as a line, can be generated this way).

Or, concerning thermal emission:

Glowing solids (or massive gaseous bodies such as stars) give different perceived colours with rising temperature (and a long range of "whiteness"), but they never look greenish.
Berthold

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### Re: A question about light and color...

It's more complicated than screen versus printer, computer screens can really only show red, green and blue, so you can never properly get all the colors of the rainbow from them either.

What you can get is something that tickles the receptors in our eyes in just the right way so that it looks to human eyes like any color you could want. Mixing paints (or printing with tiny dots close together) is really just another way to fake it in a way that tickles our eyes just right. In theory, you should be able to get a perfect match, although it would depend on lots of details like lighting, etc.

It is pretty fascinating stuff, with a lot more details and complications; definitely worth flipping through an anatomy textbook for.
magetoo

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### Re: A question about light and color...

I agree with magetoo.

You won't be able to get spectral violet on a computer screen (at least as computer screens are made now), because they work by mixing red, green and blue.

When you print images, a further problem is that printed images are not made out of red, green and blue. Red, green and blue are used for emission. When you combine them on a screen, the light emitted in each frequency adds up to make what you see. When you mix dyes together on paper, the light is not being emitted from the dye: It is being reflected from the paper, and some of it is being absorbed by the dye. Each dye absorbs different frequencies to a different degree. When you mix dyes together, the absorption at different frequencies adds up, so the way dyes combine is totally different from the way emitting elements on a computer screen combine.

This is often described by saying that when emission is involved, colors combine additively, and when things like dye are involved they combine subtractively. This is because when combining colors additively (emission) you add up the light that each color contributes, and that gives you the resulting light, but when combining colors subtractively (absorption) you start with the light source (the light being reflected from the surface before any dye is put on it - typically white for paper) and subtract the light that each color removes by absorption, and that gives you the resultant light.

For this reason, red, green and blue are not suitable primary colors to use when absorption, rather than emission, is involved. If you tried to use a red, green, blue (RGB) system, things would get totally joshed up. Instead a cyan, yellow and magenta system is used. (Black is also typically available as a separate color as well, although you can get it by mixing the other three.) This is known as CMYK. (K means black.) You can see this by opening a color printer and looking at the different colored cartridges.

Note that this does not apply to all cases of reflection. For example, if you have a black wall, and you paint red paint on it, you can clearly make the wall red because the red paint is going to be reflecting red light that the bare wall did not reflect. However, this is not generally the case for printer dyes, which are regarded as just contributing to absorption and combining subtractively.

One thing from all this is that mixing colors is not very simple.
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Paul Almond

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### Re: A question about light and color...

The topic becomes even more fascinating when we consider that some women may be tetrachromats;

http://en.wikipedia.org/wiki/Tetrachrom ... rachromats

Scarlett and Ironclad wrote:Campermon,...a middle aged, middle class, Guardian reading, dad of four, knackered hippy, woolly jumper wearing wino and science teacher.

campermon
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### Re: A question about light and color...

campermon wrote:The topic becomes even more fascinating when we consider that some women may be tetrachromats;

http://en.wikipedia.org/wiki/Tetrachrom ... rachromats

Well, I'm pretty sure I'm not a tetrachromat. Although I did get a perfect score on this test... But that just tests how good you are at distinguishing colors. ;P

paceetrate

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### Re: A question about light and color...

paceetrate wrote:
campermon wrote:The topic becomes even more fascinating when we consider that some women may be tetrachromats;

http://en.wikipedia.org/wiki/Tetrachrom ... rachromats

Well, I'm pretty sure I'm not a tetrachromat. Although I did get a perfect score on this test... But that just tests how good you are at distinguishing colors. ;P

Just took at look at it!

I'm feckin' useless, I didn't know where to start! Mind you, I could never see the numbers in those spot tests and I was told that I am colour 'deficient', whatever that means!

Scarlett and Ironclad wrote:Campermon,...a middle aged, middle class, Guardian reading, dad of four, knackered hippy, woolly jumper wearing wino and science teacher.

campermon
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### Re: A question about light and color...

campermon wrote:

Just took at look at it!

I'm feckin' useless, I didn't know where to start! Mind you, I could never see the numbers in those spot tests and I was told that I am colour 'deficient', whatever that means!

Oh I hated those spot tests. I mean, I could see the different colored spots, but I always had a hard time making out what number they were forming.

Does anybody know of a test to see if you are a tetrachromat? Is it even possible to do over a computer screen?? I found one on some random blog, but it seemed a little dodgy.

paceetrate

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### Re: A question about light and color...

paceetrate wrote:
campermon wrote:

Just took at look at it!

I'm feckin' useless, I didn't know where to start! Mind you, I could never see the numbers in those spot tests and I was told that I am colour 'deficient', whatever that means!

Oh I hated those spot tests. I mean, I could see the different colored spots, but I always had a hard time making out what number they were forming.

Does anybody know of a test to see if you are a tetrachromat? Is it even possible to do over a computer screen?? I found one on some random blog, but it seemed a little dodgy.

It appears that you'd need an anomaloscope to test this; http://scienceray.com/biology/human-bio ... -you-cant/

I don't know if there are any technical reasons why a computer screen could be used. You'd need to speak to an uber-geek!

Scarlett and Ironclad wrote:Campermon,...a middle aged, middle class, Guardian reading, dad of four, knackered hippy, woolly jumper wearing wino and science teacher.

campermon
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### Re: A question about light and color...

campermon wrote:
paceetrate wrote:Does anybody know of a test to see if you are a tetrachromat? Is it even possible to do over a computer screen?? I found one on some random blog, but it seemed a little dodgy.

It appears that you'd need an anomaloscope to test this; http://scienceray.com/biology/human-bio ... -you-cant/

I don't know if there are any technical reasons why a computer screen could be used.

I think you'd need pretty precise control over the wavelengths involved, and that there is no way a computer screen can be used to do that. The differences of the two variants of cones would be tiny...

Unless there is some "side effect" you can check.
magetoo

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### Re: A question about light and color...

campermon wrote:
I'm feckin' useless, I didn't know where to start! Mind you, I could never see the numbers in those spot tests and I was told that I am colour 'deficient', whatever that means!

It means that you shouldn't be the one who cuts the wire when defusing the explosive device with the red LED time display !
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### Re: A question about light and color...

twistor59 wrote:
campermon wrote:
I'm feckin' useless, I didn't know where to start! Mind you, I could never see the numbers in those spot tests and I was told that I am colour 'deficient', whatever that means!

It means that you shouldn't be the one who cuts the wire when defusing the explosive device with the red LED time display !

Scarlett and Ironclad wrote:Campermon,...a middle aged, middle class, Guardian reading, dad of four, knackered hippy, woolly jumper wearing wino and science teacher.

campermon
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### Re: A question about light and color...

paceetrate wrote:
campermon wrote:The topic becomes even more fascinating when we consider that some women may be tetrachromats;

http://en.wikipedia.org/wiki/Tetrachrom ... rachromats

Well, I'm pretty sure I'm not a tetrachromat. Although I did get a perfect score on this test... But that just tests how good you are at distinguishing colors. ;P

Online ColorIQ Challenge Results

Aw
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Horwood Beer-Master

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### Re: A question about light and color...

Horwood Beer-Master wrote:
paceetrate wrote:
campermon wrote:The topic becomes even more fascinating when we consider that some women may be tetrachromats;

http://en.wikipedia.org/wiki/Tetrachrom ... rachromats

Well, I'm pretty sure I'm not a tetrachromat. Although I did get a perfect score on this test... But that just tests how good you are at distinguishing colors. ;P

Online ColorIQ Challenge Results

Aw

I got 37. That still seems fairly low. I imagine that it is the midrange hues that most people have problems distinguishing between if they aren't tetrachromats.
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MedGen

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### Re: A question about light and color...

Wow! I got a score of 4... Which means that I am near the perfect colour acuity of 0.

Does that mean I may be a tetrachromat?

Darkchilde
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### Re: A question about light and color...

Joe09

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### Re: A question about light and color...

Based on your information, below is how your score compares to those of others with similar demographic information.

* Gender: Male
* Age range: 50-59
* Best score for your gender and age range: 0
* Highest score for your gender and age range: 1241

0 ( Perfect Color Acuity )
99 ( Low Color Acuity )

klazmon

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### Re: A question about light and color...

paceetrate wrote:Well, I'm pretty sure I'm not a tetrachromat. Although I did get a perfect score on this test... But that just tests how good you are at distinguishing colors. ;P

I got a perfect score also, which surprised me since there were various colour gradations where I thought I could perceive no difference. For those areas I was going on intuition: and it obviously worked.

Pombolo

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