Additive and subtractive color?

[John Salerno]

I've been reading these two pages on Wikipedia and my head feels like it's spinning. I understand the whole concept of light being absorbed/reflected to create color (subtractive). And I understand what additive color is, but I don't understand how these two interact, or how to tell which is the one producing any given color you might see.

 

For example, if I have a red shirt, how was it created? Is it absorbing all light except red, thus it looks red to me? Or was it created by *combining* colors to make red? (Although in that case, it would just be the single color red. But say the shirt is yellow. Is it absorbing all light except yellow? Or was the shirt made by combining red and green colors?)

 

The example given for additive color was a computer monitor. There are three pixels close together (RGB) which turn on and off to create colors. But is subtractive color not playing any part in this? Why do those colors even *look* red, green, and blue in the first place if not for absorption/reflection of light? Or is this maybe the crucial difference between the two?

 

Hmm, I just had another thought. Would it be correct to say that additive color requires its own light source to make? Such as the computer monitor, or projectors, or theater lights? So in other words, additive color is not possible except for things that are backlit or light-producing on their own? And that subtractive color requires normal light only (i.e. I wouldn't be able to see the red or yellow shirt without the light on, so it can't be additive color)?

[swansont]

I've been reading these two pages on Wikipedia and my head feels like it's spinning. I understand the whole concept of light being absorbed/reflected to create color (subtractive). And I understand what additive color is, but I don't understand how these two interact, or how to tell which is the one producing any given color you might see.

 

For example, if I have a red shirt, how was it created? Is it absorbing all light except red, thus it looks red to me? Or was it created by *combining* colors to make red? (Although in that case, it would just be the single color red. But say the shirt is yellow. Is it absorbing all light except yellow? Or was the shirt made by combining red and green colors?)

 

The example given for additive color was a computer monitor. There are three pixels close together (RGB) which turn on and off to create colors. But is subtractive color not playing any part in this? Why do those colors even *look* red, green, and blue in the first place if not for absorption/reflection of light? Or is this maybe the crucial difference between the two?

 

Hmm, I just had another thought. Would it be correct to say that additive color requires its own light source to make? Such as the computer monitor, or projectors, or theater lights? So in other words, additive color is not possible except for things that are backlit or light-producing on their own? And that subtractive color requires normal light only (i.e. I wouldn't be able to see the red or yellow shirt without the light on, so it can't be additive color)?

 

Yes, that's it. Additive color involves a source, and subtractive color involves absorption (which generally accompanies reflection). Computer monitors/TVs aren't technically additive, since the pixels don't overlap, but instead rely on diffraction present in the eye to give the effect.

 

Color perception is intimately tied in with how the eye works. The yellow shirt can be perceived as yellow because it reflects yellow light, or because it reflects both green and red.

[John Salerno]

I think I'm starting to understand it better now. When I was lying in bed last night in the dark I started thinking of it this way: I can't see the color of anything in my room because there is no light present to reflect off the objects. However, if I had my TV on, I would see colors despite the absence of other light, because the TV itself is producing specified colors. So when I see red on TV, it's because the actual color red is being shown in the TV, rather than absorbing outside light (which isn't present anyway) and reflecting red. Does that make sense?

 

The yellow shirt can be perceived as yellow because it reflects yellow light, or because it reflects both green and red.

 

This is sort of what led me to even ask the question in the first place, and I still don't quite understand this. If I can't see the yellow shirt in the dark, wouldn't that suggest that it's only subtractive color? Or are you saying that perhaps the shirt was made to reflect red and green, which is still a subtractive process, but then my eye uses the additive color process to combine them to yellow?

 

At the most fundamental level, I suppose I'm sort of asking, how is the shirt made? What goes on in the factory to make it? Is it made with dyes that reflect yellow? Or that reflect red and green? Is it at this point (the making) that determines whether I'm seeing a subtractive or additive color later when I buy the shirt? Or does the type of color I'm seeing (subtractive or additive) change based on the conditions in which I'm viewing the shirt?

Edited August 29, 2010 by John Salerno

[Fuzzwood]

If I can't see the yellow shirt in the dark, wouldn't that suggest that it's only subtractive color? <-- it is. And whether I shine white light or red/green light on it, it doesnt matter. If I shine blue light on it, it should stay invisible.

[Sisyphus]

The human eye has no way to distinguish between a source that produces both red and green light, and a source that produces yellow light.

 

Similarly, under white light (a mixture of all colors), the human eye has no way to distinguish between an objects which reflects both red and green light (and absorbs all others) and an object which reflects yellow light (and absorbs all others). This makes sense - it's still just red+green photons and yellow photons entering your eye, whether or not it's direct from the source or reflected. However, those two objects would look different under different colored lights. For example, under pure red light, the first objects would look red because it reflects red light, and the second object would look black because it doesn't.

[John Salerno]

If I can't see the yellow shirt in the dark, wouldn't that suggest that it's only subtractive color? <-- it is. And whether I shine white light or red/green light on it, it doesnt matter. If I shine blue light on it, it should stay invisible.

 

What do you mean by "invisible"? If you shine blue light on a yellow shirt, wouldn't it be black? Or do you mean "invisible" in the dark, i.e. it would appear black in the dark?

 

But if you shine red or green light on it, wouldn't that change it somehow? If you shine just red, would the shirt now look red (if it's reflecting red and green light)? Or would it still appear black if the shirt is only reflecting yellow light?

 

This is why I'm still confused about what exactly is making us see yellow on the shirt. I understand that our eyes can tell the difference between yellow light and red/green light combined, but still, there has to be an answer to what is producing the yellow shirt, doesn't there? Either it's reflecting yellow light, or it's reflecting red/green light.

 

Would shining just a red light or just a green light on the shirt answer this question?

[Sisyphus]

What do you mean by "invisible"? If you shine blue light on a yellow shirt, wouldn't it be black? Or do you mean "invisible" in the dark, i.e. it would appear black in the dark?

 

If the shirt doesn't reflect any blue light and you shine pure blue light on it, it will look black.

 

But if you shine red or green light on it, wouldn't that change it somehow? If you shine just red, would the shirt now look red (if it's reflecting red and green light)?

 

Yes.

 

Or would it still appear black if the shirt is only reflecting yellow light?

 

Yes.

 

This is why I'm still confused about what exactly is making us see yellow on the shirt. I understand that our eyes can tell the difference between yellow light and red/green light combined, but still, there has to be an answer to what is producing the yellow shirt, doesn't there? Either it's reflecting yellow light, or it's reflecting red/green light.

 

Would shining just a red light or just a green light on the shirt answer this question?

 

Yes, exactly. You could tell the difference using different colored lights. What makes the shirt yellow depends on the shirt in question.

[John Salerno]

Yes, exactly. You could tell the difference using different colored lights. What makes the shirt yellow depends on the shirt in question.

 

Ok, it's all sinking in now. I would try this experiment if I had access to all these colored lights, but first just a hypothetical situation. Let's say you have two different yellow shirts:

 

Shirt #1: In the dark (no white light), you shine a red light on the shirt. The spot where the red light is shining now makes that portion of the shirt look red. This means the shirt is yellow because it is reflecting red and green light (when white light is present).

 

Shirt #2: Again, in the dark (no white light), you shine a red light on the shirt. The spot where the red light is shining now makes that portion of the shirt look black. This means the shirt is yellow because it is reflecting yellow light (when white light is present).

 

Given the above, I have three questions:

 

1. Is it possible to get both of these results with two different shirts? Or can it only be one or the other all the time?

 

2. If it is possible, what does each of the two cases mean for the shirt? What do the cases say about how the shirt was made, or what makes it look yellow to us?

 

3. If it is possible, why would there even be two different ways to produce a yellow shirt? Again I guess this comes down to how shirts are actually manufactured. Are they made with yellow-reflecting dyes, or with red/green-reflecting dyes, or both? If both, why are there two methods to do the same thing?

Edited August 29, 2010 by John Salerno