Slight gripe about the

Cecil's Columns/Staff Reports
1

This is actually not so much a gripe about the colors of birds in general, but the description of the creation of green in parots that Cecil mentioned. Although I understand the gist of Cecil’s statement in his article, it was misleading. The statement was:

This sentence seems to me to imply that if you take a blue gel and a yellow gel and pass light through them, masking the blue gel with the yellow one, the resulting light will be green. This is actually not true, since this is subtractive mixing. Simply masking blue with yellow will block all the light. I believe that what Cecil meant in this case is that because the light is being reflected by two different layers, one yellow and one blue, when the light is recombined, it mixes to green.

In detail, what happens is that the originating light hits the outside layer, yellow, and is split. Part of it is reflected at the yellow portion of the spectrum, the rest is allowed to transmit and then hit the second layer where it is again split, this time at blue. The blue part of it is reflected back through the first layer, which allows it to transmit because of its structure. Then the blue mixes with the yellow. It is the additive mixing of the two reflected colors that creates the green, not the masking of the blue by the yellow.

DISCLAIMER: The above statement regarding a blue gel and a yellow gel will depend on the purity of the blue and yellow and the quality of the gel.

2

I am a little confused with your reply. What is the difference between a blue light and a yellow light mixing together, and a single light passing through a blue and then a yellow gel? I know that Red Green and Blue are the primary colors of light, versus the Red, Yellow and Blue of pigment.

Am I right in understanding, then, that a single light through 2 gels follows the color mixing behavior of light, while 2 colored lights mixing together follow the color mixing behavior of pigment?

3

The column being discussed can be found here : Are blue jays not really blue?

4

Dang, I was going to post this, too. The problem is with transmitted vs absorbed light.

A blue filter removes all colors except blue. A yellow filter removes all colors except yellow. So in the system described in the column, only blue light comes off the feathers. That blue light then passes through a yellow filter, and somehow we get green. Nope. You’d get black. If you pass purely blue light through a yellow filter, none of the light would get through. Of course, this is a simplification. No filter allows only one wavelength through. I think what they really meant was that the blue filter passes mostly blue, with some purple and green thrown in, while the yellow filter passes mostly yellow, with some green and orange thrown in. Thus, only green light could pass through both filters.

Contrast this with transmitted light. If both yellow and blue are reaching your eyes, you see green, which is why we say they mix.

5

No. Light will always follow the mixing behavior of light. The difference is additive mixing versus subtractive mixing. In reality, mixing pigment is only an extension of mixing light, but I won’t get in to that.

Additive mixing means you take different light sources with different filters and combine them to mix a new color. Subtractive mixing means you take one light source and pass it through two or more filters. When this happens, each time a filter transmits the light, it removes colors. For example, if you have a cyan filter, and a magenta filter, the light passes through the cyan filter which removes all but cyan (made of green and blue). It then hits the magenta (made of blue and red) which will remove all but magenta. In this case, because the red had been removed by the cyan filter, you are left with blue. Your resulting light is blue.

In reality, there are few gels that are so perfect as to not allow transmission of any unwanted wavelengths of light, but ideally this is how it works.

Actually, there are filters (very expensive ones) that will block a suficient amount of other wavelengths to call them perfect.

The feathers of birds, however, are not simple filters. They are not pigment based, as Cecil mentioned. It is the physical construction of the feather that acts as a filter by reflecting one wavelength of light and allowing the others to pass through. However, it only reflects that wavelength on one side of the feather. So, reflected light from the second feather (in this case blue) is allowed to pass back through. After is has done that it mixes with the reflected light from the first feather (yellow) to make green.

Hope that made sense this time. :slight_smile:

6

Well, now you’ve got my interest. By all means, if you have the inclination, go into the details of how mxing pigment is an extension of mixing light. It make intuitive sense on some level, since ultimately you are seeing the light, but I’d like to understand the theory behind it

-G

7

I have to interject here regarding L. Guardian’s comments:

My bird Arthur is a blue and gold macaw. He has a green patch on the crown of his head. There are no blue feathers underneath to reflect blue light back through the top feather; also, his feathers are pretty opaque unless held up to a strong light. I have lots of loose feathers around. Arthur’s green feathers, as well as feathers from Amazons, still appear green by themselves. Something else appears to be at work here.
When Arthur’s blue feathers are turned over, the back side is yellow. The yellow is actual pigment and is still yellow when the feathers are wet. Most of his feathers apparently scatter enough light to appear blue; the thicker the feather, the darker the blue. His green feathers may )a be thin enough to show more yellow with the blue, making green, or b) scatter more towards the low (green) end of blue. Come to think of it, his green feathers are pretty thin.
When held up to a light, his and other parrots’ feathers appear to be dusky brown to charcoal grey. No substitute for field work, eh?

8

Sineater, in case it wasn’t really clear from my OP, my objective was to simply clarify the process of color mixing in Cecil’s column. I am not an expert on birds, and I was merely building on Cecil’s description of how the feathers of a parrot work to create green.

However, my OP about the fact that masking blue with yellow does not make green is still valid.

Perhaps someone with more knowledge of the exact construction of a birds feathers can clarify the reality of that situation. It seems to me that the question would be, “Are a parrot’s green feathers yellow on blue, or blue on yellow?” Not having a parrot, I really can’t answer that. It would seem that a parrot’s feathers actually have a yellow pigment base, with a blue “filter” masking it, which would make green. If the top layer is truly blue, then the layer has to scatter the blue light and allow the reflected light from the lower, yellow layer to transmit and mix with the blue to create green. This is what Cecil said, although with yellow on top instead of blue, but that doesn’t matter much.

The only way to actually mask the yellow pigment to create green is to use a cyan filter, not blue. Then, both colors have green, and only green will transmit. However, this is completely different from what Cecil’s column said.
For gdebeer:

I can’t go in to much detail on the pigment vs light thing, but I’ll try to give you some information. Essentially, your intuition is correct. Because the color of a pigment is simply the wavelength(s) of light it reflects, pigment color mixing is nothing more than combining filters to create different colors of light. Pigment colors are subtractive, however, in that they reduce the amount of light. For this reason, artists use a greater number of colors on their palette than lighting designers. In light, you can mix to any hue with just the three light primaries (red, green, blue). In pigment, you can very quickly acheive a neutral color if you’re not careful, especially using primaries. Artists tend to favor the use of partially saturated secondaries and tertiaries for mixing (I think, I’m not really a painter, so this may not be exactly right) for this reason. There are, I think, actually 12 standard pigment colors that artists use.

However, basically, pigment mixing is just subtractive light mixing. Note that above, I used the term “saturated” in the more common, but less correct way. Saturated actually means, I think, how close a color is to primary. More commonly, it’s used to mean how rich or dark the color is, at least in lighting.

9

I must apologize for not stating my position more clearly in my last post. Instead of clarifying the issue, I left the question still open to misunderstanding.

What I had attempted to make clear in my previous post is that the actual case is neither.
Green birds whose green coloration is normal for their breed have feathers which scatter light at a longer wavelength, reflecting green light. This is not a result of masking colors or pigments. Their feathers simply reflect greener light. Arthur’s green feathers contain no yellow pigment. Some birds, such as the turaco, have glossy black feathers that reflect an iridescent, dark green. This is exactly as Cecil described blue refection in the article, just at a longer wavelength.
There are as always exceptions. Selective breeding of yellow or light-colored birds can result in a green variation which is actually caused by blue light scattered by a layer above yellow pigment, thus producing blue through color mixing.
Since the blue in the case of feathers is produced by refraction and not by pigment, “masking” or having a layer of yellow over the refractive layer cannot produce green. In the case of for instance green cockatiels, the green is produced by light reflected from the yellow pigment and being largely scattered. Some light is not scattered and is perceived as yellow; some light is scattered and is perceived as blue. The net perception is green, usually light green.
You can read more about this process at http://www.seacoastpub.com/cpw_mag.html .
As an experiment, try viewing blue feathers with a strong light through a yellow gel. Dyed feathers will appear green. Blue parrot feathers will appear muddy brown.
Once again I apologize for any misunderstanding.