Two of my enlargers work with sharp-cutting narrow-band RGB filters. There is nothing subjective
about this whatsoever. These are true primaries. However, for convenience, the actual control
panel is YMC, which makes it easier to relate to starting points on paper batches, densitometer readings, and similarity to conventional subtractive enlargers. All this is related to the basic physiology and physics of color. Back when they taught YRB as finger paint primaries, it was to kids eating Elmer's glue.
As you can see, there are a multitude of ways to look at this problem, and not every way will click for every person.
Since you said 'let's get back to cones', that's what I'll do. Our eyes have rods and cones; cones are for color, rods are b&w and used in low light.
For whatever reason, our eyes are capable of seeing a region of the electromagnetic spectrum from about 400-700nm; blue-violet to deep red. This is probably a very useful range if you live on this planet and live like we do, whereas some animals (gulls for instance) can see into the UV. So in this respect, the color theory problem is a uniquely human concern. Our color prints might not look like reproductions of reality to an alien with a different range of sensitivity. Likewise, an example of two-color photography might look like a perfect reproduction of reality to someone who is color blind (missing 1 channel of sensitivity).
But back to Earth... evolution has seen fit to give us this range, and it doesn't do so by making our eye continuously sensitive to every wavelength between 400-700nm, it achieves it in a way that's quite similar to color photography, with 3 sensitive elements that each have a sensitivity curve. Through varying proportions of these 3 sensitivities, we get the sensation of all colors. This is the Young-Helmholtz theory of color vision, and these "curves" were an intense area of interest to color photography researchers around the turn of the century. See A Handbook of Photography in Colours by Tallent, Bolas, Senior.
3 cones and 3 sensitivities, with peaks at 420–440nm (B), 534–545nm (G), and 564–580nm (R). Curious that they resemble our color separation filters, and also the senstivities of the 3 layers in a color film...
RGB are the primaries, CMY are the secondaries (or if you must, subtractive primaries).
Additive systems assume the use of 3 separate lights to modulate RGB. Subtractive systems only have 1 light to work with, and thus can only subtract from white light.
As Steve pointed out, CMY colors will "minus" one color as light passes through them. Whereas RGB colors will minus two. So any two colors from RGB will minus everything, making black. Whereas mixing two "minus-one" colors will make an intermediate color. An RGB set can never make a color print on paper (or transparency) because of the preceding fact, and no system has ever used this.
If we want to make red on a print, we need to overlap yellow & magenta, which will subtract blue and green, leaving red.
The difference between additive and subtractive can be summed up like this: are we trying to create black or create white? With additive systems, black is the default, and we add lights to create white. Subtractive processes default to white and we have to subtract from this to get black.
I hope this is clear and I haven't just further muddied the water...
Last edited by holmburgers; 04-13-2012 at 12:16 PM. Click to view previous post history.
Reason: clean up
What's really amazing is how close the human Red and Green cones are in spectral response.
It's a little late now, but I believe we could have gotten away with making the three primaries Blue, Green and Greenish-Yellow.
Yes it is very surprising that the so-called "red" cone has its sensitivity peak at yellow*, not red. I suppose that because the human vision is more complex than that, and because there is crosstalk between different color channels (different cone types), the purest colors can be achieved with primaries differing somewhat from what the peaks would be if there was only one type of cones present at a time. Yellow light, while it indeed peaks the red cone, also causes a significant signal in green cones at the same time. This simultaneous signal causes the yellow vision.
Remember, we are not seeking for maximally bright vision with minimum energy (or maximum efficiency); we just want a maximum number of different colors representable with minimum number of color components possible. If we were to design an energy-saving computer screen, we might want to select those cone peak wavelengths. We would get red output much higher with much less power, but we wouldn't be able to produce pure red, it would be orange.
In fact, this is clearly noticeable in the response level in practice; at a constant power, 660 nm light, while being very pure red, is quite inefficient (dark to human eye) compared to 600-620 orangish-red light which is bright. This was the key difference when "super bright" red LEDs and laser diodes were invented; it was not a power up.
If you want to represent all visible colors with just three primaries, you have to choose them very closely right and there is no option. This has been proven numerous times experimentally and anyone can repeat that very easily. In fact, the current primaries widely used are not optimum. There were problems finding suitable green phosphor for CRT tubes, and the green which was selected is way too yellow. This is why you cannot see certain vivid bluish-green hues on a typical CRT TV or computer screen, but can see those in a cinema projection on a film.
Originally Posted by Bill Burk
*) And note, when I say yellow here, it means a single wavelength between red and green, a different thing that yellow in subtractive CMY where it is a wide-banded yellow, including red, yellow and green wavelengths.
Last edited by hrst; 04-13-2012 at 05:01 PM. Click to view previous post history.
Thanks SIWA for helping me to understand how colors are formed in Helsinki. Fooling, really, thank you for the good description and the dethroning of the 'major' difference between 'primary' and 'secondary'. It really devloves into semantics. And, the lack of purity was also worthy to bring into the equation: how versed in the scientific valuation of hues is the average high school art teacher?
But I do wish there was a 'standard' that both artists and projectionists could finally agree upon. Most, here, are saying that, essentially, there really is no difference between the two, only subjective evaluation.
And, holmberger, you confirm that there is a more solid aspect to the RGB as it removes not one but two colors. Thank you.
Seriously, I am not up to your collective sensitometric level but I did round out my limited knowledge here. - David Lyga
Last edited by David Lyga; 04-13-2012 at 05:25 PM. Click to view previous post history.
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Just to add a response to the OP, Many men are red/green colour blind.
“The contemplation of things as they are, without error or confusion, without substitution or imposture, is in itself a nobler thing than a whole harvest of invention”
Originally Posted by David Lyga
The additive primaries are Red, Green, and Blue, commonly abbreviated RGB. Light of those three colors, when projected together, appear white to the human eye.
The subtractive (or complimentary) primaries are Cyan, Magenta, and Yellow. These plus black* are used for printing and color reproduction, and abbreviated CMYK.
Both of those abbreviations, with their underlying meaning, are in common usage in industry.
The problem that arises is that the subtractive primaries are presented in school as "red, blue, and yellow", when in fact they're no such thing. They're magenta, cyan, and yellow.
*nb. Black is included because combining cyan, magenta, and yellow yields a yucky dark color that is not black, nor any other definable color, and it looks terrible.
Last edited by Leigh B; 04-13-2012 at 05:52 PM. Click to view previous post history.
“Wise men talk because they have something to say; fools, because they have to say something.” - Plato
wavelength vs color
An important distinction:
Light of a single (or narrow band) of wavelength, such as a laser, appears to color vision as having a color.
However, natural color, such as light reflected from grass, has a reasonably wide range of wavelengths.
The perception of color starts from the stimulation of the color sensors in the eye, whose wavelength responses overlap.
Similarly, the color-sensitive chemicals in color photographic film have wavelength responses that overlap, as graphed in the film data sheets.
I agree with Leigh, it's pretty simple. The only problem is the school. Just like the taste map thing. It's funny to hear we have exactly the same misinformation and the same legends all over the world .
These are all scientific problems from 1700-1800. They were all clear in the beginning of 1900, but they are still unclear to the teachers whose responsibility is to teach those things to kids, and even to some authors who have been taught by these kind of teachers.
It's understandable that "laymen" do not check every "fact" they hear. However, teachers and authors should be a bit more careful.
To further elaborate on this, yes, it's because the C, M & Y pigments that are used are not perfect or pure, so, these all together are not able to remove some wavelengths even when they should.
Originally Posted by Leigh B
But there is an another reason for CMYK, too, and it's even more important. It's because of the registration problems in offset printing. You usually cannot match the positions of the records perfectly with each other. With photos, a small misalignment is not a huge problem, but it makes small text and fine graphics unreadable. That's why they are printed only in black ink.
And, to further escalate the story, so called "spot inks" can be used, especially in product packaging, to create graphics in any color without unwanted halftoning artifacts (raster of small dots to create "intermediate" brightness values in ink based printing) .
Now, this does not relate to the color theory or photography anymore . But I think the color theory is already quite well described here by many .