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Monochrome yellow vs. filtered or red+green mixing

May 1, 2012

Following the development of LED technology (from red, yellow and green only, to the addition of blue, making possible all the rest of the colors), I learned about “additive mixing” versus “subtractive” mixing. The standard color-wheel based system, by which paints and inks are mixed, and the primary colors are red, blue and yellow, differs from the way light is mixed, where the primaries are red, green and blue. I always wondered why the primaries of the color TV screen (usually printed on the set as part of the TV brand’s logo) had green, instead of yellow. Green is yellow mixed with blue, isn’t it? But in reality, even the red and blue of the TV screen are slightly different than the red and blue you see on those ink bars next to the pictures on some of the color pages of the newspaper (such as the Sunday funnies, or ad circulars). The red appears slightly yellowed, and the blue looks “electric”, like it has a tinge of red.

These however are the primary colors of light, at least to our eyes. What you think is a primary, non-yellowish red ink actually is reflecting a small amount of blue light along with the red. (reflecting, hence “subtractive”, because color is made by filtering out what is not reflected). This is called “magenta” or “fuschsia”, though the primary ink color is deeper than the color with the hexadecimal RRGGBB value of FF00FF of the color screen. It is really more like FF0080. Likewise, the “pure” looking blue is reflecting a small amout of green. This is called cyan, and likewise differs from the 00FFFF of the screen. It is really a color called azure, and is 00C0FF. So yellow is an equal mix of red and green: FFFF00. That’s why yellow light reflects so well off of red and green surfaces. Here are examples I am talking about:
Subtractive: ; Additive:
If you look closely at these bars, you can see two primary colors of each pixel lit for the subtractive colors which mix to form the overall color when you do not look at it closely; while the additive ones have only one pixel lit, representing the color you see upfront. The white background has all three colors lit. So if you’re mixing light, such as LED’s or TV pixels, the primaries are on the right. But to get those colors on a printed page, you mix the ones on the left; in which case you will see the colors on the right are made from small dots of different colors mixed together, or sometimes a transparent “stain”, especially the yellow being printed over the blue to turn it green, or fuchsia to turn it to normal fire engine red.

But what about those brilliant amber light tubes that are seen under overpasses and other places? (A few places use them as street lights, such as Oyster Bay and Babylon townships in Long Island, and they have lined entire tunnels at times, such as the Brooklyn Battery Tunnel!) They distort ALL colors, including red and green, which become dull brownish looking colors. It looks so funny going through a tunnel bathed in that light, and you have on a bright red shirt, that now looks little different from any other color. You’re basically in a black and yellow (“B/Y”) world! (Much like the old Game Boy!)
Well, yellow can be made by mixing red and green, but it can also be a monochrome wavelength of light. In looking at LED’s, you see this, because the color is measured by the nanometer. The bright 590nm LED’s, that began appearing on message signs in the 90’s, are the same color as the low-pressure sodiums, which are the awkward street light bulbs (The peach colored more common street lights are high-pressured sodium).
So I wondered how could that be, if yellow is actually not a prime color. I then realized, and thought up this nice rhyming illustration: vision is caused by


Light is emitted by whichever source, then it travels through whatever medium (air, vacuum, glass, etc. in which it can be altered somehow), and then enters the eye, and then an image is sent through the nerves to the brain. The eyes consist of the red, green and blue “cones”, which receive the waves of light. Light of any wavelength will stimulate certain cones more than others.

So an LED that makes yellow by mixing red (600’s nm) and green (lower 500’s) will send both wavelengths to your eye, and stimulate both the red and green cones, and you’ll perceive “yellow”. Also, if you take white light, and place a “yellow” lens over it, it will filter out the blue, and allow the red and green to pass, and reach your eye, and you’ll again see yellow.
The monochrome amber of the 590nm LED or low pressure sodium will consist of one wavelength, however, it will still stimulate both red and green cones. That is why you perceive it as yellow like the other lights. This is called “metamerism”. However, it will look much more “saturated” than the other yellows, and not reflect off of as many things.

(This page suggests that the “fundamental primary” green which lies way off of the normal spectrum of visibility, is closest to 497 nm on the spectrum, which is basically light blue! So the primary colors are . Looks like a “washed out” version of the regular additive set, and too skewed toward the blue. I would think the fundamental primaries would be something more like
But again, this is the “reception” end we’re talking about, and it’s only what the cones pick up and transmit to the brain. If the most fundamental primaries conceivable; beyond even the eye cone gamut, located on the l,m coordinates of red=1, 0; blue=0, 0; and green=0,1; then this is ultimately a four way symmetry, omitting 1,1; which would be squarely in the “yellow” range.
Perhaps, this is why red, green blue and yellow are treated as “primaries” in brightly colored plastic objects and such, and even a new TV pixel technology called “Quattron”; being said to be the most visible colors to the eye. We just don’t have the cone for the yellow).

This definition of perception also answers the old question: “If a [noise] goes off somewhere where no one hears it, does it really make a sound?”
Well, it would still vibrate the air molecules in waves radiating outward. Even if these waves never encounter any eardrums, they still did vibrate the airwaves, which is what “sound” is in one sense! We could insist on defining “sound” only as when it vibrates an eardum and is perceived by a living brain. So all a person has to do is pick from the two definitions, and they will have their answer.

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