"If blue, red, and yellow are primary colors, why do color TVs use blue, red, and green?"

Sharp just released a new RGBY TV (or Red, Green, Blue, Yellow TV).

In a previous Cecil article he addressed how to to use RGB to create all the colors as well as RBY, the primary colors. Now, Sharp is touting this new 4-color technology saying that it has more colors.

Is this a marketing gimmick or is it the real deal? How do you even measure the number of colors a particular palette will yield?

If you think of the hue of colors (that is, their actual “color”, not brightness or saturation) being spread out on a 2-d plane like this,the having three primaries basically lets you describe anything in the triangle defined by those colors.

If you have four primaries, you get a quadrangle of some sort, which might or might not contain more space (and therefore more colors), but they probably wouldn’t bother if it didn’t.

“Number of colors” basically equates to area covered by the perimiter of the primaries on such a chart.

(Note that real color space charts (based on wavelength) have a different shape than this circle, but the idea is the same).

Fantastic, straight-forward answer! I dig the visual aids.

So would the RBY hue range be bigger or smaller than the RGB hue range?

RBY can’t create green, if you’re mixing light. RGBY may well give you more vivid yellows though, just as when you’re printing CMYK gives you a clearer black than CMY alone.

Um… Doesn’t blue and yellow make green?

Not when you’re adding colours like a TV does with light, rather than subtracting like when you mix pigment.

No, sure you can. Just checkerboard a screen with blue and yellow pixels, stand far enough back and - badoomp - green. Think pointilist.

Maybe if you use cyan, you’ll see something resembling green. According to color theory, if you mix pure blue light with yellow light (a mixture of green and red), you’ll get white.
Powers &8^]

Regarding color mixing, there’s a few things to keep in mind. The whole “yellow and green makes blue” idea comes from subtractive coloring, like what happens with paints and dyes; this is NOT the case with additive coloring. You will NOT see green when you mix yellow and blue light because, as far as the human eye is concerned, they’re opposing colors and will cancel out, so unless there’s other colors in the same area, you’ll most likely see white, or some shade of gray.

Anyway, to the OP, I think the visual aid in the first response pretty much nails what you’re after. I think one part that really makes the technology make sense is considering the way the eye sees colors (blue/yellow are opposites, and red/green are opposites), so it ought to allow this new television to create virtually any color that the human eye can perceive. That said, I’m unsure how much difference it will make, mostly because that additional information is lacking in the traditional video/image format. I suspect we won’t really get to see the technology in action until a new video format comes into popular usage that can take advantage of it.

Well, as the Land Experiment showed, these matters are surprising complex.

Maybe RGBY looks better to tetrachromats…

I’ve heard that the eye is can distinguish between a larger number of green shades than shades of the other colors. A standard RGB display with 256 values for each color is not very good at showing pictures of nature, for instance. Maybe they’ve added yellow to make up for that.

I just tried. I got grey. I expect pointilists need to think in additive colours as well.

light bulb appears above head

I had no idea that displays were that restricted. I thought there were many more values possible. Is that another way my analog CRT will look better?

I’m not sure how an RGBY TV can be better in this regard, anyways, as the signal it receives will still be RGB. It’s not like it’s analog and can be switched to whatever.

Well, except that the human eye perceives red, blue and green - it has receptors for each of them, and the brain mixes the signal to build the colour picture. That’s kinda sorta the reason why the colour screen is made the way it is. Blue and yellow *are *opposites, but the opposite of green is magenta, not red, and the opposite of red is cyan, not green.

Of course the eye’s response curves are lumpy, and at least one of them has at least two maxima (not to mention the problem of tetrachromatism), so there’s no such thing as a phosphor, LED, or whatever that exactly matches them. Perhaps this RGBY scheme offers a pragmatic correction there.

I have to point out, though, that 256 levels for each of red, green, and blue makes for a total of 16,777,216 colors, which is a lot more than you ever saw in a Crayola[sup]®[/sup] box.

I want one with ultraviolet. Plus UV-transparent lenses so I can see it. More colours! Must have more colours!

Yes, if we’re talking about light. But Blaster Master was talking about the way colors are perceived by human brains. The inputs into the red, green, and blue cone cells are evaluated by our visual systems on three axes – light/dark, red/green, and blue/yellow.

See http://en.wikipedia.org/wiki/Opponent_process for more information. Significantly, it’s the reason that red, green, blue, and yellow (and black and white) are often considered “primary” colors even though they are not all so under traditional color theory.

That said, although I had the same lightbulb as Sinisterniik at first, the points raised subsequently – that the signal is still RGB and thus the Y component can only be synthesized somehow, and that we don’t actually have Y-detecting cone cells in our eyes – are quite valid and make me wonder how this new Y channel is supposed to help anything.
Powers &8^]

Not really. People talk about red, green and blue sensitive cones, but in fact the peak response of the so-called red cones is at a wavelength of 560nm, which is actually a greenish yellow, and the peak responses of the other two cone types are not at wavelengths that you would experience as perfect, unmixed blue or green either (although closer). Also, all the cone types respond to some degree to light across most of the visible spectrum. The colors we see depend on the differences between the responses of the different cone types, but not in a straightforward way. As others have pointed out, even before the color signal leaves the retina it has been organized into two opponent channels that code the degree of red versus green, and blue versus yellow respectively.

My guess would be that the extra color plane is used to compensate for inadequacies in the underlying LCD technology.

It’s worse than that, as compressed video typically utilizes a YCbCr colorspace with subsampled chroma planes. More like RGB converted to luma/chroma with 75% of the chroma information discarded then converted back to RGB.