These still produce a single [adjustable] wavelength, which means some colors that are displayable on displays of today are not representable using just one of these, and multiples will be required.
Yes, it’d be two subpixels instead of the current three. It’s not clear that that’s worth the added complexity of having to control each subpixel across two dimensions (brightness and wavelength) instead of just one (brightness).
Yes, mix two complementary colors like orange and cyan. You just need two wavelengths that hit all three cone types [0] in the right ratio. There’s the possibility that it’s subject to more variation across individuals though, as not everyone has exactly the same sensitivity curves.
Human vision in the yellow (~590nm) region is known to be extremely sensitive to particular wavelengths. Observe how quickly things go from green through yellow to amber/orange!
Every single white LED bulb you buy for your light fixtures is a mix of blue LED and yellow phosphor, so in practice it's no problem at all. Although I do concede that the yellow is probably not monochromatic.
It is 100% not monochromatic and that makes all the difference.
Here's one model I'm fairly familar with, having evaluated it for design-in to a product a few years back: https://www.lightstec.com/wp-content/uploads/2018/10/Philips... (apologies for the non-authoritative link, their entire datasheet server appears to be down....)
Take a look at page 8 (PDF page 9), Figure 4, "Relative Spectral Distribution vs. Wavelength". Look at those spectral curves and what that phosphor really does. See that nice broad peak, that's pretty insensitive to the exact details? A little shift in the peak doesn't change the output much. And yet, they still bin white LEDs intensively!
These things just do not work with monochromatic emission in the orange. And the phosphor isn't even that good at low color temperatures (CCTs). Below about 2000K-2400K (ish), this approach doesn't work: the resulting LED looks like yellow trash, not like you'd expect (it should look something like a candle flame). So even phosphors can't get you down all that far in CCT. (There are probably expensive phosphors that can do it... but none were in mass production five or six years ago when I did a deep search.)
Or if pixel density is high enough, adjacent pixels could display the colors to combine with no flickering. Unlike regular RGB subpixels, this would only be needed for areas where the color cannot be displayed by an individual pixel alone.
Yeah, and both techniques can be combined, which common with LCD screens, although it does sometimes lead to visible moving patterns when viewed close up.
There’s more flexibility with tunable wavelengths, though, since there will often be multiple solutions for what colors and intensities can be combined to create a particular photoreceptor response. By cycling through different solutions, I wonder if you could disrupt the brain’s ability to spot any patterns, so that it’s just a very faint noise that you mostly filter out.
Sure, but that’s assuming you need a higher rate than is already used for brightness. That’s a question I think can only be determined experimentally by putting real human eyes on it, although I think you could do the experiment with traditional RGB LEDs. But the other question is whether the wavelength tuning can be changed at the same rate as intensity.
I bet you might run into some interesting problems trying to represent white with two wavelengths. For example, colorblind people (7% of the population) might not perceive your white as white. And I wonder if there is more widespread variation in human eye responses to single wavelengths between primary colors that is not classified as colorblindness but could affect the perception of color balance in a 2-wavelength display.
