Why can’t they reduce voltage to each pixel to reduce brightness? I don’t see this as a fundamental limitation of the display technology, PWM is just easy and cheap to implement.
This bandgap corresponds to the photon energy of the emitted light as electrons get excited due to the applied voltage as electrons are excited to the conduction band and then relax back to the ground state giving off light.
The bandgap energy changes as a function of temperature. The primary reason for this is that the lattice constants increases as temperature increases. This causes the bandgap to decrease, meaning the energy of the photons is less giving a longer wavelength.
The opposite effect is also true, cooling a LED will lead to a shorter wavelength. Here is a cool video showing the effect![1]
Increasing the current through the LED may change the temperature by a little bit but you need large temperature changes to have any effect.
The temperature has a much greater impact on the intensity of light emitted by the LED. I have seen a typically 1% decrease in intensity per degree C for the LEDs I have tested. This is the effect that matters most when using RGB leds as if the red led gets dimmer cause it is hot, than the green or blue, it will be seen as a color change, even though the center wave length of its emission is unchanged.
I mostly just wanted to share things I have learned about LEDs over the past year or two and your comment gave me a good opportunity!
I could plausibly see some color shift at close to bandgap voltage if there isn't a perfect uniformity in bandgap across a diode, inconsistent or even just gaussian distributed doping would result in some holes being preferentially excited if there isn't a sufficient surplus V?
[1] https://www.researchgate.net/publication/250139383_Dependenc...
Really? How does JPEG get away with severely degrading color information when compressing then? I thought it’s brightness that we are sensitive to.
> Digital pulse-width modulation is well-suited to driving microLED displays. MicroLEDs experience a color shift as the current magnitude changes. Analog schemes change current to change brightness. With a digital pulse, only one current value is used for the on state. Thus, there is no color shift that occurs as brightness changes.
In this regard, AMOLED displays have a strong disadvantage. If you feed less voltage to the organic diodes, not only do they limit their brightness, but their color also changes, so that there might suddenly be visible differences in the color reproduction.
Based on that article, non-PWM based solutions seem to be very hard to implement for smaller devices anyway.
Apple has already unfortunately abandoned PWM flicker free displays on almost all their devices. So I have no reason to believe this will change.
[0]: https://www.notebookcheck.net/PWM-Ranking-Notebooks-Smartpho...
The bigger challenge here is pixel architecture, but if apple is actually slicing up wafers into a couple million pieces to build these displays, they are already sort of moving away from the typical TFT architecture and may be able to integrate more complex pixel drivers, potentially including things like touch sensors directly onto the pixels.
I don’t understand what you mean with the GPU. It is has no information about the exact color of the LED.
The shift in wavelength is primarily determined by temperature and current, and they work in opposite directions so sort of cancel each other out. And in any case, we're talking about well-characterized shifts on the order of a few nm over the operating range. The eye's cones are broadband, so you're not going to notice wavelength shifts, especially compared to the brightness variations over the same range.
This is a big deal for white LEDs because you have no control of the resulting color temperature (the phosphor emission and blue component wholly determine the output), but for an RGB structure, you have pixel-level control over each component.
As to binning LEDs that works because it is constant. You can calibrate it once and done. But if you change the brightness by changing the current, it means your calibration is out of wack. Perhaps you can make a calibration at multiple current settings, but that seems inconvenient when using PWM will achieve the same thing.
So if the color shift is noticeable enough to require correction, then it's definitely enough to substantially decrease the color gamut as well. And so a range of wider-gamut colors simply can't be compensated for at all.
If you have a display 1000 pixels tall, then the brightest LED's are on for 1/1000th of the time.
And manufacturing limitations mean you can't easily have those per-pixel while still keeping the whole thing cheap.
“can’t easily” seems to imply it is possible. If that’s true, Apple, with its deep pockets, should be able to do it.
Also, I don’t think Apple will be bothered much with “keeping the whole thing cheap”. They will want to prevent it from getting expensive, but likely will accept intermediate costs if the result is much better.