\* or have LED for each sub-pixel of screen.
In: 3
If you use a light emitter for every pixel, you have an OLED or MicroLED. Other LED/LCD panels use a range of techniques from edge-lighting to full array local dimming (basically an array grid of lights behind the picture instead of lighting from the edge) to light the panel.
The biggest problem is cost, which is due to the difficulty of manufacturing the OLED and MicroLED panels.
We do, called MicroLED (not to be confused with MiniLED).
The issue though is that for high resolution (and pixel density) there doesn’t exist LEDs small enough. So we are waiting for LED manufactures to meet demand. Currently the only MicroLED displays are large and lower resolution.
Per-pixel lighting behind an LCD panel is probably technically possible, but the cost and complexity are great enough that it makes more sense to move on to OLED, which uses multiple colored LED sub-pixels rather than light shining _through_ LCD sub-pixels.
OLED and LCDs have something in common – you can make them by sticking a film of something (either the liquid crystal in LCDs or the organic light emitting layer in OLEDs) on top of a grid of transparent transistors.
Nice and easy to manufacture a screen because you’re making giant sheets of the stuff.
Not so with LEDs, microLED screens exist but each individual tiny LED needs to be manufactured separately. No-one has figured out how to make a whole sheet of the stuff in one go. So it’s very expensive.
As for why they don’t have LCDs with each pixel as a microLED? Well once you’re making LEDs that small they’re still expensive so you may as well just make 3 in R,G and B instead of 1 white one behind an LCD panel that kills some of the benefits of microLED anyway.
Like this? https://www.planar.com/products/led-video-walls/ says “pixel pitches from 0.6mm to 20mm”. A 1024×768 15″ display has a [dot pitch](https://en.wikipedia.org/wiki/Dot_pitch) of ~0.3mm.
We can, it’s just that they’re not the best fit for the form factors *and price points* that LCDs are useful for. If you’re talking about a desktop or personal device display to be viewed up to 3m or so, then you want a finer dot pitch because that’s what the existing products have.
microLED is in development.
After quite the rabbit hole I think I finally understand enough to give a definitive answer.
Fundamentally you need light to be broken down into subpixels. You can either do this with emitters larger than a subpixel passing light through LCD subpixels, or you can use subpixel-size emitters.
Inorganic LEDs are good for LCD panels because of their brightness, but until recently it was impossible to make a backlight of pixel-sized emitters. Panels can be made with extremely tiny Organic LEDs, but they aren’t bright enough to be good as an LCD backlight. However, it’s not significantly harder to make an OLED panel with subpixel-size emitters than it is to make an OLED panel with pixel-size emitters, so we skipped entirely from globally backlit LCDs to OLED TVs with each subpixel having its own emitter.
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LCDs are still less expensive than OLED, so we started making LCD screens with multiple dimmable zones to improve contrast. As LED technology has improved we’ve been able to increase the number and decrease the size of those zones. LCDs only make sense as long as they are less expensive than OLED though, so there’s a limit to how far that can go.
Micro LED is a new (extremely expensive) technology using inorganic LEDs that are small enough to be used as subpixels. This provides some advantages over OLED as LEDs are significantly brighter than Organic LEDs and OLED manufacturing limitations create an upper limit on the size of an OLED screen that doesn’t apply to Micro LED.
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