There used to be a time when digital cameras were marketed by their increasing resolutions in Megapixels (MPs). That's a sensor resolution. The other marketing term when it comes to displays is also display resolution in pixels. HD / FHD / 4K / 8K ....But what exactly is a pixel physically when it comes to display resolution?
A pixel is fundamental unit of resolution that tells how much dense display information is packed in a given area. Physically a pixel is represented in some way on the underlying electronics. It represents a Red / Green/ Blue (RGB) light generation mechanism that together when mixed constitutes the final color we see.
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In a LCD display there is a backlight and this backlight has a panel on top it (LCD panel) whose light passing properties can be controlled by current by switching individual areas of pixel size physical locations to let light pass through RGB filters which then acts as a pixel. However as you can see a pixel here is like a gate controlling light to create a pixel. There is always leakage of light as you cant switch off fully, so you cant get deep black/ contrast in LCD displays.
In comparison if you use OLED (o stands for organic)(also marketed by apple as Retina displays) or micro OLED or micro LED displays (latest), there are actually super tiny LEDs that acts as individual physical pixels. So each LED is pixel and since each LED can be individually switched on/off, there is deep black/ high contrast because LED off means black unlike in LCD where you still have leaks due to backlight. Each LED can thus emit RGB color depending on input signal for media being played. Hence termed emissive tech.
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So that's how a pixel is defined for a LCD display or an OLED / micro OLED / micro LED displays. Higher numbers of LEDs you pack in smaller areas, better your resolution gets. But what's the limit?
For the micro OLEDS and micro LEDs the distance between two pixels could be in range of a few micrometer distances away from eachother. To get such small distances these LEDs are directly printed on silicon materials (called silicon wafers) using very advanced manufacturing methods.
For OLEDs the distance between two LEDs is higher and manufacturing is more easier due to large size. That's why OLEDs can be seen around easily though at higher prices than LCD. In comparison micro LED and micro OLEDs are yet still evolving and in intial stages so incredibly expensive. Also because during manufacturing so tiny micro OLEDs / micro LEDs there is lots of wastage to get one good batch of good LEDs.
The ability to pack a large number of LEDs in tiny areas with high brightness makes micro OLEDs and micro LEDs very useful for wearable AR / VR displays where compactness, high brightness matters. But as they are still evolving they are still very expensive. Micro LEDs r further found better than micro OLEDs due to ultra high brightness and longevity.