LED displays
Pixel pitch and legibility: connecting dots into an image
Pixel pitch is arguably the most important property of an LED screen. It determines how sharp the image is, how close you can view it, and how much it costs. Here you read how that all hangs together.
On an LED screen the points of light are not stuck up against one another: there is a little space between them. That space is called the pixel pitch.
If there is little space between them (small pitch), the image looks sharp and you can stand close. If there is a lot of space between them (large pitch), you have to stand further away to see the separate dots as a single image. Bright light helps too: it ‘fills in’ the space between the dots a little.
Pixel pitch is the centre-to-centre distance between two neighbouring pixels, in millimetres (e.g. P10 = 10 mm, P2.5 = 2.5 mm). The smaller the pitch, the more pixels per square metre, the higher the resolution, and the closer you can view the screen without seeing separate dots.
Pitch, viewing distance and cost
There is a direct trade-off. A small pitch gives a sharp image up close, but requires far more LEDs per square metre and is therefore more expensive. A large pitch is cheaper and often brighter, but demands a greater minimum viewing distance. A common rule of thumb for the minimum viewing distance is roughly as many metres as the pitch in millimetres (P10 ≈ from 10 m). For text, character height is what counts most; see calculating viewing distance.
Connecting the dots
Characters on a screen consist of separate LEDs with space between them. Our brain ‘connects the dots’ into legible letters (see perception). Two things help: a smaller pitch and sufficient brightness, because light bridges the gaps between the LEDs. If the pitch becomes too large for the viewing distance, the text falls apart into separate dots and is no longer legible.
Pixel pitch sets the native resolution (pixels = screen area / pitch²) and thereby the angular resolution at a given viewing distance. Under good conditions the human eye distinguishes detail down to about 1 arcminute; from this follows the common rule of thumb that the minimum comfortable viewing distance in metres is roughly equal to the pitch in millimetres.
Cost scaling
The number of LEDs (and therefore the cost of LEDs, drivers and processing) scales with 1/pitch². Halving the pitch roughly quadruples the number of pixels per m². The design choice is thus an optimisation between the desired minimum viewing distance, the content (text vs fine imagery) and the budget.
Brightness bridges the pitch
Legibility is a perceptual grouping process: the angular distance between neighbouring LEDs determines whether the brain still sees them as a single character. Higher luminance lowers the grouping threshold because light visually bridges the gaps. That is why a bright screen with the same pitch can remain legible from slightly further away, and why brightness plays a part in the viewing distance, not only the pitch.