Three different numbers
When a data sheet says “viewing distance: 1.5 m”, it could mean any of three things, and the three disagree by a factor of seven. This is the single most common source of confusion in an LED specification, and it is entirely avoidable once you know which is which.
- Minimum viewing distance. The closest a viewer can get before the pixel structure becomes obtrusive. A convention, not a measurement — usually pitch × 0.5 or pitch × 1.
- Recommended viewing distance. Where the image reads comfortably. Also a convention, usually pitch × 1 to pitch × 2.
- Retina distance. Where individual pixels become optically unresolvable. Not a convention at all: it falls out of human visual acuity.
If two suppliers quote different viewing distances for the same pitch, neither is lying. They are printing different conventions. Ask which, and the disagreement disappears.
Where pixels disappear
A person with 20/20 vision resolves detail down to about one arcminute of arc — one sixtieth of a degree, which is roughly 1/3438 of a radian. Two points closer together than that angle merge.
Turn that around and a pitch of p millimetres becomes unresolvable at p × 3.438 metres. A 1.2 mm wall is seamless from 4.1 m. A 6.67 mm billboard is seamless from 22.9 m — which is roughly where a car is when the driver reads it.
This is the only one of the three figures that is a physical fact rather than a house style, which makes it the one to argue from when a client asks why you are not quoting a finer product.
The comfort figure
Nobody actually stands at the retina distance. In a boardroom people sit at three metres from a wall whose retina distance is five, and the wall looks fine, because content moves, the eye is not hunting for structure, and the brain is reading a chart rather than inspecting a surface.
The convention the original easyLED catalogue used — and a reasonable default — is that the minimum comfortable distance is about pitch × 0.5 metres and the recommended distance about pitch × 1 metre. On a 1.2 mm product that is 0.6 m and 1.2 m, which reads absurdly close until you remember it describes an installation where someone occasionally walks past the wall, not one where they stand and study it.
Use the comfort figures to describe a room. Use the retina figure to defend a pitch. They are tools for different arguments.
How far is too far
The other end of the problem gets less attention and causes more disappointment: a wall that is too small for the room. Pitch does not help here at all — image size does.
AVIXA’s display sizing standard (ANSI/AVIXA V202.01, usually called DISCAS) sets image height from the farthest viewer and the smallest element that has to be legible, and separates content you merely have to see from content you have to analyse. The practical shorthand most integrators carry in their head:
- For general content — video, imagery, large figures — the farthest viewer should be no more than about six times the image height away.
- For detailed content — spreadsheets, plans, dashboards — that comes down to about four times the image height.
A wall 2 m high therefore serves general content out to roughly 12 m, and analytical content out to roughly 8 m. If the back of the room is 20 m away, no pitch will save you; the wall needs to be taller.
Making text legible
Text is where the two ends of the problem meet, and it chains together neatly:
- A character needs roughly ten pixels of height before it is cleanly formed on an LED wall. Below that the strokes break up.
- So the smallest usable character height in millimetres is about pitch × 10.
- A character is comfortably readable out to roughly 150 times its height, and legible at a push out to 200 times.
Multiply those together and a pitch of p supports comfortable text out to about p × 1.5 metres — but only for text set at the minimum size. Bigger type reaches further, which is why a 10 mm billboard works: nothing on it is set at ten pixels tall.
A worked example
A shopping centre concourse. The wall will be 4.8 m wide and 2.7 m high. The nearest shopper passes at 2 m; the farthest useful viewer is at the far side of the atrium, 14 m away. The content is a mix of video and promotional text.
| Question | Working | Answer |
|---|---|---|
| Is the wall big enough? | 2.7 m high × 6 for general content | Serves to 16.2 m — yes |
| What pitch does the nearest viewer need? | 2 m ÷ 3.438 | 0.58 mm for a truly seamless image |
| Is that sensible? | Shoppers pass, they do not inspect | No — use the comfort convention instead |
| What does the comfort convention give? | 2 m at pitch × 1 | About 2 mm |
| Nearest standard product | 1.875 mm or 2.5 mm | Quote 2.5 mm, offer 1.875 mm as an upgrade |
| Smallest usable text? | 2.5 mm × 10 characters, × 150 | 25 mm characters, readable to 3.75 m |
That last row is the one worth putting in front of the client: at 2.5 mm, anything they want read from the far side of the atrium has to be set large. It is a content constraint, and finding it during the quotation is much cheaper than finding it at commissioning.
Four common mistakes
- Quoting the retina distance as the recommended one. It makes your product look worse than a competitor quoting the comfort figure for the same pitch.
- Specifying for the closest person who will ever exist. Someone can always press their nose to a wall. Specify for where people actually stand and stay.
- Ignoring the far end. A wall that is too small cannot be fixed with a finer pitch, only with more cabinets.
- Forgetting the camera. If the wall will be filmed, refresh rate and scan rate matter more than pitch, and the calculation is done for the lens rather than the eye.
The free configurator prints the minimum and retina distances for whatever pitch you pick, so you can check a claim in about four seconds — including one somebody else has made.