Note 004 · S1 Parameters · 29 Oct 2026
How many nits a sun-facing wall needs — and why the number is not on the sheet
Brightness on a datasheet is measured in a dark room. A wall facing the sun is judged against whatever light is falling on the glass. The second number is never on the sheet, and it is the one that decides.
Industry guidance for a wall in direct sun runs 5,000–7,500 cd/m². Fort outdoor cabinets are rated 6,000 cd/m² at P4.81 and P3.91 and 6,500 cd/m² at P10 and P6.6, measured at the panel. Which end of that range you need depends on site illuminance, which we will not estimate.
What a brightness figure actually measures
Nits on a datasheet are measured on the panel itself, in a dark or controlled room, at full white. That is a fair way to compare two panels. It says nothing about whether the wall will be readable on a west-facing elevation at four in the afternoon, because readability is a ratio between lit pixels and the light reflected off the glass.
Two walls with the same 6,000 cd/m² rating can look entirely different. In shade, the rated figure is far more than needed. Facing a low sun, the same wall can read as grey. The missing variable is the illuminance arriving at the surface, in lux, and the reflectance of whatever is in front of it.
The ladder we can quote
| Model | Pitch (mm) | Brightness (cd/m²) | IP front / rear |
|---|---|---|---|
| Fort Lite P4.81 | 4.81 | 6,000 | IP65 / IP54 |
| Fort Standard P3.91 | 3.91 | 6,000 | IP65 / IP54 |
| Fort Lite P10 | 10 | 6,500 | IP65 / IP54 |
| Fort Standard P6.6 | 6.6 | 6,500 | IP65 / IP54 |
| Fort Ultra P2.976 | 2.976 | 4,500 | IP65 / IP54 |
| Fort Ultra P5 | 5 | 5,000 | IP65 / IP54 |
Higher brightness sits on the coarser pitches, because a larger pixel has more emitting area per unit of drive current.
The rule we will defend
If the wall faces the sun inside a 45 ° cone for more than an hour a day, then specify the 6,500 cd/m² variants and treat auto-brightness as mandatory. If it faces away, sits under a canopy, or is seen mostly after dark, then 4,500 cd/m² is enough and the extra nits buy nothing but power and heat.
The 45 ° cone matters more than the compass point. A wall facing east is only sunlit before noon, and a wall facing west is dark all morning and unreadable at six in the evening in summer. Ask for the elevation, the overhang, and the hours of direct sun — not for the orientation.
Where the sheet stops helping
1. The glass is glossy and faces a bright surface. Reflected light from paving, a glazed façade or water adds to the ambient load without appearing in any orientation drawing. A wall that reads well in the factory can wash out in front of a white building.
2. The sun is low and near-parallel to the surface. Grazing light produces specular reflection rather than diffuse wash, and the answer is a louvre, a hood or a change of angle — not more nits. No brightness figure compensates for a mirror reflection of the sun.
3. The wall runs at night only. At 6,500 cd/m² in a dark street the wall becomes a nuisance and a planning risk. The right specification is a 6,500 cd/m² panel run at a low setting, with the dimming curve agreed in writing before installation.
4. Nobody has measured the site. This is where we stop. The 5,000–7,500 cd/m² range is industry guidance, not a reading of your wall. Without a lux measurement at the wall face at the worst hour, the position inside that range is still an opinion — so we quote the panel rating and leave the site figure open rather than pick a number for you.
One thing we got wrong ourselves
We used to answer this question with a rule of thumb borrowed from projector advice, and occasionally with a number that made the wall sound adequate. Neither was traceable to anything. The current position is narrower and less comfortable: we quote the panel rating we can document, and we refuse to quote a required brightness until somebody has measured the light on the wall.
- Datasheet — Fort series sheets: P4.81 and P3.91 at 6,000 cd/m², P10 and P6.6 at 6,500 cd/m², P2.976 at 4,500 cd/m², P5 at 5,000 cd/m², all IP65 front / IP54 rear.
- Practice — Industry guidance of 5,000–7,500 cd/m² for a wall in direct sun, and the practice of specifying against worst-case ambient light at the wall face rather than against a fixed nits figure. The range is a working figure, not a published standard, and manufacturers differ.
Questions we get on this
It sits at the top of the industry range for direct sun, which runs 5,000–7,500 cd/m², and it is the top of our outdoor range. Whether it is enough on your wall still needs a lux reading at that wall: the range tells you where to aim, not what your site measures.
A coarse pixel has more emitting area for the same drive current, so it reaches higher brightness more easily. P10 and P6.6 reach 6,500 cd/m²; the 2.976 mm wall tops out at 4,500 cd/m².
At the same content and the same setting, yes — and the Fort range shows it: the 6,500 cd/m² models are rated 750 W/m² maximum against 650 W/m² for the 6,000 cd/m² models.
Send the elevation, the overhang and a lux reading if you have one.
We will return the panel rating we can document and, where the site figure is missing, say so in writing rather than estimate it.