Brightness is one half of a ratio
A nit is one candela per square metre: light leaving the surface. Lux is light arriving at it. The eye does not respond to either number on its own. It responds to the ratio between what the screen emits and what the screen reflects back at it.
That ratio has a name — ambient contrast ratio, ACR — and it is the only brightness figure that predicts whether a person can read a screen in a given room:
ACR = (screen luminance + reflected luminance) ÷ (black luminance + reflected luminance)
Notice what sits in both halves. Reflected light is added to the white and to the black equally, which is why it compresses contrast towards 1:1 no matter how bright the panel is. Doubling nits does not double legibility. Cutting reflectance can.
Reflected luminance, in one line
For a diffuse front surface, the reflected luminance is ambient illuminance times reflectance, divided by π:
Lref = E × ρ ÷ 3.14, with E in lux and ρ the reflectance of the finished front.
Plug in a summer pavement at 100,000 lux and you get the numbers people find hard to believe:
| Front surface | ρ | Reflected at 2,000 lux (office) | 20,000 lux (canopy) | 100,000 lux (direct sun) |
|---|---|---|---|---|
| Bare float glass over LCD | 8% | 51 cd/m² | 509 cd/m² | 2,546 cd/m² |
| Clear cover, no treatment | 5% | 32 | 318 | 1,592 |
| Anti-glare etched, bonded | 2% | 13 | 127 | 637 |
| Anti-reflective coated, bonded | 1.5% | 10 | 95 | 477 |
Read the last column again. In direct sun, an untreated front reflects more light than a 2,500-nit panel emits. At that point the screen is a mirror with an image behind it.
So what does that do to a real panel
Take a 1,000:1 panel, which in a dark room is genuinely 1,000:1. Put it in the sun with a 5% front. White is 1,000 nits plus 1,592 of reflection; black is 1 nit plus the same 1,592.
ACR = (1,000 + 1,592) ÷ (1 + 1,592) = 1.6:1. That is not a dim picture. That is a grey rectangle, which is exactly what customers describe when they say the screen “disappeared” at noon.
Now the same 1,000 nits behind an anti-reflective front at 1.5%: ACR = (1,000 + 477) ÷ (1 + 477) = 3.1:1. Still not comfortable, but text becomes readable.
And 2,500 nits with the same 1.5% front: (2,500 + 477) ÷ (2.5 + 477) = 6.2:1. The brightness tripled and the contrast only doubled, because the reflection did not change. That is the whole argument for spending on the front surface first.
For reference, we work to 4:1 for anything a person has to read at a metre, 10:1 where the content is photographic, and 20:1 and up for signage that has to stay legible from across a road.
The ladder we quote, by where the unit is going
| Where it stands | Ambient | Panel brightness | Front surface |
|---|---|---|---|
| Meeting room, corridor | 200–500 lux | 300–350 cd/m² | Any |
| Bright office, retail interior | 500–1,000 lux | 500–700 | Anti-glare helps |
| Indoor facing a window | 2,000–5,000 lux | 700–1,000 | Anti-glare, bonded |
| Under a canopy, station platform | 10,000–20,000 lux | 1,500–2,500 | Anti-reflective, bonded |
| Direct sun, south facing | 50,000–100,000 lux | 2,500–3,500 | Anti-reflective ≤2%, bonded |
| Direct sun with video content | 100,000 lux | 5,000 | Anti-reflective, bonded, active cooling |
Those are minimums for static content with a sensible palette. White text on white backgrounds fails at every row of that table.
Buying nits has a bill attached
Power and heat. A 2,500-nit 55-inch LCD draws roughly 2.5 to 3 times what a 500-nit indoor unit does. In a sealed outdoor totem that heat has to leave through the same enclosure the sun is heating, which is why the fan and the air path become part of the specification.
Backlight life. The familiar 50,000-hour figure is measured at a reduced drive level. Run the same LEDs at maximum for eight hours a day in a hot enclosure and the useful number is shorter, in the way described in our note on reading lifetime figures.
Black level. A 2,500-nit panel at 1,000:1 has a black of 2.5 nits. In a dark indoor room that reads as glowing grey, which is why high-brightness units need a wide dimming range and a sensor, not just a bright backlight.
Noise. Bright panels are cooled panels. A 3,000-nit double-sided totem next to a cafe table is an audible object.
Flicker. Dimming a bright panel down to 30% by chopping the backlight at a low frequency makes it flicker on phone cameras and in peripheral vision. Ask for a dimming frequency above 1 kHz, or analogue dimming.
Two things that make the datasheet number meaningless
Where it was measured. A single centre reading with full white at 25 °C is the friendly way to publish. Nine-point uniformity on a finished unit is the honest way, and it is usually 10–15% lower at the corners.
Whether it is sustained. Some panels boost the backlight for a few seconds, or quote the panel module rather than the assembled display with its cover glass and bonding losses. A 2,500-nit module behind an air gap and an 8% float glass can deliver under 2,000 nits at the viewer. Ask for the finished-unit figure, measured through the front.
Ten minutes on site before you specify anything
You do not need a luminance meter to settle most of these arguments.
1. Put a phone lux meter against the front glass at the worst hour. You will get a rough but usable number, and it is almost always higher than the project assumed.
2. Photograph a 10% grey field on the screen from where the audience stands. If the grey looks like the wall behind it, no brightness increase will save the installation.
3. Move the unit. Half a metre behind a mullion, or ten degrees off the sun line, often costs nothing and beats the next brightness tier.
4. Ask for the reflectance of the finished front, not the transmittance of the sensor. If the supplier cannot give it, that is your answer.
5. Check the content. Dark backgrounds with high-contrast type read at a third of the brightness. If the artwork is a white catalogue page, buy reflectance, not nits.
The one-line version
A screen has to out-shine the light it reflects. Every design decision that lowers reflectance buys contrast at a better exchange rate than adding nits, and the two stack: bond the front, treat the front, then raise the backlight only as far as the thermal design can carry.
Two case notes on this site cover the same physics from the field side: a shop window reading 1,900 nits through glass, and a lobby that needed 700 nits less than it needed a different wall position.
Where to go next
Outdoor display range The 55-inch zero-bonding outdoor unit Where the air gap hurts: bonding explained