Light
Brightness and nits: how do you measure the light output of an LED screen?
‘Nits’ is the unit that tells you how bright a screen is. But what exactly does that mean, and why does an outdoor screen need thousands of them while an indoor one needs far fewer? A clear explanation of how light is measured.
A nit is a way of saying how bright a screen looks to your eyes. The more nits, the brighter the light you see.
Think of a flashlight. Indoors it looks really bright. But switch it on outside in the sun and you can barely see the light any more, because the sun is much brighter. A screen works the same way: outdoors a screen needs a great many nits to be stronger than the sun, otherwise you cannot see the picture. Indoors, where it is darker, a screen needs far fewer nits.
That is why the big screens along the motorway have thousands of nits, while your tablet’s little screen has far fewer.
When we say a screen is “bright”, we actually mean one specific quantity: the luminance. Its unit is candela per square metre (cd/m²), and “nit” is simply a shorter name for it: 1 nit = 1 cd/m².
Four quantities that are often confused
Measuring light involves four related but distinct quantities. The difference lies in what you measure: the source, a direction, or a surface.
| Quantity | Unit | What it tells you |
|---|---|---|
| Luminous flux | lumen (lm) | The total amount of light a source emits in all directions. |
| Luminous intensity | candela (cd) | How much light travels in one particular direction. |
| Illuminance | lux (lx) | How much light arrives on a surface. |
| Luminance | cd/m² = nit | How bright a surface appears — this comes closest to what people call “brightness”. |
Why so many more nits are needed outdoors
A screen has to stand out against its surroundings. Indoors those surroundings are gently lit, so a luminance of around 600 to 1,000 nits is more than enough. Outdoors, direct sunlight falls on the screen; to stay bright and legible even then, an outdoor screen typically needs 5,000 to 7,500 nits.
It is not only brightness that matters, but also contrast: the difference between the bright points and the dark background of the screen. A black housing and black LED packages increase that contrast, making the image look sharper even in bright light.
There are two measurement systems for light intensity. Radiometry measures physical radiant energy (in watts), regardless of the eye. Photometry weights that energy according to the sensitivity of the human eye and is the relevant method for displays. The conversion is governed by the photopic luminosity function V(λ), which peaks at 555 nm (yellow-green): for equal radiant energy we perceive yellow-green light as far brighter than deep red or blue.
The four photometric SI quantities
| Quantity | Symbol | SI unit | Definition |
|---|---|---|---|
| Luminous flux | Φv | lumen (lm) | Total emitted quantity of light, derived from the radiometric radiant flux. |
| Luminous intensity | Iv | candela (cd) = lm/sr | Luminous flux per solid angle (steradian). The candela is an SI base unit. |
| Illuminance | Ev | lux (lx) = lm/m² | Luminous flux incident on a surface. |
| Luminance | Lv | cd/m² (nit) | Luminous intensity per unit area of an extended source, in a given direction. |
Illuminance decreases with the square of the distance: E = I / d². A point source of 36 cd produces an illuminance of 36/3² = 4 lux at 3 m.
Luminance is independent of distance — and so is luminous intensity
A crucial and often misunderstood point: luminance and luminous intensity do not depend on the distance to the observer. A screen of 6,000 nits is 6,000 nits whether you stand at 5 or 50 metres. What does decrease with distance is the illuminance (lux) the screen casts onto your eye. This explains why “nits” is the right measure for comparing screens with one another, independent of viewing distance.
Why outdoor brightness is a question of contrast
In direct sunlight the illuminance on a vertical plane quickly reaches tens of thousands of lux. Part of that is reflected by the screen surface and lowers the effective contrast. Legibility is therefore determined not by luminance alone, but by the ratio between the screen’s luminance and the reflected ambient luminance (external contrast). That is why outdoor technology combines high luminance (5,000–7,500 nits) with low reflection: black SMD or COB packages and matte masks.