Nits, Candela, and what it all means

[Twinbee]

Hello all,

Out of all the types of light measurement, none has given so much headache and confusion as the candela unit.

Nits are supposedly candela per meter squared. However, this would imply that to measure the candela of a light source, we would need to specify an angle (sub-portion or sub-area) of the light, not just the direction!!

You can see my confusion, since I've often seen that only the direction is the needed variable. However, then we might as well be speaking in nits again! (well apart from one can cherry pick the brightest part of a beam).

So what gives?

[jtr1962]

I've never understood why things like display brightness are specified in candela per square meter or nits either. Why not simply lumens per square meter? That would be a lot more intuitive.

BTW, in case you're interested in converting to nits just divide the lumens per square meter by pi. For example, suppose a display screen has a surface area of 0.1 square meters and the total luminous flux exiting the screen is 100 lumens. The brightness is 100/(0.1*pi) = 318.3 candela/m² = 318.3 nits.

[Twinbee]

Yeah, this is where it gets even more confusing. Does "lumens per square meter" (lux) mean square meter of light, or the object it lights up? It has to be the object it lights up, because lux depends on how far the object is away from the light source.

However, when we talk of "candela per square meter" (nits), then I think it's per square meter of the light source itself. Does this help at all? I may be wrong, but from the tons of info I've read, that what seems to make most sense.

Therefore in theory, candela should be referring to the light reflecting from a given object, rather than the light source directly.

It's easy for this to lead round in a massive whirlpool of contradiction and confusion, but I'm trying to get to the bottom of it

BTW, in case you're interested in converting to nits just divide the lumens per square meter by pi

If what I say above is correct, then nits and lumens per square meter (lux) are very different creatures measuring different things. As said previously, and as far as I know, the 'per square meter bit' refers to the object reflection bit, rather than what the light source gives off, otherwise you would indeed be correct in saying lpsm=nits.

[Twinbee]

I would still very much appreciate answers to the questions in my two posts above if possible.

[Twinbee]

I will try one more time to bump this thread up. Again, I would still very much appreciate an answer. If not, I'll have to try elsewhere, but I know there are people who know this stuff...

If for nits, we measure brightness, and average that over a metre square of the light source, then for candela, one should take into account brightness and also the cone angle size of the light (and not just the direction!).

If direction is the only variable, then we may as well be speaking in nits again (apart from one can pick the lightest or dullest part of the beam, and ignore the weaker/stronger parts of the beam).

I hope I've made myself clear. If not, then please say where I'm ambiguous.

Merry Christmas to everyone!

[2xTrinity]

It is a bit confusing, but the reason lm/m^2 (lux) is not used is that it does not take into account the angle of projeciton. Lumens are defined as candelas multiplied by the angle of the cone through which they are projected. A screen that emits 100 lumens with a surface area of .1 m^2 (1000 lux) will appear to be a much higher intensity (candela) if those lumens are direction to a narrow viewing angle than if they spread out evenly in all directions. For looking at a screen, it is how bright that the source itself appears when looked at (intensity, or candelas) that matters, not how much that source will light up the room (lumens), as would be relevant for a light bulb.

[Kinnza]

The reason some devices are rated in nits (luminance) is because they are intended to be viewed, no to light any area.

Although the unit used for luminance (nit=cd/m2) seems similar to cd alone, both units refers to different things. Luminance refer to a surface, wich receive the light or emits light with low intensity (as a reflecting surface). Its useful to know how we perceive it, but fully unuseful to know how it lights the surrounding space, wich devices rated in nits arnt intended to do.

Cd is a radiance units, related to the light source, not to the surface/space receiving the light. Radiance units (lm, cd) refer to the light source, and describes how much light the source emits (lm) or the intensity wich it emits (cd).

Irradiance units, lux (lm/m2), fc (lm/sq ft) describes the light reaching a lighted surface. Luminance refers to the light reflecting from the surface (or internally lighted in a diffuse way, so it looks like it is reflecting light).

[Twinbee]

Thanks for your replies! I'm a little confused, as I always associated the intensity with the "nits" unit (candela per metre squared).

Just to make sure I've got this right, I'll paste in four concise descriptions for each of the units, and if you can alter/adjust them, that would be great. I imagine nothing could be clearer than stating what variables need to be taken into account for each unit.

"matters" below means to "take into account" as variables. Also, "object area" could be translated as the 'viewer', or the surface that the light is shining on.

Candela/m^2 or Nit ::: light-source-intensity matters
Candela ::: light-source-intensity matters AND light-source-size matters
Lumen/m^2 or Lux ::: light-source-intensity matters AND light-source-size matters AND light-source-distance-from-object matters
Lumen ::: light-source-intensity matters AND light-source-size matters AND light-source-distance-from-object matters AND size-of-object-area matters

For the last two, the terms light-source-distance-from-object and size-of-object-area can be combined to form the translation: "angle of the light source's cone".