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Hello.
They’re related units with different meaning, nit is cd/m^2 a unit of radiance – radiated energy. Lux is unit of luminous flow, lm/m^2 = cd*sr/m^2. As it is a unit of received energy, the receiving optical angle is important.
So measure in lux depends on optical angle of the sensor. For ideally parallel infinite or very far distance light source, such as when the sensor is tiny in comparison to source and far enough, they’re equivalent. Likewise if emitted light is collimated. If the sensor is close, the steradians have to be actually measured to convert.
The scale factor is Pi. For far away small enough sensor, lux = nit * Pi, nit = lux/Pi.
Colorimeters and spectrometers use collimating optics, which is how they can output level in nits from so close.
Which way am I suppose to point the ambient scensor.
If it measures 21 lux on my table pointed at the ceiling. Does that mean it’s 21/pi = nits 6.68 nits ?
Does the distance to ceiling make a difference in the measurement ?
Should I point it at the wall where the TV is mounted instead ?
In normal non-flare conditions the results will be identical no matter where you put it, as long as it’s not shadowed. Especially if a diffuser cap is used.
Sometimes light meters say they output lux, when they actually output nits. I have one of those in my cellphone.
6 nits would be a very darkened room, as in feeling dark, but not pitch black.
Hello AstralStorm.
Still about the conversion from lux to nits
May I ask you to help me find out if/where I am wrong.
I see many web sites that compute a conversion with a Pi factor, however, starting from the definitions I get another result (2Pi). One website computes like i do.
I pretend that the luminance of a perfectly diffuse white rectangle lit by X lux is X/(2Pi) candela/m² (aka nits).
Bear With me :
Given a surface S lit by X lux
X lux = X lumen / m² –> XS lumen are received by the surface S.
Assuming that, that surface is perfectly white and diffuse, it reemits all this light in 2PI steradians.
Which makes it possible to compute its luminosity in lumen/sr = candela
Its luminous intensity is XS/(2PI) candela.
And its luminance is X/(2PI) candela/m². (just divided by S)
Do you agree ?
See also https://www.justfreetools.com/en/lumen-to-candela-calculator
