Adding up illuminations in lux

[zemiacik2]

TL;DR Summary

lux

Hello,
I just found on google, that 1,5lx + 0,5lx=2lx. I wanted to check if it´ s true, but i couldn´´ t find anything on the internet. So was Google right or not?

 

[hutchphd]

Why would it not be true? Even if the theory was nonlinear this equation is always true as a mathematical entity. Also the theory is linear (2 sources produce twice as much light) in general.

 

[zemiacik2]

thank You for Your reply, I wasn´´ t sure, because one man told me that i can´ t add it up

 

[zemiacik2]

And when I have 1,5lx on one window and 0,5lx on another, smaller window, does it add up?

 

[hutchphd]

If they are beams of light falling onto the same surface, yes. Lux characterizes the luminous intensity per unit area onto a surface (it is a photometric measure meaning it is weighted to match your eye's color response)

 

[jbriggs444]

And when I have 1,5lx on one window and 0,5lx on another, smaller window, does it add up?

In this case we have two different surfaces subject to two different luminous intensities. You can add the two. But the sum has no physical significance that I can see.

A sum weighted by area could give you the total illumination entering the room.

 

[zemiacik2]

Thank you all so much, for help.
Actually it has no physical significance, but if I can add it up normally, than it´´ s over the limit set by the law, which that is the meaning.

 

[sophiecentaur]

thank You for Your reply, I wasn´´ t sure, because one man told me that i can´ t add it up

There are some things that you can't 'just add up'. For instance, vector addition involves knowing the angle between two Forces if you want to find the effect of both. Also, coherent light beams (say, light from a laser source) can combine to give Interference patterns, in which case, where there are two equal contributions, you can get four times the power in some places and none in others.
But, for two independent light sources you just add them together.

 

[zemiacik2]

There are some things that you can't 'just add up'. For instance, vector addition involves knowing the angle between two Forces if you want to find the effect of both. Also, coherent light beams (say, light from a laser source) can combine to give Interference patterns, in which case, where there are two equal contributions, you can get four times the power in some places and none in others.
But, for two independent light sources you just add them together.

thank you, but when windows arent light sources, but there is some light falling on them, can I simply add it up?

 

[sophiecentaur]

thank you, but when windows arent light sources, but there is some light falling on them, can I simply add it up?

Yes, for a very simple (‘ideal’) system but what layout is yours? I was thinking in terms of a Physics experiment.
What is the directional sensitivity of your meter? Are the two window blinds(?) reflective and are there and significant reflections in the room.
Also (importantly), what are your measured values? How bad are your answers?

 

[hutchphd]

To add them up you need to be talking about one particular surface. Lux onto one window would not be added to lux onto the other unless that light was falling in (roughly parallel) beams onto a third surface (like a table or a painting) from both windows. Even then you might need to know the transmissivity through the windows, which is not quite 100%. But if it is just diffuse ambient light they would not be simply added.