# Units of measurements in RADIANCE

Hello,
I was looking for an answer to this question and I stumbled upon this old post:

I would like to use RADIANCE in meters. According to what Roland_Schregle said, there is no particular care I have to put, right?
If, for example, my test room and the objects it contains come from a 3D modeling software where they were modeled in meters and my ies files are also expressed in meters there shouldn’t be any problems since the radiance and irradiance in RADIANCE are expressed in meters^2.
Would this be right?

World units fall out of all lighting calculations, which is why scale models work for daylight prediction. The radiance units (watts/m^2) do not imply that the world be measured in meters, only that lighting density is. Without going into too much detail, you can use inches or centimeters or feet or whatever you like for your world dimensional units and the lighting results will be the same.

Thank you Greg. I know my questions might look trivial but, just to understand, let’s say that I am looking for lux values in the model, the system has to look at my physical model and refer to an area expressed in square meters. How does it know if my room is in feet or meters?

Radiance never converts from flux (watts/m^2) to watts or the reverse, so it never needs to know the sizes of surfaces in the space. If you think about it, scaling the space also scales the size of the light source or window, which means the same relative dimensions are in play. The flux (area density) of photon energy is unaffected by scaling.

Ok, I understand. So when the falsecolor function is called and lux values appear on the screen, the software is not actually dividing the flux by the area. Something similar to the lux values we read on sensors in a scaled model where they are calculated in direct relation with the light reaching them regardless of the area they are placed on (?)

Yes, that’s a good way of looking at it. The lux meter doesn’t know the scale of the environment being measured, and it doesn’t matter in the end.