How ies2rad convert the ies file in the part of the luminous data of luminaires?

Hi everyone,

I am new to Radiance. Recently, I read the .rad and .dat files generated by ies2rad. Based on my understanding, the data in .dat is the radiance value of artificial lighting sources. But based on my observation, the data in .dat file = date in .ies / 179.

But I am confused about this conversion because the data in .ies file are the luminous intensity (lm/sr), and the 179 is the luminous efficacy (lm/w). So the unit of result is w/sr which is not the irradiance or radiance. Therefore, I would like to ask how ies2rad converts the data in .ies file.



Hi Hao,

This is a good question, and shows you are thinking about things in the right way.

Indeed, there is another factor in the equation, which is the projected area of the light source. This must be divided into the luminous emmissivity value, which is accomplished by the boxcorr() or lboxcorr() function in, making the units watts/sr/meter^2 as required for radiance.

I hope this is clear enough.


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Hi Greg,

Thank you so much for your reply to my question. Your reply is clear to this question and gives the clue to understand the detailed process of ies2rad.

Besides, I want to say that I am new to Radiance. I don’t have a background in artificial lighting illuminance analyses or daylight simulation. It’s not long for me to start my academic research in this area. In this case, I have been asking a lot of questions in this forum. Some questions may be a bit simple or even fool for you. But they are really important to me to understand the way the Radicane works. Thank you again for your reply and effort. I appreciate your help and your patience.

Best regards,


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