Hello everyone,
I am trying to find out how illuminance in a point in a room is influenced separatly by direct sunlight, diffuse light from a clouded sky, and diffuse light from a clear sky.
As a first attempt, I manipulated the .epw file for my location: I made three separate files, one in which 'direct normal radiation' values are all set to zero, one in which 'diffuse horizontal radiation' values are all set to zero, and one in which 'total sky cover' and 'opaque sky cover' values are all set to zero. 
The results I get, however, don't make much sense to me.
1) When the diffuse horizontal radiation is set to zero, all annual daylight values are zero. This makes it seem like direct normal radiation has no influence at all?
2) When the direct normal radiation is set to zero, all annual daylight values are slightly lower than the results from the regular .epw file. This suggests that there is some influence of direct normal radiation, but it is small.
3) When the sky cover is set to zero, all annual daylight values are slightly different (higher or lower) than the results from the regular .epw file. This suggests that cloudiness has very low impact on illuminance? 
Can anyone help me with the interpretation of these outcomes?
Further, it seems like DIVA creates a .wea file of the .epw file. I interpret this data as (Month, Day, Hour, DirectNormalRadiation, DiffuseHorizontalRadiation). Is this all information used for the calculation of illuminance values? Then how is sky cover taken into account?
Thanks in advance!

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Hi Andrea,

Indeed, direct normal and diffuse horizontal irradiance are the two fields taken from the EPW file.

Sky cover is not used at all.  (Any change in results is merely stochasticity.)

I'm guessing that you are hoping to separate direct and diffuse sources by manipulating the EPW file.  I would advise against this.  Radiance and Daysim use the ratio of direct and diffuse irradiance to determine the distribution of light over the sky dome.  So removing the direct component won't merely remove the sun -- it will dramatically (and unrealistically) alter the appearance of the rest of the sky as well.

Direct and diffuse sources can be separated, but not through the current DIVA UI.


Hi Jon,

Thank you for your insightfull reply. I am indeed trying to separate these sources, but I obviously don't know much about the way in which illuminance values are calculated.

I am a student in Architecture, and hoped to be able to retrieve this kind of light information from a user friendly interface like DIVA. Apparently, it is not that simple.
Do you have any suggestions on how to do this?
Do I have to use the visual basic component for this? 
Or can I better just learn how to work with Radiance directly?

I am new to these programs, so it will probably take a while for me to figure out how to do this. However, it would be nice to know where to start. 

Thanks in advance,


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