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# How does Radiance use lux values from epw file to calculate daylighting

I'm running a Radiance day-lighting calculation and I can't understand why I receive the result below when comparing the day-lighting for Dec 28 13:00 and Dec 24 at 13:00 when the direct Normal Illuminances are so different (taken from the epw file).

Furthermore I receive similar results even when the direct normal illuminance is zero, see below

I suppose what I am missing is the actual equations that Radiance uses to calculate the lux value of a point in time, we would ultimately like to do hand calculations for one point in time to quality check our simulated values. Would anyhow be able to point me to where I could find the equations and an explanation of them?

Many thanks,

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hi, In which program do we get these images? How do we get Radiance?

( 2018-10-07 07:12:23 -0500 )edit

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Yeah, there is a big disconnect here. Your results are for "CIE skies", which Mostapha (and I) are assuming were generated with gensky. Gensky takes location (lat/long) and time of day as Mo said, as well as the very general sky conditions (clear, overcast, "intermediate"...). Comparing these results to the (presumably) exterior global/direct normal/diffuse horizontal values which (again, presumably) came from an epw file is apples and oranges, since the epw data on any given day will vary significantly from these ideals of totally clear or overcast. (This is why climate based daylight modeling has come about in the first place.)

As Mo said you could use gendaylit for your sky generation, using these actual exterior values from the epw file as your inputs (climate based!). You could also use gensky as it appears you have been, and simply place a calculation point above your model, free of obstructions (or calculate for 0,0,0 with the same sky and an otherwise empty octree), and calculate illuminance for that point. That way you'd truly have two apples, of the same variety, even. You'd have (total) exterior daylight illuminance (sun+sky), with which to compare to your interior values.

To your last point about validating your results with hand calculations, you can easily do this for the sky model (see pp354-356 in "Rendering with Radiance"). When it comes to the interior points where you're dealing with interreflection though, things can get nasty in a hurry. Again if you're simply looking to validate the process you'd want to use a really simple model (shallow box of a room with totally diffuse surfaces, clear specularly transmitting glazing, and a centrally located calculation point). Otherwise you're validating at least a couple different kind of apples, if not two different fruits altogether.

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excellent use of fruit analogies

( 2016-08-03 17:48:30 -0500 )edit

CIE sky is NOT generated based on the data from epw file. It only takes the location data (e.g latitude, longitude, etc). Check gensky manual for an example. If you are looking for a climate-based sky then use gendaylit.

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@rpg777 thanks for your very informative comment that does make things a great deal clearer. I am still somewhat confused as to the difference between how sun and sky illuminance is calculated however can you point me in the right direction? Much appreciated thanks!

( 2016-08-05 17:02:12 -0500 )edit