Direct Radiation includes Circumsolar Radiation?


After running a single hour simulation for the direct radiation, I had a question why the result shows more than two different radiation values on my facade.
I checked with SkyDome component, and then realized that the single hour direct radiation includes more than one sun-vector or radiation value on the patched sky!

I tried to understand the python code, but failed…
Will you explain how LadyBug calculates the direct radiation on the geometry?
What is the reference for this approach?
Was it intended to simulate circumsolar radiation?
If then, what is the calculation process in LadyBug?

I attached my gh file and weather data to be selected on “file path” component.


Best regards,

Jaeman PARK (444.4 KB) (250.7 KB)

The discretized sky is based on Tregenza Sky subdivisions. . The original subdivision scheme proposed by Peter Tregenza had circular patches and therefore there were gaps in the coverage of the celestial hemisphere.

Christoph Reinhart had discussed this somewhat extensively in his dissertation work in 2000-2001. The sky dome that is implemented in Ladybug/Honeybee is based on this subdivision scheme (mostly).

No, its just solar radiation. Since the size of the patches is much larger than the size of the solar disc (which is around 0.533 steradians), it would be a gross simplification in terms of - sun position and radiant intensity - to simply assign all the radiation to single sky patch. So, the solar radiation is assigned to multiple patches instead.

Thanks sarith! Well understood!
And is it right that LadyBug uses anisotropic sky model as default (when I see the sky dome of diffuse radiation)?

For the sky domes that are generated based on radiation data and geographical variables, the model being employed is the Perez Sky Model (@mostapha would be able to confirm this). So yes, the sky is anisotropic.

Hi @sarith, About the subdivision of skies, what is the default subdivision number of CIE sky and Climate based sky, and if I could increase the sky patch numbers for these skies to improve the accuracy of my simulation?

It’s 145.

This is an interesting question. I thought the accuracy would improve, however, what I found was that for the simulation type implemented in HB[+], the difference in accuracy based on sky discretization alone was not significant. This was found to be the case for both illuminance and image-based simulations. The proper positioning of sun-patches has a greater impact.
Eleonora Brembilla independently arrived at a similar conclusion with her research as well (see section 5.2.4).

If you run a simulation with just sky patches and a no solar dics, then the accuracy would improve (as Eleonora points out in her paper). However, there is really no justificiation for leaving out solar discs in the simulation.


Thanks for the response.
I am a little confused with “the difference in accuracy based on sky discretization alone was not significant… The proper positioning of sun-patches has a greater impact”. I want to increase the number of sky patches to have a more accurate position of the sun for my PIT simulation on a CFS. Hope that my explanation is clear enough.

Sky patches are not applicable for PIT simulation as the sky used is a continuous sky. Skies are only discretized for annual simulations.

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