[Radiance-general] how to split vertical illuminance level into light coming from sky and light reflected by surface?

[Ji Zhang]

Thanks, Thomas!


On Mon, Jul 26, 2010 at 4:20 AM, Ji Zhang <hope.zh at gmail.com> wrote:
> > Dear Radiance experts,
> >
> > We're trying to explore the relationship between Vertical Daylight Factor
> > and Facade Sky View Factor.
>
> In the UK we have the "Vertical Sky Component" which quantifies the
> _direct_ contribution of the sky to a point on a vertical plane (ie.
> wall). Because it's only the direct component that matters here it is
> related to the portion of sky visible from this point. It's also based
> on the CIE overcast sky so it matters which part of the sky you can
> see. Is that the same concept as your "Facade Sky View Factor"?
>

Ji: Yes, Thomas, the concept as explained by you is the same as the "Facade
Sky View Factor" mentioned by me.



>
> > By Vertical Daylight Factor we mean the ratio of the illuminance level of
> a
> > point on vertical building facade (i.e. the vector of the sensor of that
> > point is perpendicular to the facade surface) to the illuminance level of
> a
> > point on horizontal plane under standard CIE overcast sky (please correct
> me
> > if the definition is wrong).
>
> Does that include the reflected part?
>

Ji: Yes, Thomas, by our understanding Vertical Daylight Factor is calculated
based on the illuminance value of a point on vertical surface that is the
sum of the light coming directly from the CIE overcast sky and the light
reflected by surrounding surfaces and ground plane.



>
> > Via Radiance, is there a way to split the illuminance level of a point on
> > vertical surface into 1) the light coming directly from the sky dome and
> 2)
> > the light reflected by surrounding building surfaces and ground plane?
>
> For the direct part simply use "-I -ab 1" in your rtrace command (or
> "-i -ab 1" if you want to calculate pictures):
>
> echo "0 0 2 0 1 0" | rtrace -w -h -ab 1 -I scene.oct | rcalc -e
> '$1=($1*0.265+$2*0.67+$3*0.065)*179' > facade_direct.lux
>
> Then calculate the total of direct and reflected component by
> increasing "-ab" to an appropriate value:
>
> echo "0 0 2 0 1 0" | rtrace -w -h -ab 4 -I scene.oct | rcalc -e
> '$1=($1*0.265+$2*0.67+$3*0.065)*179' > facade_total.lux
>
> The reflected light is the difference between total and and direct
> component. You can also render images and subtract the images from
> each other. See the 'pcomb' command for details.
>

Thank you very much for your detailed suggestion! We'll try accordingly
later.

Ji
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