# how to split vertical illuminance level into light coming from sky and light reflected by surface?

Ji.

Interesting concept.

We're trying to explore the relationship between Vertical Daylight 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"?

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?

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

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

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.

Regards,
Thomas

···

On Mon, Jul 26, 2010 at 4:20 AM, Ji Zhang <hope.zh@gmail.com> wrote:

Thanks, Thomas!

>
> 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

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

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

···

On Mon, Jul 26, 2010 at 4:20 AM, Ji Zhang <hope.zh@gmail.com> wrote:

Thank you for your suggestions, Christopher!

Ji

···

On Mon, Jul 26, 2010 at 9:46 PM, Christopher Rush <Christopher.Rush@arup.com > wrote:

Note that setting -ab 1 may not count sky component diffusely transmitted
through trans materials, as that's considered another bounce. Maybe this is
fine, depending on interpretation of your intention - "direct contribution
of the sky to a point on a vertical plane."

If you need to set -ab 2 or higher to count diffuse transmission in this
first part, you could set all your non transmitting materials to zero
reflectance, or the ambient include/exclude options may serve the purpose,
see -ae, -ai, -aE, and -aI.

-----Original Message-----
Sent: Monday, July 26, 2010 4:32 AM

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

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

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.

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