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

We're trying to explore the relationship between Vertical Daylight 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).

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?

Thank you!

Ji

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.

ยทยทยท

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Thomas Bleicher
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 '\$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.

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