[Radiance-general] the vector of sensor in SVF calculation

[Ji Zhang]

Dear Radiance experts,

According to the paper "Compagnon, R. (2004). Solar and daylight
availability in the urban fabric. Energy and Buildings, 36(4), 321-328", we
can use Radiance to calculate Sky View Factor by "using a uniform sky model
of arbitrary luminance L and by computing the illuminance Ei without taking
into account the contribution of interreflections to ensure the integration
is performed over the unobstructed part of the sky vault only".

The formula is: SVF = Ei / (PI * L).

We use the following to define the uniform sky with a horizontal diffuse
irradiation of 100 w/m^2:
#########################################################
!gensky 6 20 +15.0  -a 1.0 -o 103.0 -m 105 -u -B 100.0 -g 0.0

skyfunc glow sky_glow 0 0 4 1.0 1.0 1.0 0

skyfunc glow ground_glow 0 0 4 1.0 1.0 1.0 0

sky_glow source sky 0 0 4 0 0 1 180

ground_glow source ground 0 0 4 0 0 -1 180
#########################################################

We use the following specification for rtrace to calculate the irradiation
of a sensor point:
aa=0.1, ab=1, ad=4096, ar=512, as=1024

We then divide the irradiation value of the sensor point as obtained by 100
(the unobstructed horizontal diffuse irradiation as specified for the "-B"
option of gensky) and this will give us the SVF of that sensor point.


According to the way of calculating SVF as introduced by Compagnon (2004),
my question is:
Should the vector of the sensor point being measured always point upward to
the zenith (in other words, the xyz vector for the sensor is 0 0 1) no
matter how the normal of that sensor point might oriented to?

For example, if we try to calculate the SVF of the centroid of a tilted
polygon (a non-horizontal and non-vertical plane), should we specify the
vector of the sensor as 0 0 1 or as the xyz vector values of the actual
normal of that polygon?


Thanks!

Ji
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