[Radiance-general] Daylight factor

Thomas Bleicher tbleicher at googlemail.com
Wed Dec 16 05:09:51 PST 2009

On Wed, Dec 16, 2009 at 9:57 AM, Victor Li <victorpermanent at gmail.com> wrote:
> Actually i just export the model from ECOTECT to RADIANCE, and i chang some
> parameters and run the simulation manually like Marija has shown above.
> The first time i calculated the daylgith factor in a grid
> I run it by define: gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000
> -B 0.558659,
> -B 0.558659 means the diffuse horizontal illuminance is 100lux outside then
> the internal illuminance is the dylight factor.

In principle that is correct. But other options in your rtrace command
introduce a rather high inaccuracy and some additional light which
will have a bigger effect on the results if you use an illuminance of
only 100 lux. If you use 10000 lux and devide by 100 you reduce
some of these effects.

> The second time i just calculated the internal illuminance in a grid under
> overcast sky in the same time as the first time. I defined the sky : gensky
> 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000
> In the grid, i can calculate the external diffuse horizontal illuminance at
> that time under overcast sky in the same point by  the external diffuse
> horizontal illuminance = the internal illuminance/ daylight factor.
> However, i calculated the the external diffuse horizontal illuminance by
> different points the results are different. Actually the results should be
> the same because it the external diffuse horizontal illuminance. So why is
> it different?

Radiance has some built-in randomisation. Therefore the result of
two identical calculations will be slightly different. Learning to choose
the correct options to minimise this difference is one of the challanges
of Radiance.

In your example I assume that the first set of daylight factors is rather
inaccurate. You now multiply this error by your internal illuminances
(which might be inaccurate, too). If your daylight factor is off by 1%
this can easily result in a difference of a few hundret lux.

It would realy help us if you could mention the actual results
of your calculations in your emails. Then we could see if the
errors are within the expected margin of your settings.

You should try your calculations with modified parameters
as suggested and with an increased sky illuminance to see
if your errors are still the same.


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