[Radiance-general] Gendaylit command

[Jack de Valpine]

Hi Krystyna,

I suggest having a look at at Axel Jacobs document on "Understanding 
rtcontrib":

    http://luminance.londonmet.ac.uk/learnix/docs/rtcontrib_lesson.pdf

On pages 30-32 he provide some excellent explanation of processing 
weather data for Radiance and gendaylit. According to this, the best 
data points to use are direct normal and diffuse horizontal irradiance 
(W/m^2) which should be provided to gendaylit AS IS (eg without any 
additional factoring for Radiance). Irradiance data points are suggested 
as the best option as apparently not all weather stations actually 
measure photometric data, meaning that such values if not measured are 
derived from the radiometric data points.

Note reading further will provide some potential cautions on gendaylit 
as well....

-Jack

-- 
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# president
#
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On 3/16/2011 3:11 AM, Krystyna Zelenay wrote:
> Hi,
>
> I am trying to define a clear and overcast sky for a particular
> location using gendaylit. I have collected global horizontal exterior
> illuminance data, but no diffuse illum. or irradiance data, so I would
> thus like to use TMY3 data. I am using DIVA-for-Rhino which requires
> an input in W/m2 for gendaylit, so I have two options:
>
> a) use the direct and diffuse horizontal irradiance values from the TMY3 file
> b) divide the direct and diffuse horizontal illuminances by 179 lm/W
> to obtain the irradiances
>
> Which of these is the correct input for gendaylit?
>
> What is the significance of the luminous efficacy used in Radiance
> (179 lm/W)? The book "Rendering with Radiance" states that luminous
> efficacy can vary from 50 to 150 lm/W depending on the type of sky or
> light considered (p357). And indeed when I divide for example, diffuse
> horizontal illum. data by 179, the irradiances I get are quite
> different from those in the TMY3 file.
>
> Does this mean that I should use the irradiance data from the TMY3 file?
>
> Thanks,
>
> Krystyna Zelenay
>
>
>
>
> from the site (but not irradiance data).The default DIVA input is in terms of
> direct horizontal irradiance (W/m^2) and
> diffuse horizontalirradiance (W/m^2)
>
> Is there a way to change the default input to
>
> NORMAL horizontal ILLUMINANCE (lm/m^2) and
> diffuse horizontal ILLUMINANCE (lm/m^2)
> (i.e. similar to the -L option in the gendaylit command)
>
> Alternatively, does anyone know how to go about converting illuminance
> to irradiance data?
>
> Thank you,
>
> Krystyna
>
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