[Radiance-general] gensky: what's the unit of the value specified for the "-b" option?

[Ji Zhang]

Ok, got it, Greg, thank you very much!

Ji



On Fri, Jul 2, 2010 at 12:43 AM, Greg Ward <gregoryjward at gmail.com> wrote:

> Hi Ji,
>
> Axel's tutorial is correct, but it is slightly simpler to use the gensky -B
> option (rather than "-b") if you are starting from the horizontal
> illuminance.  First, apply the 179 lumens/watt conversion factor:
>
>        10,000 lumens/m^2 * 1 watt / 179 lumens => 55.87 watts/m^2
>
> Using this in your gensky command:
>
>        gensky 12 4 +12:00 -a 51 -o 0 -m 0 -c -B 55.87
>
> You should get the output below.  (I'm not sure why the tutorial has "21"
> as the month for the first argument -- I'm assuming this is a typo.)
>
> # gensky 12 4 +12:00 -a 51 -o 0 -m 0 -c -B 55.87
> # Local solar time: 12.00
> # Solar altitude and azimuth: 16.8 -0.0
> # Ground ambient level: 17.8
>
> void brightfunc skyfunc
> 2 skybr skybright.cal
> 0
> 3 2 2.29e+01 3.56e+00
>
> The reported "Ground ambient level" is actually the diffuse horizontal
> illumination in disguise.  If you multiply 17.8 by pi and 179, you should
> get what you started with as your horizontal illuminance.  This is one way
> to check.  Another method is to run rtrace with just the sky and compute the
> horizontal irradiance using the -I option.
>
> I hope this clears things up.
>
> -Greg
>
>  From: Ji Zhang <hope.zh at gmail.com>
>> Date: June 30, 2010 10:22:08 PM PDT
>>
>>
>> Dear Radiance experts,
>>
>> May I ask what is the unit for the value specified after the "-b" option
>> of the "gensky" command?
>>
>> According to Axel Jacob's "Radiance Tutorial" p. 24 and "Radiance
>> Cookbook" p.35, if we want to define an overcast sky with the horizontal
>> illumination level being 10,000 lx, we need to specify the zenith brightness
>> with an irradiance value of 22.86(W/m2) for the "-b" option in gensky
>> command.
>>
>> However, according to the gensky menu, what we need to specify for the
>> "-b" option is the zenith brightness as an "radiance" value of which the
>> unit should be watts/steradian/m2.
>>
>> I assume that the term "radiance" and "irradiance" should not be used
>> interchangeably as they are different concepts, am I right?
>>
>> So, which one is correct? Axel's tutorial or the gensky menu?
>>
>>
>> Or more specifically, for "Formula 4" in Axel Jacob's "Radiance Tutorial"
>> p. 24 that specify the relationship between zenith brightness and horizontal
>> illuminance: R_zenith = (9/7)*(E_horiz/179/PI)
>>
>> What are the units for "R_zenith" and "E_horiz" exactly?
>>
>> If the unit for "R_zenith" is watts/steradian/m2, then it's a "radiance"
>> value.
>>
>> But if the unit for "R_zenith" is watts/m2, then it's a "irradiance"
>> value.
>>
>> Correct me if I'm wrong.
>>
>> Thanks!
>>
>> Ji
>>
>
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-- 
ZHANG Ji 张冀 (PhD) :: Research Fellow :: Centre for Sustainable Asian Cities
:: School of Design and Environment :: National University of Singapore :: 4
Architecture Drive,  Singapore, 117566 :: Contact: 65-6516 5046 :: Email:
sdezj at nus.edu.sg
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