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

[Greg Ward]

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