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

[Ji Zhang]

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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