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

May I ask what is the unit for the value specified after the "-b" option of
the "gensky" command?

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

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?

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

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 <[email protected]>
Date: June 30, 2010 10:22:08 PM PDT

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

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

Ji

···

On Fri, Jul 2, 2010 at 12:43 AM, Greg Ward <[email protected]> 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 <[email protected]>

Date: June 30, 2010 10:22:08 PM PDT

May I ask what is the unit for the value specified after the "-b" option
of the "gensky" command?

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

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?

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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