# Daylight factor

Dear:

Daylight factors are expressed as the percentage of natural light falling on
a work surface compared to that which would have fallen on a completely
unobstructed horizontal surface under exactly the same sky conditions. Thus,
a daylight factor of 5% on an internal surface means that it received only
1/20th of the maximum available natural light.

so we can calculate daylight factor by RADIANCE with a overcast sky. Then i
calculated the illuminance under overcast sky. I found the distribution of
illuminance and skyfactor in the room is different. For example, in the room
ratio of minimum to average of daylight and illuminance are quite different.
Would you tell me the reason?

Best Regards!

Hi Victor,

For daylight factor calculation in Radiance you can use *dayfact* script
It has interactive input ,so you can input all data you need, and it will
calculate Df and various related images.

If you want to calculate in only in few points, you can do it simple,
calculate illuminance in points inside the building for overcast sky (with
rtrace program), and just divide this values with overcast sky horizontal
illuminance value.
For example overcast sky for Paris for diffuse horizontal illuminance
10000lux
gensky 6 22 +11 -c -a 48.817 -o -2.483 -m -15 -B 55.866

-B is irradiance an is calculated as 10000/179=55.866

And rtrace command would be like:
rtrace -I+ -h- -oov -ab 5 -ad 1024 -as 256 -aa 0.15 -ar 1000 octree.oct|
rcalc -e '\$1=\$1; \$2=\$2; \$3=\$3;
\$4=(0.265*\$4+0.670*\$5+0.065*\$6)*179'<points.txt >output.txt

Note that you should put appropriate calculation parameters in rtrace to
have accurate results.
rcalc command above converts rtrace output from RGB to 1 illuminance value.

So when you got illuminances inside the building just divide them with
10000, to get DF values, or directly in rcalc divide with 10000 to get Df
values on output:
\$4=(0.265*\$4+0.670*\$5+0.065*\$6)*179/10000

Hope this helps,
Marija

···

On Thu, Dec 10, 2009 at 10:04 AM, Victor Li <[email protected]>wrote:

Dear:

Daylight factors are expressed as the percentage of natural light falling
on a work surface compared to that which would have fallen on a completely
unobstructed horizontal surface under exactly the same sky conditions. Thus,
a daylight factor of 5% on an internal surface means that it received only
1/20th of the maximum available natural light.

so we can calculate daylight factor by RADIANCE with a overcast sky. Then i
calculated the illuminance under overcast sky. I found the distribution of
illuminance and skyfactor in the room is different. For example, in the room
ratio of minimum to average of daylight and illuminance are quite different.
Would you tell me the reason?

Best Regards!

It would help to know your command process for calculating illuminance
and daylight factor to troubleshoot.

Also, if your rpict/rtrace ambient sampling settings are not stringent
enough, differences could be due to random sampling variation between
two different calculation runs.

···

From: [email protected]
[mailto:[email protected]] On Behalf Of
Victor Li
Sent: Thursday, December 10, 2009 4:04 AM

Dear:

Daylight factors are expressed as the percentage of natural light
falling on a work surface compared to that which would have fallen on a
completely unobstructed horizontal surface under exactly the same sky
conditions. Thus, a daylight factor of 5% on an internal surface means
that it received only 1/20th of the maximum available natural light.

so we can calculate daylight factor by RADIANCE with a overcast sky.
Then i calculated the illuminance under overcast sky. I found the
distribution of illuminance and skyfactor in the room is different. For
example, in the room ratio of minimum to average of daylight and
illuminance are quite different. Would you tell me the reason?

Best Regards!

____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

The rtrace setting is "rtrace -I -h -dp 2048 -ar 32 -ms 0.063 -ds .2 -dt .05
-dc .75 -dr 3 -sj 1 -st .01 -ab 8 -aa .1 -ad 512 -as 256 -av 0.01 0.01 0.01
-lr 12 -lw .0005 -af 11.amb 11.oct < 11.pts > 11.dat" And the two
calculations of daylight factor and illuminance are all the same settings.
What do you think?

···

2009/12/10 Christopher Rush <[email protected]>

It would help to know your command process for calculating illuminance
and daylight factor to troubleshoot.

Also, if your rpict/rtrace ambient sampling settings are not stringent
enough, differences could be due to random sampling variation between two
different calculation runs.

*From:* [email protected] [mailto:
[email protected]] *On Behalf Of *Victor Li
*Sent:* Thursday, December 10, 2009 4:04 AM

Dear:

Daylight factors are expressed as the percentage of natural light falling
on a work surface compared to that which would have fallen on a completely
unobstructed horizontal surface under exactly the same sky conditions. Thus,
a daylight factor of 5% on an internal surface means that it received only
1/20th of the maximum available natural light.

so we can calculate daylight factor by RADIANCE with a overcast sky. Then i
calculated the illuminance under overcast sky. I found the distribution of
illuminance and skyfactor in the room is different. For example, in the room
ratio of minimum to average of daylight and illuminance are quite different.
Would you tell me the reason?

Best Regards!

____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

_______________________________________________
[email protected]

What are the dimensions and resolution of your grid in 11.pts compared
to the full boundary of your scene (including ground plane and anything
else) in 11.oct? Maybe -ar is not high enough.

What is your sky definition going into 11.oct?

Do you have an rcalc command to convert the output in 11.dat?

···

From: [email protected]
[mailto:[email protected]] On Behalf Of
Victor Li
Sent: Thursday, December 10, 2009 8:34 PM

The rtrace setting is "rtrace -I -h -dp 2048 -ar 32 -ms 0.063 -ds .2 -dt
.05 -dc .75 -dr 3 -sj 1 -st .01 -ab 8 -aa .1 -ad 512 -as 256 -av 0.01
0.01 0.01 -lr 12 -lw .0005 -af 11.amb 11.oct < 11.pts > 11.dat" And
the two calculations of daylight factor and illuminance are all the same
settings. What do you think?

2009/12/10 Christopher Rush <[email protected]>

It would help to know your command process for calculating illuminance
and daylight factor to troubleshoot.

Also, if your rpict/rtrace ambient sampling settings are not stringent
enough, differences could be due to random sampling variation between
two different calculation runs.

From: [email protected]
[mailto:[email protected]] On Behalf Of
Victor Li
Sent: Thursday, December 10, 2009 4:04 AM

Dear:

Daylight factors are expressed as the percentage of natural light
falling on a work surface compared to that which would have fallen on a
completely unobstructed horizontal surface under exactly the same sky
conditions. Thus, a daylight factor of 5% on an internal surface means
that it received only 1/20th of the maximum available natural light.

so we can calculate daylight factor by RADIANCE with a overcast sky.
Then i calculated the illuminance under overcast sky. I found the
distribution of illuminance and skyfactor in the room is different. For
example, in the room ratio of minimum to average of daylight and
illuminance are quite different. Would you tell me the reason?

Best Regards!

____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

_______________________________________________
[email protected]

Thank you very much!

Actually i uesed ECOTET exported to DESKTOP RADIANCE to calculate. The only
differernce between Daylight factor and Illuminance calculation is the -B
In Daylight factor calculation the sky is defined as following:

!gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 | xform
-rz -27.000

skyfunc glow sky_mat
0
0
4
1 1 1 0

sky_mat source sky
0
0
4
0 0 1 180

skyfunc glow ground_glow
0
0
4
1 .8 .5 0

ground_glow source ground
0
0
4
0 0 -1 180

In Illuminance calculation, the definition is :

!gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 | xform
-rz -27.000

The rest is all the same.

For converting the output, the ECOTECT can convert it automatically.

···

2009/12/11 Christopher Rush <[email protected]>

What are the dimensions and resolution of your grid in 11.pts compared to
the full boundary of your scene (including ground plane and anything else)
in 11.oct? Maybe –ar is not high enough.

What is your sky definition going into 11.oct?

Do you have an rcalc command to convert the output in 11.dat?

*From:* [email protected] [mailto:
[email protected]] *On Behalf Of *Victor Li
*Sent:* Thursday, December 10, 2009 8:34 PM

The rtrace setting is "rtrace -I -h -dp 2048 -ar 32 -ms 0.063 -ds .2 -dt
.05 -dc .75 -dr 3 -sj 1 -st .01 -ab 8 -aa .1 -ad 512 -as 256 -av 0.01 0.01
0.01 -lr 12 -lw .0005 -af 11.amb 11.oct < 11.pts > 11.dat" And the two
calculations of daylight factor and illuminance are all the same settings.
What do you think?

2009/12/10 Christopher Rush <[email protected]>

It would help to know your command process for calculating illuminance and
daylight factor to troubleshoot.

Also, if your rpict/rtrace ambient sampling settings are not stringent
enough, differences could be due to random sampling variation between two
different calculation runs.

*From:* [email protected] [mailto:
[email protected]] *On Behalf Of *Victor Li
*Sent:* Thursday, December 10, 2009 4:04 AM

Dear:

Daylight factors are expressed as the percentage of natural light falling
on a work surface compared to that which would have fallen on a completely
unobstructed horizontal surface under exactly the same sky conditions. Thus,
a daylight factor of 5% on an internal surface means that it received only
1/20th of the maximum available natural light.

so we can calculate daylight factor by RADIANCE with a overcast sky. Then i
calculated the illuminance under overcast sky. I found the distribution of
illuminance and skyfactor in the room is different. For example, in the room
ratio of minimum to average of daylight and illuminance are quite different.
Would you tell me the reason?

Best Regards!

____________________________________________________________

Electronic mail messages entering and leaving Arup business

systems are scanned for acceptability of content and viruses

_______________________________________________
[email protected]

_______________________________________________
[email protected]

I have increased the -ar to 300, but the results are all the same.

···

2009/12/13 Victor Li <[email protected]>

Thank you very much!

Actually i uesed ECOTET exported to DESKTOP RADIANCE to calculate. The only
differernce between Daylight factor and Illuminance calculation is the -B
In Daylight factor calculation the sky is defined as following:

!gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 |
xform -rz -27.000

skyfunc glow sky_mat
0
0
4
1 1 1 0

sky_mat source sky
0
0
4
0 0 1 180

skyfunc glow ground_glow
0
0
4
1 .8 .5 0

ground_glow source ground
0
0
4
0 0 -1 180

In Illuminance calculation, the definition is :

!gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 |
xform -rz -27.000

The rest is all the same.

For converting the output, the ECOTECT can convert it automatically.

2009/12/11 Christopher Rush <[email protected]>

What are the dimensions and resolution of your grid in 11.pts compared to

the full boundary of your scene (including ground plane and anything else)
in 11.oct? Maybe –ar is not high enough.

What is your sky definition going into 11.oct?

Do you have an rcalc command to convert the output in 11.dat?

*From:* [email protected] [mailto:
[email protected]] *On Behalf Of *Victor Li
*Sent:* Thursday, December 10, 2009 8:34 PM

The rtrace setting is "rtrace -I -h -dp 2048 -ar 32 -ms 0.063 -ds .2 -dt
.05 -dc .75 -dr 3 -sj 1 -st .01 -ab 8 -aa .1 -ad 512 -as 256 -av 0.01 0.01
0.01 -lr 12 -lw .0005 -af 11.amb 11.oct < 11.pts > 11.dat" And the two
calculations of daylight factor and illuminance are all the same settings.
What do you think?

2009/12/10 Christopher Rush <[email protected]>

It would help to know your command process for calculating illuminance and
daylight factor to troubleshoot.

Also, if your rpict/rtrace ambient sampling settings are not stringent
enough, differences could be due to random sampling variation between two
different calculation runs.

*From:* [email protected] [mailto:
[email protected]] *On Behalf Of *Victor Li
*Sent:* Thursday, December 10, 2009 4:04 AM

Dear:

Daylight factors are expressed as the percentage of natural light falling
on a work surface compared to that which would have fallen on a completely
unobstructed horizontal surface under exactly the same sky conditions. Thus,
a daylight factor of 5% on an internal surface means that it received only
1/20th of the maximum available natural light.

so we can calculate daylight factor by RADIANCE with a overcast sky. Then
i calculated the illuminance under overcast sky. I found the distribution of
illuminance and skyfactor in the room is different. For example, in the room
ratio of minimum to average of daylight and illuminance are quite different.
Would you tell me the reason?

Best Regards!

____________________________________________________________

Electronic mail messages entering and leaving Arup business

systems are scanned for acceptability of content and viruses

_______________________________________________
[email protected]

_______________________________________________
[email protected]

You have the same -B 0.558659 in both the daylight factor and
illuminance calculations. I'm afraid I can't help much more, as I'm not
sure what Ecotect does with the data that comes out of your Radiance
process.

Also, I'm still not sure what command you're actually using to generate

-Chris

···

From: [email protected]
[mailto:[email protected]] On Behalf Of
Victor Li
Sent: Saturday, December 12, 2009 11:29 PM

Thank you very much!

Actually i uesed ECOTET exported to DESKTOP RADIANCE to calculate. The
only differernce between Daylight factor and Illuminance calculation is
the -B 0.558659 definition in 11_SKY.RAD
In Daylight factor calculation the sky is defined as following:

!gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 |
xform -rz -27.000

skyfunc glow sky_mat
0
0
4
1 1 1 0

sky_mat source sky
0
0
4
0 0 1 180

skyfunc glow ground_glow
0
0
4
1 .8 .5 0

ground_glow source ground
0
0
4
0 0 -1 180

In Illuminance calculation, the definition is :

!gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 |
xform -rz -27.000

The rest is all the same.

For converting the output, the ECOTECT can convert it automatically.

2009/12/11 Christopher Rush <[email protected]>

What are the dimensions and resolution of your grid in 11.pts compared
to the full boundary of your scene (including ground plane and anything
else) in 11.oct? Maybe -ar is not high enough.

What is your sky definition going into 11.oct?

Do you have an rcalc command to convert the output in 11.dat?

From: [email protected]
[mailto:[email protected]] On Behalf Of
Victor Li
Sent: Thursday, December 10, 2009 8:34 PM

The rtrace setting is "rtrace -I -h -dp 2048 -ar 32 -ms 0.063 -ds .2 -dt
.05 -dc .75 -dr 3 -sj 1 -st .01 -ab 8 -aa .1 -ad 512 -as 256 -av 0.01
0.01 0.01 -lr 12 -lw .0005 -af 11.amb 11.oct < 11.pts > 11.dat" And
the two calculations of daylight factor and illuminance are all the same
settings. What do you think?

2009/12/10 Christopher Rush <[email protected]>

It would help to know your command process for calculating illuminance
and daylight factor to troubleshoot.

Also, if your rpict/rtrace ambient sampling settings are not stringent
enough, differences could be due to random sampling variation between
two different calculation runs.

From: [email protected]
[mailto:[email protected]] On Behalf Of
Victor Li
Sent: Thursday, December 10, 2009 4:04 AM

Dear:

Daylight factors are expressed as the percentage of natural light
falling on a work surface compared to that which would have fallen on a
completely unobstructed horizontal surface under exactly the same sky
conditions. Thus, a daylight factor of 5% on an internal surface means
that it received only 1/20th of the maximum available natural light.

so we can calculate daylight factor by RADIANCE with a overcast sky.
Then i calculated the illuminance under overcast sky. I found the
distribution of illuminance and skyfactor in the room is different. For
example, in the room ratio of minimum to average of daylight and
illuminance are quite different. Would you tell me the reason?

Best Regards!

____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

_______________________________________________
[email protected]

_______________________________________________
[email protected]

Victor.

There are a couple of things you should change. If you can't change the
options ECOTECT uses you can run the simulation manually as Marija
has shown above and then read the results back into ECOTECT.

More details within:

Thank you very much!

Actually i uesed ECOTET exported to DESKTOP RADIANCE to calculate.
The only differernce between Daylight factor and Illuminance calculation is
the -B 0.558659 definition in 11_SKY.RAD

In Daylight factor calculation the sky is defined as following:

!gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 | xform
-rz -27.000

I can't see a difference between the two gensky commands below but the
"-B 0.558659" will create a sky with 100 Lux diffuse vertical illuminance. This
is very low if you want to calculate Lux values. The only reason to do this
is that you don't have to convert from a 10000 Lux baseline to 100 Lux (or %)
if you want to calculate daylight factors.

Since both setups and scene files (skies) are the same you would
expect the same values for both calculations. However, your settings
for the rtrace command are not very accurate so the absolute results
are going to be different. You haven't told us yet how big the difference
is, btw.

[copied from an older mail]:

The rtrace setting is "rtrace -I -h -dp 2048 -ar 32 -ms 0.063 -ds .2 -dt
.05 -dc .75 -dr 3 -sj 1 -st .01 -ab 8 -aa .1 -ad 512 -as 256 -av 0.01 0.01
0.01 -lr 12 -lw .0005 -af 11.amb 11.oct < 11.pts > 11.dat" And the two

For more accuracy I would use something like:
"-ar 128 -ab 6 -aa 0.01 -ad 2048 -as 256"

Mind you, I don't know anything about your scene so these may not
be the right settings at all.

The remaining settings are less important in a daylight calculation
without sun. I would also remove the "-av 0.01 0.01 0.01" because
that would introduce a large amount of artificial ambient light that
is too high for you 100Lux outdoor illuminance.

[...]

skyfunc glow ground_glow
0
0
4
1 .8 .5 0

ground_glow source ground
0
0
4
0 0 -1 180

This creates a "ground" hemisphere that is nearly as bright as
the sky (about 80%). You should reduce the values in the
ground_glow definition to a quarter of their current setting
(unless you have ground covered in snow):

skyfunc glow ground_glow
0
0
4
0.25 0.2 0.125 0

[...]

In Illuminance calculation, the definition is :

!gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 | xform
-rz -27.000

The rest is all the same.

For converting the output, the ECOTECT can convert it automatically.

ECOTECT can read "single" value and "[r,g,b]" value output, but the
values have to be in the right scale. If you want your DF values
to mean anything you have to know the outside illuminance of the
sky and scale all the values if it is anything else than 100 lux.

Hth,
Thomas

···

On Sun, Dec 13, 2009 at 4:28 AM, Victor Li <[email protected]> wrote:

Actually i just export the model from ECOTECT to RADIANCE, and i chang some
parameters and run the simulation manually like Marija has shown above.

The first time i calculated the daylgith factor in a grid
I run it by define: gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000
-B 0.558659,
-B 0.558659 means the diffuse horizontal illuminance is 100lux outside then
the internal illuminance is the dylight factor.

The second time i just calculated the internal illuminance in a grid under
overcast sky in the same time as the first time. I defined the sky : gensky
12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000

In the grid, i can calculate the external diffuse horizontal illuminance at
that time under overcast sky in the same point by the external diffuse
horizontal illuminance = the internal illuminance/ daylight factor.
However, i calculated the the external diffuse horizontal illuminance by
different points the results are different. Actually the results should be
the same because it the external diffuse horizontal illuminance. So why is
it different?

···

2009/12/13 Thomas Bleicher <[email protected]>

Victor.

There are a couple of things you should change. If you can't change the
options ECOTECT uses you can run the simulation manually as Marija
has shown above and then read the results back into ECOTECT.

More details within:

On Sun, Dec 13, 2009 at 4:28 AM, Victor Li <[email protected]> > wrote:
> Thank you very much!
>
> Actually i uesed ECOTET exported to DESKTOP RADIANCE to calculate.
> The only differernce between Daylight factor and Illuminance calculation
is
> the -B 0.558659 definition in 11_SKY.RAD
>
> In Daylight factor calculation the sky is defined as following:
>
> !gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 |
xform
> -rz -27.000

I can't see a difference between the two gensky commands below but the
"-B 0.558659" will create a sky with 100 Lux diffuse vertical illuminance.
This
is very low if you want to calculate Lux values. The only reason to do this
is that you don't have to convert from a 10000 Lux baseline to 100 Lux (or
%)
if you want to calculate daylight factors.

Since both setups and scene files (skies) are the same you would
expect the same values for both calculations. However, your settings
for the rtrace command are not very accurate so the absolute results
are going to be different. You haven't told us yet how big the difference
is, btw.

[copied from an older mail]:
>> The rtrace setting is "rtrace -I -h -dp 2048 -ar 32 -ms 0.063 -ds .2 -dt
>> .05 -dc .75 -dr 3 -sj 1 -st .01 -ab 8 -aa .1 -ad 512 -as 256 -av 0.01
0.01
>> 0.01 -lr 12 -lw .0005 -af 11.amb 11.oct < 11.pts > 11.dat" And the two

For more accuracy I would use something like:
"-ar 128 -ab 6 -aa 0.01 -ad 2048 -as 256"

Mind you, I don't know anything about your scene so these may not
be the right settings at all.

The remaining settings are less important in a daylight calculation
without sun. I would also remove the "-av 0.01 0.01 0.01" because
that would introduce a large amount of artificial ambient light that
is too high for you 100Lux outdoor illuminance.

[...]

> skyfunc glow ground_glow
> 0
> 0
> 4
> 1 .8 .5 0
>
> ground_glow source ground
> 0
> 0
> 4
> 0 0 -1 180

This creates a "ground" hemisphere that is nearly as bright as
the sky (about 80%). You should reduce the values in the
ground_glow definition to a quarter of their current setting
(unless you have ground covered in snow):

skyfunc glow ground_glow
0
0
4
0.25 0.2 0.125 0

[...]

> In Illuminance calculation, the definition is :
>
> !gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659 |
xform
> -rz -27.000
>
> The rest is all the same.
>
> For converting the output, the ECOTECT can convert it automatically.

ECOTECT can read "single" value and "[r,g,b]" value output, but the
values have to be in the right scale. If you want your DF values
to mean anything you have to know the outside illuminance of the
sky and scale all the values if it is anything else than 100 lux.

Hth,
Thomas

_______________________________________________
[email protected]

Hi Victor,

You don't need 2 calculations, just one.
With the overcast sky:
gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000 -B 0.558659,
calculate internal illuminances with rtrace, and then just divide this
values with 100lux (sky diffuse horizontal illuminance).
No need to calculate illuminances in the same point when there is no
building, since diffuse horizontal illuminance means exactly that:

···

**
*diffuse horizontal
illuminance*<http://www.satel-light.com/guide/glosatod.htm#evd>:Illuminance
produced by the visible part of the diffuse solar radiation on
a horizontal surface on the earth.

Also, I think you should use sky with higher diffuse illuminance, at least
1000lux, because with such low outside lux, like the ones you have, results
inside may be too low, and errors due to ambient calculation may be more
visible.

Hope this helps,
Marija

On Wed, Dec 16, 2009 at 10:57 AM, Victor Li <[email protected]>wrote:

Actually i just export the model from ECOTECT to RADIANCE, and i chang some
parameters and run the simulation manually like Marija has shown above.

The first time i calculated the daylgith factor in a grid
I run it by define: gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000
-B 0.558659,
-B 0.558659 means the diffuse horizontal illuminance is 100lux outside then
the internal illuminance is the dylight factor.

The second time i just calculated the internal illuminance in a grid under
overcast sky in the same time as the first time. I defined the sky : gensky
12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000

In the grid, i can calculate the external diffuse horizontal illuminance at
that time under overcast sky in the same point by the external diffuse
horizontal illuminance = the internal illuminance/ daylight factor.
However, i calculated the the external diffuse horizontal illuminance by
different points the results are different. Actually the results should be
the same because it the external diffuse horizontal illuminance. So why is
it different?

Actually i just export the model from ECOTECT to RADIANCE, and i chang some
parameters and run the simulation manually like Marija has shown above.

The first time i calculated the daylgith factor in a grid
I run it by define: gensky 12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000
-B 0.558659,
-B 0.558659 means the diffuse horizontal illuminance is 100lux outside then
the internal illuminance is the dylight factor.

In principle that is correct. But other options in your rtrace command
introduce a rather high inaccuracy and some additional light which
will have a bigger effect on the results if you use an illuminance of
only 100 lux. If you use 10000 lux and devide by 100 you reduce
some of these effects.

The second time i just calculated the internal illuminance in a grid under
overcast sky in the same time as the first time. I defined the sky : gensky
12 21 12.00 -c -a 31.400 -o -121.400 -m -120.000

In the grid, i can calculate the external diffuse horizontal illuminance at
that time under overcast sky in the same point by the external diffuse
horizontal illuminance = the internal illuminance/ daylight factor.
However, i calculated the the external diffuse horizontal illuminance by
different points the results are different. Actually the results should be
the same because it the external diffuse horizontal illuminance. So why is
it different?

Radiance has some built-in randomisation. Therefore the result of
two identical calculations will be slightly different. Learning to choose
the correct options to minimise this difference is one of the challanges

In your example I assume that the first set of daylight factors is rather
inaccurate. You now multiply this error by your internal illuminances
(which might be inaccurate, too). If your daylight factor is off by 1%
this can easily result in a difference of a few hundret lux.

It would realy help us if you could mention the actual results
of your calculations in your emails. Then we could see if the
errors are within the expected margin of your settings.

You should try your calculations with modified parameters
as suggested and with an increased sky illuminance to see
if your errors are still the same.

Regards,
Thomas

···

On Wed, Dec 16, 2009 at 9:57 AM, Victor Li <[email protected]> wrote: