# Gendaylit Visible radiation

Hi everyone,

I 'm using gendaylit function to generate sky with visible spectrum only. I introduce horizontal direct irradiance and horizontal diffuse irradiance (-G parameter) which are computed from horizontal global irradiance (meteo data : full spectrum).

I don't understand why sky irrandiance in Radiance is approximatively 60% of horizontal global irradiance. In my mind, the visible spectrum is approximatively 40-45% of full spectrum, is it wrong?

Example gendaylit command :

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -G 500 200

Thanks for your answer.

Cheers
Julien

Hi Julien,

Using the default option (-O 0 ) gendaylit "transfers" the solar radiation into visible radiation, using a luminance efficacy model.
Therefore the "sky irradiation" (=equals visible irradiation) differs also from situation to situation. The luminance efficacy of the sun is much lower than for the sky and this is taken into account by the model.

So for your case

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -G 500 200

you get
425 W/m² visible radiation

if you use

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -O 1 -G 500 200
you get
695 W/m² solar radiation which is +- the input radiation of 500+200=700W/m²

So the results are as expected.

The included luiminance efficacy model works properly within gendaylit - at least a recently internal performed validation study on measured data showed this.

By the way, my colleague will announce later today a major update of gendaylit, which have some significant improvements included.

Cheers,

Jan

···

On 01/31/2013 11:56 AM, Julien Boutillier wrote:

Hi everyone,

I 'm using gendaylit function to generate sky with visible spectrum only. I introduce horizontal direct irradiance and horizontal diffuse irradiance (-G parameter) which are computed from horizontal global irradiance (meteo data : full spectrum).

I don't understand why sky irrandiance in Radiance is approximatively 60% of horizontal global irradiance. In my mind, the visible spectrum is approximatively 40-45% of full spectrum, is it wrong?

Example gendaylit command :

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -G 500 200

Thanks for your answer.

Cheers
Julien

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

--
Dr.-Ing. Jan Wienold
Head of Team Passive Systems and Daylighting
Fraunhofer-Institut für Solare Energiesysteme
Thermal Systems and Buildings
Heidenhofstr. 2, 79110 Freiburg, Germany
Phone: +49(0)761 4588 5133 Fax:+49(0)761 4588 9133
[email protected]

In office:
Mo,Tue: 8:30-18:00
We,Thu: 8:30-16:00
Fr: 8:30-15:30

Hi Julien,

you did not forget to apply the skyfunc generated by gendaylit to a glow modifier and use that with a source object? How do you calculate the irradiance in Radiance? -ab is greater than 0?

Cheers, Lars.

···

Hi everyone,

I 'm using gendaylit function to generate sky with visible spectrum only. I introduce horizontal direct irradiance and horizontal diffuse irradiance (-G parameter) which are computed from horizontal global irradiance (meteo data : full spectrum).

I don't understand why sky irrandiance in Radiance is approximatively 60% of horizontal global irradiance. In my mind, the visible spectrum is approximatively 40-45% of full spectrum, is it wrong?

Example gendaylit command :

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -G 500 200

Thanks for your answer.

Cheers
Julien

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

Hi Lars,

I don't forget to apply the skyfunc to a glow modifier. Here is 2 files which describe my sky. Did I make something wrong?

···

-----------------------------
sky.rad
-----------------------------
# gendaylit 6 21 12:00GMT -a 46.517402 -o -6.562417 -m -15 -G 500 200
# Ground ambient level: 39.2

void light solar
0
0
3 4.510e+06 4.510e+06 4.510e+06

solar source sun
0
0
4 0.140166 -0.384035 0.912617 0.533000

void brightfunc skyfunc
2 skybright perezlum.cal
0
10 4.182e+01 2.566e+01 -0.905314 -0.339010 12.922843 -3.513900 0.118111 0.140166 -0.384035 0.912617

------------------------------
skyglow.rad
------------------------------
skyfunc glow sky_glow
0
0
4 1 1 1 0

sky_glow source sky
0
0
4 0 0 1 180

skyfunc glow ground_glow
0
0
4 1 1 1 0

ground_glow source ground
0
0
4 0 0 -1 180

--------------------------------

I use rtrace to calculate the irradiance in a point high in the sky (100meter). -ab parameter is equal to 0, I don't need interreflexion for this case, is it wrong?

Thanks for your help.

Cheers,

Julien

Le 31 janv. 2013 à 15:01, Lars O. Grobe a écrit :

Hi Julien,

you did not forget to apply the skyfunc generated by gendaylit to a glow modifier and use that with a source object? How do you calculate the irradiance in Radiance? -ab is greater than 0?

Cheers, Lars.

Hi everyone,

I 'm using gendaylit function to generate sky with visible spectrum only. I introduce horizontal direct irradiance and horizontal diffuse irradiance (-G parameter) which are computed from horizontal global irradiance (meteo data : full spectrum).

I don't understand why sky irrandiance in Radiance is approximatively 60% of horizontal global irradiance. In my mind, the visible spectrum is approximatively 40-45% of full spectrum, is it wrong?

Example gendaylit command :

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -G 500 200

Thanks for your answer.

Cheers
Julien

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

Hi Jan,

Thanks for the explanation.

I'll test this gendaylit update.

Cheers,

Julien

···

Le 31 janv. 2013 à 15:01, Jan Wienold a écrit :

Hi Julien,

Using the default option (-O 0 ) gendaylit "transfers" the solar radiation into visible radiation, using a luminance efficacy model.
Therefore the "sky irradiation" (=equals visible irradiation) differs also from situation to situation. The luminance efficacy of the sun is much lower than for the sky and this is taken into account by the model.

So for your case

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -G 500 200

you get
425 W/m² visible radiation

if you use

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -O 1 -G 500 200
you get
695 W/m² solar radiation which is +- the input radiation of 500+200=700W/m²

So the results are as expected.

The included luiminance efficacy model works properly within gendaylit - at least a recently internal performed validation study on measured data showed this.

By the way, my colleague will announce later today a major update of gendaylit, which have some significant improvements included.

Cheers,

Jan

On 01/31/2013 11:56 AM, Julien Boutillier wrote:

Hi everyone,

I 'm using gendaylit function to generate sky with visible spectrum only. I introduce horizontal direct irradiance and horizontal diffuse irradiance (-G parameter) which are computed from horizontal global irradiance (meteo data : full spectrum).

I don't understand why sky irrandiance in Radiance is approximatively 60% of horizontal global irradiance. In my mind, the visible spectrum is approximatively 40-45% of full spectrum, is it wrong?

Example gendaylit command :

gendaylit 6 21 12:00GMT -a 46.51 -o -6.56 -m -15 -G 500 200

Thanks for your answer.

Cheers
Julien

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

--
Dr.-Ing. Jan Wienold
Head of Team Passive Systems and Daylighting
Fraunhofer-Institut für Solare Energiesysteme
Thermal Systems and Buildings
Heidenhofstr. 2, 79110 Freiburg, Germany
Phone: +49(0)761 4588 5133 Fax:+49(0)761 4588 9133
[email protected]
http://www.ise.fraunhofer.de

In office:
Mo,Tue: 8:30-18:00
We,Thu: 8:30-16:00
Fr: 8:30-15:30

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

Hi Julien!

I don't forget to apply the skyfunc to a glow modifier. Here is 2 files which describe my sky. Did I make something wrong?

Seams to be ok in there.

I use rtrace to calculate the irradiance in a point high in the sky (100meter). -ab parameter is equal to 0, I don't need interreflexion for this case (...)

Could you please try with -ab 1? Your glow is effective only in the indirect calculation. rtrace calculates irradiance on a virtual diffuse surface, so the glow is hit over at least one bounce.

I have to admit that I tend to forget the exact behavior of rtrace here and cannot test it. The -I may internally increase -ab, but I would not expect it.

Cheers, Lars.

Hi Lars,

I have try with -ab 1 and the result is the same.

I search information about Perey algorithm which is used in gendaylit, and I understand my result now!

Thanks for your help,

Julien

···

Le 2 févr. 2013 à 07:12, Lars O. Grobe a écrit :

Hi Julien!

I don't forget to apply the skyfunc to a glow modifier. Here is 2 files which describe my sky. Did I make something wrong?

Seams to be ok in there.

I use rtrace to calculate the irradiance in a point high in the sky (100meter). -ab parameter is equal to 0, I don't need interreflexion for this case (...)

Could you please try with -ab 1? Your glow is effective only in the indirect calculation. rtrace calculates irradiance on a virtual diffuse surface, so the glow is hit over at least one bounce.

I have to admit that I tend to forget the exact behavior of rtrace here and cannot test it. The -I may internally increase -ab, but I would not expect it.

Cheers, Lars.
_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

Hi Julien,

whatever result you get there, something is wrong with the scene which is not about the Perez sky model or its parameters.

If you do an irradiance calculation using rtrace -I+ -ab 0, glow sources do not contribute at all to the result. That means that whatever sky distribution you generated, it will not be considered in your calculation. If you really got the same results for your sky with -ab 0 and -ab 1, that means that you did not have any diffuse sky component in your scene. Maybe the input was out of the valid range of the Perez sky, and only the sun (which is modeled as light, not glow), was written to the sky description.

Try it out yourself - create a scene with a 180 degrees source, modified by a glow definition of RGB 1, 1, 1. This should lead to an irradiance of RGB pi. With -ab 0, you will get zero irradiance, with -ab 1 and anything higher you get the expected 3.1416.... W/m2.

I have try with -ab 1 and the result is the same.

Cheers, Lars.

Hi Lars,

Thanks for this explanation.

I tried again to put -ab 0 and effectively I only have sun contribution. I don't know what I have done last time (I thought I had tried this case already).

With 500 W/m2 for direct irradiance and 200 W/m2 for diffus irradiance I obtain 280 W/m2 with -ab 0 and 425 W/m2 with -ab 1 or higher.

With 0 W/m2 for direct irradiance and 200 W/m2 for diffus irradiance I obtain 0 W/m2 with -ab 0 and 136 W/m2 with -ab 1 or higher.

I am not sure to understand why the diffus contribution is not the same (425-280 = 145 W/m2 not equal to 136 W/m2). I saw Perez parameters are not the same, so I think the created sky is a bit different, and this is why…?

Cheers,

Julien

···

Le 4 févr. 2013 à 21:49, Lars O. Grobe a écrit :

Hi Julien,

whatever result you get there, something is wrong with the scene which is not about the Perez sky model or its parameters.

If you do an irradiance calculation using rtrace -I+ -ab 0, glow sources do not contribute at all to the result. That means that whatever sky distribution you generated, it will not be considered in your calculation. If you really got the same results for your sky with -ab 0 and -ab 1, that means that you did not have any diffuse sky component in your scene. Maybe the input was out of the valid range of the Perez sky, and only the sun (which is modeled as light, not glow), was written to the sky description.

Try it out yourself - create a scene with a 180 degrees source, modified by a glow definition of RGB 1, 1, 1. This should lead to an irradiance of RGB pi. With -ab 0, you will get zero irradiance, with -ab 1 and anything higher you get the expected 3.1416.... W/m2.

I have try with -ab 1 and the result is the same.

Cheers, Lars.
_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

Hi Julien,

as Jan explained before, the inputs of gendaylit can be "visual irradiance" (which is probably achieved by dividing illuminance by Radiance internal luminous efficacy 179), irradiance (which is W/m2 as measured by a pyranometer without any weighing), or illuminance. You select according to your data using the -O option. If you use -O 0, gendaylit assumes that you have a typical data set as recorded by a pyranometer, which includes e.g. infrared. It will assume a luminous efficacy for sun and sky, and reduce the irradiance accordingly.

If you want to verify your method first, start simple. E.g. go with a simple model, where you assume that photo metric measurements were available. Create a basic sky with 1000lx beam (normal) direct illuminance and 1000lx diffuse horizontal. Use this with rtrace -I+ -ab 0, and you should get 1000/179 * cos(pi-sun altitude) W/m2. Render with -ab 1, and you should get this direct horizontal irradiance + 1000/179 W/m2.

The next step is to look into whether you have measured photo metric readings, or whether you need to use radio metric readings. In this case, you can let gendaylit reduce these using its luminous efficacy models.

Cheers, Lars.

Dipl.-Ing. Lars O. Grobe

···

Am 05.02.2013 um 10:13 schrieb Julien Boutillier <[email protected]>:

Hi Lars,

Thanks for this explanation.

I tried again to put -ab 0 and effectively I only have sun contribution. I don't know what I have done last time (I thought I had tried this case already).

With 500 W/m2 for direct irradiance and 200 W/m2 for diffus irradiance I obtain 280 W/m2 with -ab 0 and 425 W/m2 with -ab 1 or higher.

With 0 W/m2 for direct irradiance and 200 W/m2 for diffus irradiance I obtain 0 W/m2 with -ab 0 and 136 W/m2 with -ab 1 or higher.

I am not sure to understand why the diffus contribution is not the same (425-280 = 145 W/m2 not equal to 136 W/m2). I saw Perez parameters are not the same, so I think the created sky is a bit different, and this is why…?

Cheers,

Julien

Le 4 févr. 2013 à 21:49, Lars O. Grobe a écrit :

Hi Julien,

whatever result you get there, something is wrong with the scene which is not about the Perez sky model or its parameters.

If you do an irradiance calculation using rtrace -I+ -ab 0, glow sources do not contribute at all to the result. That means that whatever sky distribution you generated, it will not be considered in your calculation. If you really got the same results for your sky with -ab 0 and -ab 1, that means that you did not have any diffuse sky component in your scene. Maybe the input was out of the valid range of the Perez sky, and only the sun (which is modeled as light, not glow), was written to the sky description.

Try it out yourself - create a scene with a 180 degrees source, modified by a glow definition of RGB 1, 1, 1. This should lead to an irradiance of RGB pi. With -ab 0, you will get zero irradiance, with -ab 1 and anything higher you get the expected 3.1416.... W/m2.

I have try with -ab 1 and the result is the same.

Cheers, Lars.
_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

Hi Lars and Julien,

just to avoid some confusion about input and/or output:

The -O option does specify the output of gendaylit, not the input!
-O 0 -> output in visible radiation (-> visible spectrum)
-O 1 -> output in solar radiation (-> solar spectrum)
-O 2 -> output in candela/m² (-> visible spectrum)

The Input is specified by
-P epsilon delta (these are the Perez parameters, rarely used)
-W direct-normal-irradiance diffuse-horizontal-irradiance (W/m²) -> solar irradiance
-L direct-normal-illuminance diffuse-horizontal-illuminance (lux) - >illuminance (visual spectrum)
-G direct-horizontal-irradiance diffuse-horizontal-irradiance (W/m²) -> solar irradiance

Cheers,
Jan

···

On 02/05/2013 06:40 PM, Lars O. Grobe wrote:

Hi Julien,

as Jan explained before, the inputs of gendaylit can be "visual irradiance" (which is probably achieved by dividing illuminance by Radiance internal luminous efficacy 179), irradiance (which is W/m2 as measured by a pyranometer without any weighing), or illuminance. You select according to your data using the -O option. If you use -O 0, gendaylit assumes that you have a typical data set as recorded by a pyranometer, which includes e.g. infrared. It will assume a luminous efficacy for sun and sky, and reduce the irradiance accordingly.

If you want to verify your method first, start simple. E.g. go with a simple model, where you assume that photo metric measurements were available. Create a basic sky with 1000lx beam (normal) direct illuminance and 1000lx diffuse horizontal. Use this with rtrace -I+ -ab 0, and you should get 1000/179 * cos(pi-sun altitude) W/m2. Render with -ab 1, and you should get this direct horizontal irradiance + 1000/179 W/m2.

The next step is to look into whether you have measured photo metric readings, or whether you need to use radio metric readings. In this case, you can let gendaylit reduce these using its luminous efficacy models.

Cheers, Lars.

Dipl.-Ing. Lars O. Grobe

Am 05.02.2013 um 10:13 schrieb Julien Boutillier <[email protected]>:

Hi Lars,

Thanks for this explanation.

I tried again to put -ab 0 and effectively I only have sun contribution. I don't know what I have done last time (I thought I had tried this case already).

With 500 W/m2 for direct irradiance and 200 W/m2 for diffus irradiance I obtain 280 W/m2 with -ab 0 and 425 W/m2 with -ab 1 or higher.

With 0 W/m2 for direct irradiance and 200 W/m2 for diffus irradiance I obtain 0 W/m2 with -ab 0 and 136 W/m2 with -ab 1 or higher.

I am not sure to understand why the diffus contribution is not the same (425-280 = 145 W/m2 not equal to 136 W/m2). I saw Perez parameters are not the same, so I think the created sky is a bit different, and this is why…?

Cheers,

Julien

Le 4 févr. 2013 à 21:49, Lars O. Grobe a écrit :

Hi Julien,

whatever result you get there, something is wrong with the scene which is not about the Perez sky model or its parameters.

If you do an irradiance calculation using rtrace -I+ -ab 0, glow sources do not contribute at all to the result. That means that whatever sky distribution you generated, it will not be considered in your calculation. If you really got the same results for your sky with -ab 0 and -ab 1, that means that you did not have any diffuse sky component in your scene. Maybe the input was out of the valid range of the Perez sky, and only the sun (which is modeled as light, not glow), was written to the sky description.

Try it out yourself - create a scene with a 180 degrees source, modified by a glow definition of RGB 1, 1, 1. This should lead to an irradiance of RGB pi. With -ab 0, you will get zero irradiance, with -ab 1 and anything higher you get the expected 3.1416.... W/m2.

I have try with -ab 1 and the result is the same.

Cheers, Lars.
_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

--
Dr.-Ing. Jan Wienold
Head of Team Passive Systems and Daylighting
Fraunhofer-Institut für Solare Energiesysteme
Thermal Systems and Buildings
Heidenhofstr. 2, 79110 Freiburg, Germany
Phone: +49(0)761 4588 5133 Fax:+49(0)761 4588 9133
[email protected]

In office:
Mo,Tue: 8:30-18:00
We,Thu: 8:30-16:00
Fr: 8:30-15:30

Hi Jan!

just to avoid some confusion about input and/or output:

Thank you for the clearification, I was writing my latest message from what I remembered on a phone...

So far, whenever possible, I have avoided the whole conversion and its implied assumptions by using illuminance for input and -O 0, which allows me to stay with the conversion by a luminous factor of 179 lm / W as I use it everywhere else in Radiance. The weather data I use has a column with photometric units, and as I am interested in luminance and illuminance as the result of my simulations, I try to avoid introducing any additional conversion. One should however be aware that fields in weather data may actually be derived from models of luminous efficacy, too, as not every station measures both irradiance and illuminance...

Cheers, Lars.

Hi Jan and Lars,

Thanks for your explanation.

I understand well now.

Cheers

Julien

···

Le 5 févr. 2013 à 20:31, Lars O. Grobe a écrit :

Hi Jan!

just to avoid some confusion about input and/or output:

Thank you for the clearification, I was writing my latest message from what I remembered on a phone...

So far, whenever possible, I have avoided the whole conversion and its implied assumptions by using illuminance for input and -O 0, which allows me to stay with the conversion by a luminous factor of 179 lm / W as I use it everywhere else in Radiance. The weather data I use has a column with photometric units, and as I am interested in luminance and illuminance as the result of my simulations, I try to avoid introducing any additional conversion. One should however be aware that fields in weather data may actually be derived from models of luminous efficacy, too, as not every station measures both irradiance and illuminance...

Cheers, Lars.

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general