# SSLD skies and zenith radiance

Hi,

I am looking into the ssld support that Phillip made available, and I found the following in the accompagnying documentation:

"4. Select an arbitrary zenith radiance, Z (eg. 100 W/m2/sr)."

What is the meaning of arbitrary here? The value is used later by the cal-file creating the distribution. I was hoping to create a distribution from the coefficients and the horizontal illuminance, but I do not know where to get data on zenith luminance.

Cheers, Lars.

Hi,

a short update, for now I am dividing the task to set-up a sky into two steps:

1) Create a gensky-description. The zenith radiance as calculated by gensky can be found in the skyfunc definition (it is the third number on the last line of the output, so it can easily be extracted using awk). The horizontal illuminance that is to be found for the ground luminance can be either calculated from the sky model using rtrace, or, for skies WITHOUT sun, be derived by multiplying the "ground ambient level" by pi*179. The sun position can also be read from the comment lines.

2) Set-up the ssld-description using the values found in step 1).

Maybe it would be worth to create a script that can do all that (and also set the ssld-coefficients from a look-up table for a given sky type).

Cheers Lars.

···

I am looking into the ssld support that Phillip made available, and I found the following in the accompagnying documentation:

"4. Select an arbitrary zenith radiance, Z (eg. 100 W/m2/sr)."

What is the meaning of arbitrary here? The value is used later by the cal-file creating the distribution. I was hoping to create a distribution from the coefficients and the horizontal illuminance, but I do not know where to get data on zenith luminance.

Hi Lars,

The SSLD creation process is a bit old and clunky. It does require a few hand calcs and a little interaction with the text file. It really could be scripted to create it automatically... One day...

To start with, you need to know what type of sky you're trying to create, where the sun is, and the direct and diffuse horizontal illuminances.

For the diffuse sky distribution, step 4 of my old documentation says just put in any old number for the zenith radiance - it doesn't matter what it is at that stage. That model is then tested and the zenith radiance is altered to create the correct diffuse horizontal illuminance in step 12.

The ground radiance is found in step 3, requiring knowledge of the global horizontal illuminance and the ground reflectance.

I'll have a search around here for a script that automates the process. I vaguely remember someone has done this before...

Cheers,
Phil.

Dr Phillip Greenup
Senior Consultant - Lighting and Sustainability

Arup
Level 17
1 Nicholson St
Melbourne VIC 3000
Ph +61 3 9668 5669
www.arup.com/lighting

···

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Lars O. Grobe
Sent: Wednesday, 1 July 2009 9:23 PM

Hi,

a short update, for now I am dividing the task to set-up a sky into two
steps:

1) Create a gensky-description. The zenith radiance as calculated by gensky can be found in the skyfunc definition (it is the third number on the last line of the output, so it can easily be extracted using awk).
The horizontal illuminance that is to be found for the ground luminance can be either calculated from the sky model using rtrace, or, for skies WITHOUT sun, be derived by multiplying the "ground ambient level" by pi*179. The sun position can also be read from the comment lines.

2) Set-up the ssld-description using the values found in step 1).

Maybe it would be worth to create a script that can do all that (and also set the ssld-coefficients from a look-up table for a given sky type).

Cheers Lars.

I am looking into the ssld support that Phillip made available, and I
found the following in the accompagnying documentation:

"4. Select an arbitrary zenith radiance, Z (eg. 100 W/m2/sr)."

What is the meaning of arbitrary here? The value is used later by the
cal-file creating the distribution. I was hoping to create a
distribution from the coefficients and the horizontal illuminance, but
I do not know where to get data on zenith luminance.

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