Questions about uniform sky

radiance-general
1

Hi,

I'm just trying to understand a few things about using a uniform sky.

1. With a uniform sky, how can I determine the theoretical irradiance or illuminance at 0,0,0 looking straight up? I'm currently testing a polygon on the z=0 plane and I get a slight variance in the illumination across that surface, due to slight errors in the calcs I presume. What I need is the theoretical exact value...

2. With a uniform sky, will the position of my test plane matter? ie. If I translate the plane from the origin +10000 in the Z direction, will the illumination on that plane still be identical?

Thanks,

Chris

2

I'm just trying to understand a few things about using a uniform sky.

1. With a uniform sky, how can I determine the theoretical irradiance or illuminance at 0,0,0 looking straight up? I'm currently testing a polygon on the z=0 plane and I get a slight variance in the illumination across that surface, due to slight errors in the calcs I presume. What I need is the theoretical exact value...

OK I presume I have to convert this brightness value:

# Ground ambient level: 23.8

Into an illuminance value. Ummm, what is "brightness" as far as Radiance is concerned and how to convert to illuminance?

2. With a uniform sky, will the position of my test plane matter? ie. If I translate the plane from the origin +10000 in the Z direction, will the illumination on that plane still be identical?

OK, I tested it and I can easily go up many tens of kilometres and still have same illumination...

Chris

3

1. Christopher, the theoretical factor between radiance and irradiance at a
point in that case is pi, you only have to integrate a uniform radiance over
a hemisphere to obtain it. Radiance can give some lite error due to the
randomless of indirect calculation.

2. yes, the simulation is over hemisphere or you can determine the source
angle you want.ie

void glow sky_glow
0
0
4 1 1 1 0

sky_glow source sky
0
0
4 0 0 1 angle

···

----- Original Message -----
From: "Christopher Kings-Lynne" <[email protected]>
To: "Radiance-General" <[email protected]>
Sent: Friday, February 25, 2005 11:05 AM
Subject: [Radiance-general] Questions about uniform sky

Hi,

I'm just trying to understand a few things about using a uniform sky.

1. With a uniform sky, how can I determine the theoretical irradiance or
illuminance at 0,0,0 looking straight up? I'm currently testing a
polygon on the z=0 plane and I get a slight variance in the illumination
across that surface, due to slight errors in the calcs I presume. What
I need is the theoretical exact value...

2. With a uniform sky, will the position of my test plane matter? ie.
If I translate the plane from the origin +10000 in the Z direction, will
the illumination on that plane still be identical?

Thanks,

Chris

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http://www.radiance-online.org/mailman/listinfo/radiance-general

4

1. Christopher, the theoretical factor between radiance and irradiance at a
point in that case is pi, you only have to integrate a uniform radiance over
a hemisphere to obtain it. Radiance can give some lite error due to the
randomless of indirect calculation.

Ah, I think I've got it...

Execute "gensky 3 21 14:00 -a 51.3 -o 0.1 -m 0.0 -u" to see:

# Ground ambient level: 23.8

Now, take the 23.8 and multiply by 179PI:

horiz_illum = 23.8 * 179 * PI = 13383.813

Which is very close to my empirical values, so must be correct, right? :smiley:

Thanks,

Chris

5

take a look at skybright.cal to know how radiance deals with sky, it's
explained too in rendering with radiance book

···

----- Original Message -----
From: "Christopher Kings-Lynne" <[email protected]>
To: "Radiance general discussion" <[email protected]>
Sent: Friday, February 25, 2005 12:00 PM
Subject: Re: [Radiance-general] Questions about uniform sky

> 1. Christopher, the theoretical factor between radiance and irradiance

at a

> point in that case is pi, you only have to integrate a uniform radiance

over

> a hemisphere to obtain it. Radiance can give some lite error due to the
> randomless of indirect calculation.

Ah, I think I've got it...

Execute "gensky 3 21 14:00 -a 51.3 -o 0.1 -m 0.0 -u" to see:

# Ground ambient level: 23.8

Now, take the 23.8 and multiply by 179PI:

horiz_illum = 23.8 * 179 * PI = 13383.813

Which is very close to my empirical values, so must be correct, right? :smiley:

Thanks,

Chris

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6

Hi,

You may find some information regarding sky models, irradiance and illuminance calculations in my RADIANCE tutorial:
http://www.eif.ch/~compag/ref/RADIANCE_tutorial_E.pdf

Hope this will be useful for you.

Raphaël

7

Agreed; I have been reading this document this morning with my coffee, and I have learned a couple things already. Sheesh, my "Radiance for Dummies" talk at the Montreal Workshop could consist of a list of urls to these fine documents. Axel, Christoph and Raphael have compiled some great stuff here.

···

On Feb 25, 2005, at 9:02 AM, Reinhart, Christoph wrote:

Hi Raphaël,

Your tutorial looks very useful. I am giving an introductory course on
building simulation this winter term at McGill and I am struggling to find
good tutorials for beginners. Maybe we should set up a central place, e.g.
on the Radiance online web site, which points to the various
Radiance/daylight simulation documents that are out there?

=================
    Rob Guglielmetti
www.rumblestrip.org

8

Hi Raphaël,

Your tutorial looks very useful. I am giving an introductory course on
building simulation this winter term at McGill and I am struggling to find
good tutorials for beginners. Maybe we should set up a central place, e.g.
on the Radiance online web site, which points to the various
Radiance/daylight simulation documents that are out there?

For those interested, our tutorial on the use of Radiance/Daysim Simulations
for Sustainable Design is located under
http://irc.nrc-cnrc.gc.ca/ie/light/daysim/DaysimTutorial.pdf.

Christoph

···

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Raphael
Compagnon
Sent: Friday, February 25, 2005 8:47 AM
To: Radiance general discussion
Subject: Re: [Radiance-general] Questions about uniform sky

Hi,

You may find some information regarding sky models, irradiance and
illuminance calculations in my RADIANCE tutorial:
http://www.eif.ch/~compag/ref/RADIANCE_tutorial_E.pdf

Hope this will be useful for you.

Raphaël

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