CIE Version 12 definition

Hi all,

I know there have been numerous discussions here about which CIE versions are used in Gensky and the possibility/utility in updating them etc…but I’m actually just trying to track down a definition/reference for the CIE Version 12 standard for some comparisons we are doing. Does anyone know where to find this?

Thanks!

I believe this is the latest set of definitions:

This is useful – I’ve actually been looking for such a reference for some time, but never came across this. Does anyone know if these formulas/parameters were officially adopted by the CIE?

It would be possible to implement this in gensky, offering a setting of sky type number (1-15) and options to override any of the 5 parameters (a-e). I’m not entirely sure who would use this. I certainly wouldn’t have a clue how or when to apply such a model, nor where to gather data for it.

The most challenging part would be coming up with an integrated sky irradiance, which is a prerequisite for determining the ground level in gensky. I could probably come up with a precalculated table based on solar altitude for the 15 standard types, but I’d be stuck with numerical integration for the general formula.

Before I go to the effort, I’d like some assurance that the endeavor would be worthwhile…

Yeah, pretty sure that paper describes the work that went into “ISO 15469:2004(en) Spatial distribution of daylight — CIE standard general sky”:
https://www.iso.org/obp/ui/#iso:std:iso:15469:ed-2:v1:en

“The CIE Standard General Sky is a solution in search of a problem, and Professor Tregenza is doing a great service in bringing it down to earth.”

Comment by Joe Lynes on this paper by Peter Tregenza: “Analysing sky luminance scans to obtain frequency distributions of CIE Standard General Skies” [1] I couldn’t agree more at the time, and nothing since then has changed my mind. The case for the General Sky wasn’t helped by not having a means to select the ‘right sky’ from, say, weather files. Some methods did appear [2][3], but I don’t believe they gained any traction. I reckon the General Sky, as a practical tool, is still looking too overcooked to be useful.

[1] P. Tregenza. Analysing sky luminance scans to obtain frequency distributions of CIE Standard General Skies. Lighting Research and Technology, 36(4):271–279, 2004.

[2] D. Dumortier and M. Kobav. Use of the Perez all weather sky luminance model to obtain the frequency of CIE standard sky types. Lux Europa, Berlin, pages 137–140, 2005.

[3] V. Ferraro, V. Marinelli, and M. Mele. A method for selecting the cie standard general sky model with regard to calculating luminance distributions. Journal of Atmospheric and Solar-Terrestrial Physics, 95-96:59 – 64, 2013.

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… and then I was gonna say something like what John just said re: your ask, Greg. =)

I definitely think it would be worthwhile! I agree with the other comments, but also I don’t think it’s all that uncommon (whether using radiance or not) for people to use a generalized clear sky for some things, and having these options would make comparative work easier.

Even if these skies aren’t reality, they provide a common baseline to compare the effect of e.g. design iterations, or compare between different software (with or without radiance). More flexibility in these options would make it easier to rule out differences in sky generation when trying to compare different methods.

In that vein, do you know what other software tools are using? How would we make sure our implementation of the CIE general skies agreed with theirs? Is it that important to have a 15-type basis for comparison as opposed to a 3-type (or the Perez model of gendaylit)?

Just asking.

I’d ask Ian what’s used in AGI, and who cares what Autodesk uses for anything. =)

And to compare AGI to Radiance’s new gensky, we could use the test cases in CIE 171:2006 maybe?

I do agree with @mdipompe that there are definitely cases where folks want to stick with a couple of “standard” skies for certain evaluations. They are also still used in methods of LEED (for what those are worth).

I like the British way saying things nicely … so I’d better not add
more comments in German style…

But, on the other hand, I also know some research groups working with
these sky-types (e.g. TU-Berlin) - so for those folks an implementation
would be surely helpful. TU-Berlin will hopefully soon come up with a
new spectral (or at least colored) sky model - which will be based at
least in its first version on these 15 types.

The question remains: would you ever be able to detect any meaningful differences between, say, the Perez All-Weather (or an overcast/clear blend) and the 15 skies of the CIE GS formulation? This paper suggests not: Sky model blends for predicting internal illuminance: a comparison founded on the BRE-IDMP dataset. Journal of Building Performance Simulation, 1(3):163–173, 2008.

Recall also that the daylight illumination received anywhere in a space is always some mix of direct and inter-reflected light from the sky and (when present) the sun, and never that just from a patch of sky (unless the space is pitch black). Accordingly, the effect of subtle (and not so subtle) divergences between sky luminance patterns on the internal daylighting conditions gets ironed out, often such that any differences remaining are likely to be only of (ahem) academic interest.

The CIE 171:2006 tests are crude, and there were some errors in the original.

For what it’s worth, I created a cal file to model the CIE 15 standard skies, called ‘ssldlum.cal’. It use to be on radiance-online but isn’t there any more. You can find it, and description of how to use it, here: http://eprints.qut.edu.au/15936/1/Philip_Greenup_Thesis.pdf
That said, I agree that there is little clear guidance on which of the 15 sky models to use when, and how to accurately set the sky (and sun) brightness. And I agree with John’s response that the choice of sky model may make little difference to daylight distributions in interior spaces.
Let me know if the old cal file would be of use to anyone and I can share it with you.

Hi Phillip,

I just added your thesis with CIE 15 cal file here:
https://www.radiance-online.org/learning/tips-tricks/tips-tricks.html
If anyone else knows of something that used to be on the old
radiance-online but isn’t on the current site let me know, I’d be happy to
add it. Or if there’s something that you think should be on there, I can
add too.

Andy

Thanks Andy! That’s a great idea to keep this sort of stuff together.