Today's Topics:
3. Re: sky definition part 2 (Greg Ward) -gensky
9. Re: sky definition part 2 (Zack Rogers) -gensky
6. Re: Re: sky definition part 2 (Alex Summerfield) -luminous efficacy
7. FW: [Radiance-general] sky definition (Martin Moeck) -luminous
efficacy
···
>
Message: 3
Date: Thu, 16 Oct 2003 08:02:08 -0700
Cc: <[email protected]>
To: [email protected]
From: Greg Ward <[email protected]>
Subject: [Radiance-general] Re: sky definition part 2
Reply-To: [email protected]Hi Zack,
It was great to meet you at the workshop, and I really enjoyed your
presentation.I think you're making trouble for yourself by trying to adjust the
turbidity factor in gensky. If you have the horizontal and diffuse
irradiance, or facsimilies thereof, you should be giving those to
gensky directly using the -B or -b and -R or -r options as we have been
discussing.Just to clarify, gensky ALWAYS produces an accurate CIE sky
distribution. Adjusting the turbidity or these other factors only
affects the absolute levels and the ratio between solar and sky
components. It does not change the distribution of skylight.This is not to say that the CIE sky model is the best. It is just an
agreed upon standard, and therefore serves as a reasonable point for
comparison. The Perez sky is probably a better approximation to actual
skies, particularly for the intermediate case.As I said in the bit of e-mail you so aptly quoted, the absolute values
produced by gensky with the default settings are unreliable because
they are not based on weather data for your area. If you have such
measurements, by all means use them, but don't adjust the turbidity --
go straight for the horizontal and diffuse specifications to gensky.
This overrides the turbidity approximation and makes it irrelevant.I hope this helps.
-Greg
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Message: 9
Date: Thu, 16 Oct 2003 18:43:13 -0600
From: Zack Rogers <[email protected]>
To: [email protected]
CC: Greg Ward <[email protected]>
Subject: [Radiance-general] Re: sky definition part 2
Reply-To: [email protected]Hi Greg,
> I think you're making trouble for yourself by trying to adjust the
> turbidity factor in gensky. If you have the horizontal and diffuse
> irradiance, or facsimilies thereof, you should be giving those to
> gensky directly using the -B or -b and -R or -r options as we have
> been discussing.Yes, the turbidity does seem to be trouble since it doesn't affect the
sun component. I don't have measurements I am trying to match, I am
just trying to make sure I have the correct IESNA recommended sky (CIE
sky) and sun components throughout the year.> Just to clarify, gensky ALWAYS produces an accurate CIE sky
> distribution. Adjusting the turbidity or these other factors only
> affects the absolute levels and the ratio between solar and sky
> components. It does not change the distribution of skylight.I don't doubt the relative luminance distribution of the sky is
accurate, I'm just questioning how Radiance determines the correct
overall level.I should clarify what I mean when I say the CIE sky function tells
me....I am refering to both the sky and sun calculations laid out in the
latest IESNA lighting handbook which uses Kittlers CIE adopted sky
function (which I understand is from his 1967 studies) to determine the
sky component. So I probably should have said that IESNA sky/sun
functions tell me. I am not sure if this is the accepted method used by
others. The direct component is calculated as this;Edn = Ext * e ^ (-cm)
where;
Edn = direct normal solar illuminance
Ext = extraterrestrial solar illuminance = Esc * (1 + 0.034 * cos (
(2pi/365)*(J-2)))
where;
Esc = solar illumination constant = 127.5 klx (11,850 fc)
J = julian date
c = atmospheric extinction coefficient (given as c = 0.21 for clear ; c
= 0.8 for partly cloudy)
m = optical air mass = 1 / sin (solar altitude)and Direct Horizontal (Edh) would then be;
Edh = Edn * sin (solar altitude)This gives the Direct Horiz to be 8,821 fc on a clear summer solstice at
40 lat. This is what I would like Radiance to give me without having to
specify with the -r parameter. Currently it is giving me 6,836fc.The IESNA recommends calculating the diffuse component like this;
Ekh = A + B * (sin (solar altitude))^ C
where;
Ekh = horiz. sky illuminance
A = sunrise/set illuminance = given as 0.8 for clear sky
B = solar altitude illuminance coef. = given as 15.5 for clear sky
C = solar altitude illuminance exponent = given as 0.5 for clear sky
(all in terms of klux)This gives a diffuse horiz of 1484.2fc. Radiance gives me this with a
turbidity of 3.22. But as I illustrated before it can range
dramatically. Ekh is then fed into the CIE sky distribution function
like so;Lz = Ekh * ZL
where;
Lz = zenith luminance
ZL = zenith luminance factor - given as a table of valuesThen Lz is part of Kittlers distribution function. I am not going to
attempt to type this function but I think we are on the same page here.
Although I understand they, CIE, are about to adopt a new sky function
with 15 different sky types (Thanks Karen for the info!). I have not
attempted to wrap my head around this yet...for now I am happy enough
with the old function.> This is not to say that the CIE sky model is the best. It is just an
> agreed upon standard, and therefore serves as a reasonable point for
> comparison. The Perez sky is probably a better approximation to
> actual skies, particularly for the intermediate case.Agreed, although the calcs I have outlined here have generally matched
up well with my measurements with enough accuracy for my uses, but I am
usually not in need of an extremely accurate sky just good average
representations.> As I said in the bit of e-mail you so aptly quoted, the absolute
> values produced by gensky with the default settings are unreliable
> because they are not based on weather data for your area. If you have
> such measurements, by all means use them, but don't adjust the
> turbidity -- go straight for the horizontal and diffuse specifications
> to gensky. This overrides the turbidity approximation and makes it
> irrelevant.Again, I don't necessarily have measurements and I don't want to have to
give gensky horiz. or diffuse specifications for annual calculations.
Although, I guess these could be pulled from annual weather data (TMY2).
Is this what others do??? I guess I am just wanting Radiance to give
me these IESNA recommended results throughout the year without having to
specify the correct turbidity, diffuce irradiance or solar radiance
values which are not required inputs to these equations. It would be
nice to have an elevation factor as well or a turbidity factor that
adjusts both the solar and sky accordingly.Please give me feedback, let me know if I have misintepretted any of
this. Since daylighting design puts food on the table I need to make
sure I am confident in my results. I am especially concerned since one
of my current projects is getting pretty detailed discussing fc-hours
underneath a tree canopy in an atrium in Chicago, given all the other
approximated variables that will be going into this model it would be
nice to be confident I am starting with good data. In this case though
I am just creating DF's with Radiance and feeding that into TMY2 data.
And so I guess as long as my sun to sky ratios are correct it should be
fine.Huh, maybe I just got it…Does the 2.75 default turbidity give the
correct ratio for the lower sun component Radiance provides? I guess
this is what you meant when you said they are not based on measurements
for my area. But these IESNA calcs are not for my area either, they are
for a generic sea level site. It seems more and more its a discrepancy
in where the c, A, B, C values come from (these variables essentially
determine the absolute magnitude for the sun and sky components) What
other standards are there in determing the magnitude of the sun and sky
throughout the year? Is weather data just used? I guess I could just
give my skies a mulitplier outside of gensky that adjusts my skies to
the levels I want. Did I just answer my own questions? Still it would
be nice if Radiance did this on its own. Are people really seeing7970
fc on a clear summer day??? Where are you living? I guess I am just
used to our clear Colorado skies where we will see up to 11,000fc
regularly in the summer.Well, sorry for typing your eyes off? Just describing this problem has
helped me understand much better what is going on. Hopefully others
find this discussion useful. If not, I aplogize.Thanks!
Zack
______________________________________________________________________
Message: 6
Date: Thu, 16 Oct 2003 16:58:33 +0100
Subject: Re: [Radiance-general] Re: sky definition part 2
From: Alex Summerfield <[email protected]>
To: [email protected]
Reply-To: [email protected]Gidday Greg, Zack & others...
Just to chip in about measurement and efficacies -
Paul Littlefair wrote a great review paper on sky efficacies, with overall
results:
144+-7 lm/W clear sky
115+-8 lm/W overcast skyLittlefair, PJ Measurement of the Luminous Efficacy, Light. Res & Tech,
1988 4:p177-188Also:
Fontoynont, M, Perceived Performance of Daylighting Systems: Lighting
Efficacy and Agreeableness, Solar Energy 2002 73(2) p83-94It should be remembered that measurements are often not of the actual sky
spectral power distribution but are taken with equipment calibrated to
Illuminant A with 160 lm/W.And if calibrated to D65 then efficacy is 203 lm/W.
hopefully all this doesn't make things even more 'hazy'
cheers
alex*******************************************************
A. J. Summerfield [email protected]
Faculty of Architecture, University of Sydney
*******************************************************
Message: 7
Subject: FW: [Radiance-general] sky definition
Date: Thu, 16 Oct 2003 12:20:05 -0400
From: "Martin Moeck" <[email protected]>
To: <[email protected]>
Reply-To: [email protected]Greg suggested yesterday:
"so you should probably intervene with your own correction if you are
working from physical
data. Therefore, I recommend multiplying values to the -r and -R options
by:
208/179 the ratio between the sun's efficacy and the
standard
Radiance factor
Likewise, values to the -b and -B options should be multiplied by:
110/179 the ratio for sky radiation efficacy over the
standard
Radiance factor"
I wrote back:
>
> are you sure? The sun's luminous efficacy above 20 deg. altitude is
> somewhere around 95-100 lm/W, overcast skies are around 120 lmW, clear
> skies around 150 lm/W. Therefore, your multipliers for the -R/r
> options should be smaller than the multipliers for the -B/b options?Greg answered:
Now that you mention it, I'm not sure at all. I got my number from the
efficacy of Standard Illuminant B, which is simulated (rather than
real) sunlight, and it could be way off. Where do you get your
numbers? If you are sure of them, please forward this message with my
apologies to radiance-general.
My answer:
The numbers for the solar/beam lumens/watt can be obtained from the
Illuminating Engineering Society Handbook , and they are listed as 95 lm/W.
If you need more precise data, you can go to
http://idmp.entpe.fr/library/papers/evlumeff.pdf
as one example. I do not think that taking the CIE daylight illuminants
and their luminous efficacy is a good way to do it.
The following book "Daylighting in Architecture" edited by Baker,
Fanchiotti, Steemers and published by James & James (ISBN 1-873936-21-4)
lists luminous efficacy functions for various sky/cloud conditions and sun
altitudes on pages 10.4 and 10.5. Input needed is the global irradiance for
calculating the overcast efficacy. They also give a formula for calculating
luminous efficacies for skies with both clear and diffuse components by
taking their ratio.
From those values, they offer a formula to obtain internal illuminance
subdivided into the internal direct sunlight factor, the clear sky factor
and the overcast sky factor.
Nevertheless, I had good experiences with actual weather data from
www.meteotest.com (software Meteonorm) and "gendaylit".
Martin, Penn State