Exterior light levels and daylight factors

Hello,
I am a graduate student in architecture at the
University of Waterloo, Canada. My thesis centres on
the modelling of two Canadian 'sustainable' buildings
to investigate the relationship between daylighting
and energy consumption for heating and cooling. I am
modelling each building in Desktop Radiance in order
to evaluate the performance of both Ecotect and
Energy-10 in daylighting analysis.

I had hoped to use Desktop Radiance to evaluate
daylight factors in key areas of the buildings. While
I am able to model fairly realistic interior
illumination results and analyse these using the False
Colour plot I have encountered several problems:

1. When I opt for a daylight factor plot, the scale
remains at a range of 0 to 1.0 and is unaffected by
any changes I try to make through the analysis
dialogue box. The result in Radiance is a washed out
image all in the 'maximum' colour (my other software
suggests DF% between 2 and 4%) confirming DF%>1.0
How can I adjust the scale for DF% readings in Desktop
Radiance?

2. I am trying to assess the effect of changing
glazing types. For daylight studies in the late winter
afternoon (based on sun charts and other software, I'm
using 4pm Dec21, where the sun is not yet set, but
very low in the sky), where I have not modelled any
surrounding obstructions such as trees or buildings,
my internal illuminance levels are consistently about
200 lux, even on the east side of the building. Common
sense tells me there should be MUCH MUCH less light
than this...probably no more than 100 lux. Also, when
changing glazing types (ie, from clear to reflective)
a significant change is noticed for noon hour
lighting, but the 4pm Dec21 lighting remains at 200
lux. What could I be mis-assuming?

I have recently run a few simplified test models and
compared my results to true exterior light conditions
using a hand-held light meter. The results are
somewhat frustrating, but interesting nonetheless.

My questions here are particularly:
3. How does Radiance calculate the external light
levels?
4. On a perfectly clear morning in a low pollution
area (assumed turbidity=2) my light meter reading read
approx. 55600 lux. A Radiance test (modelling an
unobstructed horizontal plane outdoors) for the same
date and time, and lat. long settings, with
turbidity=2 produced an exterior light level of 47000
lux (15% off the true reading!). While there may have
been some diffuse reflection off nearby buildings,
there was certainly no direct reflection or shadow on
the light meter. This seems to be much too large a
difference. Any suggestions?

Apologies for the long email. I often read that these
software applications are tested and confirmed but my
data isn't showing an appropriate correlation of the
simulation to reality. Any remarks would be greatly
appreciated.

Caroline Prochazka
M.Arch candidate
University of Waterloo

···

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47000 comparato a 55600....
be' non e' mica male...
...boh!

···

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of
Caroline Prochazka
Sent: 06 April 2004 14:46
To: [email protected]
Subject: [Radiance-general] Exterior light levels and daylight factors

4. On a perfectly clear morning in a low pollution
area (assumed turbidity=2) my light meter reading read
approx. 55600 lux. A Radiance test (modelling an
unobstructed horizontal plane outdoors) for the same
date and time, and lat. long settings, with
turbidity=2 produced an exterior light level of 47000
lux (15% off the true reading!). While there may have
been some diffuse reflection off nearby buildings,
there was certainly no direct reflection or shadow on
the light meter. This seems to be much too large a
difference. Any suggestions?

__

___________________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses.

I did some validation measurements, some years ago, for my thesis... (not as
good as some others did before me... )
The values I had were quite different from the expected (radiance
calculated).
At the time I had not reliable information on the value of the turbidity
factor for the area I was studying.... what happened then it is another
story...
but going back to your problem:

I think that calculated illuminance can be the same only if you have a way
of measuring the sky luminance distribution.... (not CIE standard but
real...)
I think that the best paper you can find on it is the Mardaljevic PHD
thesis...
(http://www.iesd.dmu.ac.uk/~jm/sg98.htm)

The values that you have aren't so bad... compared to mine (I remember
something like 30% RER..)
;-((

PS - a recent post pointed out the limits of simulating low altitude sun
skies... (Perez sky model have the same limitation with low angles, where
uncertainty is really high) ... so the only way is to override the sky
definition and constrain it to a known illuminance value... I guess... but
you have still no way of knowing the luminance dist...

···

---------------------
about the falsecolor images you should try to use on the dos terminal:
falsecolor -s maxvalue -n numberof intervals -l label -m multiplier -i
yourpicture > finalimage

if gives you an error try this:
falsecolor -s maxvalue -n numberof intervals -l label -m multiplier -ip
yourpicture > finalimage

where multiplier is:
179 for illuminance and luminance (depending on the type of image)
179/external_illuminance for daylight factor plots

cheers,
giulio
----------------------------------------------------------------------------
------------------------

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of
Caroline Prochazka
Sent: 06 April 2004 14:46
To: [email protected]
Subject: [Radiance-general] Exterior light levels and daylight factors

Hello,
I am a graduate student in architecture at the
University of Waterloo, Canada. My thesis centres on
the modelling of two Canadian 'sustainable' buildings
to investigate the relationship between daylighting
and energy consumption for heating and cooling. I am
modelling each building in Desktop Radiance in order
to evaluate the performance of both Ecotect and
Energy-10 in daylighting analysis.

I had hoped to use Desktop Radiance to evaluate
daylight factors in key areas of the buildings. While
I am able to model fairly realistic interior
illumination results and analyse these using the False
Colour plot I have encountered several problems:

1. When I opt for a daylight factor plot, the scale
remains at a range of 0 to 1.0 and is unaffected by
any changes I try to make through the analysis
dialogue box. The result in Radiance is a washed out
image all in the 'maximum' colour (my other software
suggests DF% between 2 and 4%) confirming DF%>1.0
How can I adjust the scale for DF% readings in Desktop
Radiance?

2. I am trying to assess the effect of changing
glazing types. For daylight studies in the late winter
afternoon (based on sun charts and other software, I'm
using 4pm Dec21, where the sun is not yet set, but
very low in the sky), where I have not modelled any
surrounding obstructions such as trees or buildings,
my internal illuminance levels are consistently about
200 lux, even on the east side of the building. Common
sense tells me there should be MUCH MUCH less light
than this...probably no more than 100 lux. Also, when
changing glazing types (ie, from clear to reflective)
a significant change is noticed for noon hour
lighting, but the 4pm Dec21 lighting remains at 200
lux. What could I be mis-assuming?

I have recently run a few simplified test models and
compared my results to true exterior light conditions
using a hand-held light meter. The results are
somewhat frustrating, but interesting nonetheless.

My questions here are particularly:
3. How does Radiance calculate the external light
levels?
4. On a perfectly clear morning in a low pollution
area (assumed turbidity=2) my light meter reading read
approx. 55600 lux. A Radiance test (modelling an
unobstructed horizontal plane outdoors) for the same
date and time, and lat. long settings, with
turbidity=2 produced an exterior light level of 47000
lux (15% off the true reading!). While there may have
been some diffuse reflection off nearby buildings,
there was certainly no direct reflection or shadow on
the light meter. This seems to be much too large a
difference. Any suggestions?

Apologies for the long email. I often read that these
software applications are tested and confirmed but my
data isn't showing an appropriate correlation of the
simulation to reality. Any remarks would be greatly
appreciated.

Caroline Prochazka
M.Arch candidate
University of Waterloo

__________________________________
Do you Yahoo!?
Yahoo! Small Business $15K Web Design Giveaway
http://promotions.yahoo.com/design_giveaway/

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Hi Caroline,

First off, there is a Desktop-Radiance Yahoo forum, and you should definitely ask around on there too, especially for your falsecolor scale questions. I'm not familiar with the Windows version(s) of the various Radiance binaries.

Giulio gave you some good leads there regarding sky simulations. This kind of question pops up on here regularly, and indeed I went through the exact same experience as you. I'd read about how accurate Radiance could be (read that again, the semantics are important), then did my first daylight simulation and was disappointed. Lots of words have been put down on paper about the difference between real, measured skies and simulated ones. Papers with lots of Greek symbols fill the servers at universities & research facilities worldwide. Greg Ward can cut through the haze as well as anyone regarding this, but being all smart and "mathy", his explanations still confused me for a while. Eventually I got it:

Nature is way too complicated. To think that one or even a thousand formulas could mathematically describe the luminance distribution of the nearly infinite number of skies is pure folly.

What has been proven by Dr. Mardaljevic is that Radiance can simulate lit environments with a high degree of accuracy. What he had was an incredibly fine dataset of real sky luminance and building interior illuminance. When this data was fed to Radiance, he got very close to the same answers. But most of us do *not* have an incredibly fine dataset of real sky luminance and building interior illuminance, we have a lat/long, maybe some sky turbidity info, and the woefully inadequate suite of sky conditions descriptors "clear, intermediate and overcast". Whoopie. So, it's no wonder that our initial forays into daylight simulation with Radiance tend to be confidence shakers.

The truth is Radiance can be as accurate as you want, but most of us are starting with input that is inherently inaccurate. It's a good starting point, but you simply can't expect your results to match up to reality with any kind of high precision when using these basic CIE skies. Don't get me wrong, the CIE skies can still be used to make valid design decisions, and certainly comparative studies of building form and material will be quite valid. But it's when people want to take a light meter and have the LCD display show the same number that Radiance spat out that the disappointment creeps in. For pure numeric accuracy, which Dr. M proved is possible, you need more measured info about your site. My experience with Radiance is that it's very very good, certainly the best available tool out there for daylighting analysis. But I don't expect my numbers to be ±5%, or even ±10% sometimes. But I believe ±20% is a fair goal. Our faith in the numbers is based on experience and intuition and we have to work hard to get this point across to our clients all the time. In my view, it's been worth the effort.

Hopefully the Greek symbols types will weigh in on this too; it's an interesting topic.

···

----

      Rob Guglielmetti

e. [email protected]
w. www.rumblestrip.org

And, while I'm thinking of it, from Ian Ashdown's excellent "Thinking Photometrically" Lightfair course notes (http://www.helios32.com/Thinking%20Photometrically%20II.pdf):

"One major problem is that the calculations are necessarily based on the various CIE or IESNA sky models, which predict illuminances based on average sky conditions. It is not uncommon for instantaneous measured illuminances to be more than twice or less than half of the mean illuminances predicted by clear and overcast sky models; the situation
for partly cloudy skies is even worse.

"As for daylighting calculations, it is likely that Jongewaard (1993) is correct – the results are only as accurate as the accuracy of the input data. Done with care, it should be possible to obtain ±20 percent accuracy in the photometric predictions. However, this requires detailed knowledge and accurate modeling of both the indoor and outdoor environments. If this cannot be done, it may be advisable to walk softly and carry a calibrated photometer.