[Radiance-general] Asking for informations
Jan Wienold
jan.wienold at epfl.ch
Fri Apr 15 07:26:45 PDT 2016
some more explanations:
about gendaylit:
to understand the output you have to know what gendaylit does and on
what models it is based:
Gendaylit uses
- a sky luminance distribution model according to Perez ( details and
publication reference see manpage),
- a luminance efficacy model according to Perez(details and publication
reference see manpage), because the conversion factor between solar
spectrum units and photometric units (=luminance efficacy) is changing
depending on the sky type and situation (for example, close to sunset,
the infrared proportion of the radiation increases a lot)
- it generates automatically the skytype based on the radiation data
(and time and location)
Gendaylit assumes, that the input is given via the -W (direct
normal,diffuse horizontal) or -G (direct horizontal and diffuse
horizontal) is radiation in the solar spectrum.
The output option -O 1 does not apply any luminance efficacy model, but
is using the Perez sky distribution. Therefore when you "put in" solar
spectrum you get also radiometric units as output. e.g. if you run
rtrace -I , then the output is directly W/m2.
The output option -O 2 applies the the Perez sky distribution AND the
luminance efficacy model. The output units are directly in photometric
quantities. e.g. if you run rtrace -I you get directly a lux value.
The output option -O 0 (default) does the same as the -O 2 option EXCEPT
the output is divided by 179. This is mainly done to be consistent with
the output of gensky.
about gensky:
it generates sky distributions according to the CIE model. you have to
specify explicitly the sky type (sunny, overcast...)
gensky does not apply any luminance efficacy model (at least I don't
know any option to do so) . So you get as output unit what you put into
it! So if you put in a solar spectrum power, then your output is also
radiometric. If you put in only the power in the visible range, then you
get the photometric quantities.
But don't put in radiometric and then expect the output to be
photometric quantities!! The 179lm/W conversion is not a luminance
efficacy model, it is just a "chosen" convention to calculate from the
radiation (in the visual spectrum) to the photometric quantities. It
does not take into account, that for example the luminance efficacy of
the sun is between 80-100 lm/W (or during sunset it drops to 50 lm/W),
whereas an overcast sky has about 130 lm/W.
as conclusion:
When you are just interested in the solar spectrum, then the difference
for a specific skytype (e.g. completely sunny) between gendaylit -O 1
and gensky is "only" the sky luminance distribution.
When you are interested in photometric quantities AND your input data is
based on the solar spectrum(e.g. from a EPW file or measured), then you
should use gendaylit only (except you calculate externally the luminance
efficacy and run then gensky).
Jan
On 14/04/16 20:47, Christopher Rush wrote:
>
> To further expand on the multiplier question… here’s my interpretation
> to be corrected if anyone understands differently…
>
> -O 0 would give Watts of energy within the visible spectrum in the sky
> model, and apply the default 179 multiplier to convert to lumens in
> any outputs.
>
> -O 1 would give Watts of energy across the full solar spectrum in the
> sky model, and typically would have to set a multiplier of 1 to keep
> Watts in any outputs.
>
> -O 2 would give lumens of energy in the visible spectrum in the sky
> model, and you would have to set a multiplier of 1 to keep that in any
> outputs. I agree I’m not sure why to do this instead of the default
> 179 multiplier.
>
> *From:*Germán Molina Larrain [mailto:germolinal at gmail.com]
> *Sent:* Thursday, April 14, 2016 8:36 AM
> *To:* Radiance general discussion
> *Subject:* Re: [Radiance-general] Asking for informations
>
> From my understanding,
>
> 1. -O0 will give you a Perez Sky... similar to the Gensky output, but
> using a diferent sky model. This model does not need to be told if
> the sky is overcast or clear. It will figure that out itself.
> 2. -O1 will give you the Perez Sky but with for the solar spectrum
> (i.e. solar radiation calculation)
> 3. -O2 is something that I wonder why is there.
>
> If you want your output on Watts (or any energy unit), you should use
> a multiplier of 1. For Lighting units (lux, cd, lumens) you need a
> multiplier of 179.....I do not usually make pictures... but I hope the
> few I have done were correct.
>
> Best.
>
> 2016-04-14 8:21 GMT-03:00 Terrance McMinn <t.mcminn at ocpms.com.au
> <mailto:t.mcminn at ocpms.com.au>>:
>
> Follow up question:
>
> Using gendaylit rather than the gensky sky generator we have
> additional -O options. -O 1 for W/m^2/sr solar radiation.
>
> /From the gendaylit man page:
> The output can be set to either the radiance of the visible
> radiation, the solar radiance (full spectrum) or the
> luminance. *-O*[0|1|2] (0=output in W/m^2/sr visible radiation
> (default), 1=output in W/m^2/sr solar radiation, 2=output in
> lm/m^2/sr luminance)./
>
> Looking at the source code it appears the -O 0 option divides
> illuminance by 179 whilst the -O 1 and -O 2 don't. I couldn't see
> how the units are outputted from gensky (in relation to the 179
> multiplier) to allow me to compare it to the gendaylit options.
>
> One might assume that the -O 0 option would be set to similar
> units as the gensky generator.
>
> From this confusion what multiplier should be used with the
> falsecolor program when using gendaylit options 0, 1 and 2
> remembering that falsecolor has the default -m 179?
>
> (I all cases I would be using the rpict -i option and looking for
> units in W/m^2)
>
> Regards
> *Terrance McMinn*
>
> On 8/04/2016 10:27 am, Terrance McMinn wrote:
>
> Thank you Chris.
>
> From an irradiance image (-i) I didn't realise a multiplier of
> 1 in falsecolor provided units in W/m^2
>
> I accept the reservations. For this project a gross mapping
> over a large site with block building shapes, the results can
> only be construed as indicative.
>
> Regards
> *Terrance McMinn*
>
> Yes, there is a -m multiplier option to falsecolor, which
> defaults to -m 179 and should equate to lux or
> cd/m². Luminance in cd/m2 is the default label to display
> luminance unless you give the -l lux option for
> example for illuminance – both related to the 179
> lumen/Watt multiplier.
>
> However for Watts you can use -m 1 in your options
> to falsecolor along with -l W/m2 to change the label
> to display as W/m2. That is presuming your image is
> calculated with -i option for irradiance. I don’t think
> people would normally expect to see a Watts based radiance
> image in W/sr/m2.
>
> This is also presuming the intent of your image takes into
> account all the shortcomings that Achim noted.
>
> *From:*Terrance McMinn [mailto:t.mcminn at ocpms.com.au]
> *Sent:* Thursday, April 07, 2016 9:39 AM
>
> Can you configure falsecolor to map as w/sqm?
>
> ____________________________________________________________
> Electronic mail messages entering and leaving Arup business
> systems are scanned for acceptability of content and viruses
>
>
> _______________________________________________
> Radiance-general mailing list
> Radiance-general at radiance-online.org
> <mailto:Radiance-general at radiance-online.org>
> http://www.radiance-online.org/mailman/listinfo/radiance-general
>
>
>
> _______________________________________________
> Radiance-general mailing list
> Radiance-general at radiance-online.org
> http://www.radiance-online.org/mailman/listinfo/radiance-general
--
Dr.-Ing. Jan Wienold
Ecole Polytechnique Fédérale de Lausanne (EPFL)
EPFL ENAC IA LIPID
http://people.epfl.ch/jan.wienold
LE 1 111 (Office)
Phone +41 21 69 30849
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.radiance-online.org/pipermail/radiance-general/attachments/20160415/c1a697f1/attachment-0001.html>
More information about the Radiance-general
mailing list