[Radiance-general] Trans material - single pane of polycarbonate
D. Charlie Curcija
dccurcija at lbl.gov
Tue Apr 4 11:38:00 PDT 2017
Christopher,
I think that you bring some interesting points, but I am afraid that you
are mixing few things here. Lets try to untangle it a bit:
- Haze measurement is simplified measurement, but for relatively thin
samples it is reasonably accurate. It does not give you directional
accuracy, nor spectral info, but overall specular vs. scattering split is
pretty good.
- Thick scattering samples are difficult to measure accurately, period.
Haze meter or not. My colleague here at LBNL is working through NFRC and
ICG TC10 to establish criteria how to properly do it. As it turns out large
integrating sphere or full goniophotometer is required to correctly measure
thick samples. Potentially, a correlation could be developed that would
allow measurement using smaller integrating sphere, but that is yet to be
found.
- Fabrics measurement according to EN standard - I am presuming that you
have in mind spectral measurement using ISO 9050 (or EN equivalent). If
not, let me know what EN measurement method you have in mind. One general
observation, though. Fabrics are thin scattering products, so their
measurement is not questionable. EN does not offer any advantage to what we
have developed through the new AERC organization, which will be published
as series of standards later this year.
To summarize, thick scattering samples need to be measured using large
integrating sphere, so neither haze meter nor Spectrophotometer with 150 mm
sphere is adequate. Thin scattering samples, fabrics included can
reasonably be measured with haze meter and Spectrophotometer with 150 mm
sphere, with the provision that haze meter does not give you any spectral
level information or directional resolution, just split between specular
and scattering.
Charlie
On Fri, Mar 31, 2017 at 7:11 AM, Christopher Rush <Christopher.Rush at arup.com
> wrote:
> Hi Charlie,
>
> I think you might be right in theory, but I’ve never been convinced that
> Haze Factor is particularly accurate based on various haze factor values
> I’ve seen on product data sheets (but I can’t recall specific examples to
> substantiate that, so I might be projecting unnecessary skepticism). I
> would be quite happy to be informed of its reliability by someone with
> greater familiarity with the ASTM standard.
>
>
>
> I’m thinking of it the same way that EN testing for woven shade fabrics
> gives more accurate results than the ASTM testing. I think a Tv(n-n) value
> from EN testing would give you a more accurate idea of specular
> transmission than the Haze Factor from ASTM D-1003. There might be
> different degrees of rigor dictated by the two different testing standards
> (required measurement angles, etc.), or the calculation methods prescribed
> to translate that measured data to a single % value.
>
>
>
> I’m not an expert on either standard, but in any case I think checking
> expected results v. simulated results is a good idea.
>
>
>
> -Chris
>
>
>
> *From:* D. Charlie Curcija [mailto:dccurcija at lbl.gov]
> *Sent:* Thursday, March 30, 2017 5:04 PM
> *To:* Radiance general discussion
> *Subject:* Re: [Radiance-general] Trans material - single pane of
> polycarbonate
>
>
>
> Wouldn't simple model (based on simple haze value) for Radiance be a BSDF
> with specular (diagonal) portion being 34% and lambertian portion (uniform
> diffuse in all directions 66%)? For this specific sample Tdiff =
> 0.4686, Tspec = 0.2414. Calculation done using logic below (H=Haze as a
> fraction).
>
>
>
>
>
> Tdiff = H*Ttot
>
> Adding other fundamentals
>
> Ttot = Tdiff + Tspec
>
> Tspec = Ttot-Tdiff = (1-H)*Ttot
>
>
>
> Charlie
>
>
>
>
>
> On Thu, Mar 30, 2017 at 12:09 PM, Christopher Rush <
> Christopher.Rush at arup.com> wrote:
>
> Erika,
>
> Have you seen a sample of the material? I see you note a Haze value of
> 66%, which is somewhat telling but not directly transferrable into a
> radiance definition. Is the material almost completely translucent with
> almost no specular transmission, or is it partially clear that you can see
> objects through it? Your trans parameter #7 (you have 0.344) I think
> implies that the material is transmitting roughly one-third of its
> transmission as direct and non-diffused transmission (direct view through
> the material).
>
>
>
> I don’t think that comment would solve your transmission loss problem, but
> would be worth a test rendering to see if the light transmission
> directionality “looks” the way you’d expect.
>
>
>
> -Chris
>
>
>
> *From:* Erika Saretta [mailto:erika.saretta at supsi.ch]
> *Sent:* Saturday, March 25, 2017 4:43 AM
> *To:* radiance-general at radiance-online.org
> *Subject:* [Radiance-general] Trans material - single pane of
> polycarbonate
>
>
>
> HI!
>
>
>
> I am trying to model a panel of polycarbonate (modeled as a surface facing
> the sun).
>
> My goal is to determine the annual solar radiation behind it.
>
>
>
> Since I have the following values:
>
> - Transmission 71%
>
> - Haze 66%
>
> I am trying to model the polycarbonate as a “trans” material in Radiance.
>
>
>
> Then, I perform the radiation analysis (I use DIVA4RHINO and Daysim model)
> with the following parameters for the trans material:
>
>
>
> void trans polycarbonate
>
> 0
>
> 0
>
> 7 0.972 0.902 0.835 0.07 0 0.706 0.344
>
>
>
> But… I obtain very low radiation values (from 30% to 50% of radiation
> loss).
>
> Does this loss seem unreal as a result?
>
> Do you think that “trans” material is the right choice for the
> polycarbonate?
>
>
>
> Any suggestion?
>
>
>
> Thank you in advance
>
>
>
> Erika
>
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--
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*D. Charlie Curcija**, Ph.D. ** Tel: (510) 495-2602*
*Lawrence Berkeley National Laboratory Fax: (510) 486-4089*
*Windows & Envelope Materials Group Cell:(510) 604-8668*
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