Spectral averaging with EN 410 and NFRC 300

[Joe Roberts]

Hi all,

I need to calculate the solar transmittance for some glazing that I have spectral data for. I've calculated the transmittance as a function of wavelength, for all of the wavelengths I had data for (300 -- 2500 nm, with 1 nm intervals).

Following the standards, I understand I need to do a numerical integration considering the standard solar sources (from CIE 1989 for EN 410, and from ISO 9845 for NFRC 300). These tabulated values however have larger intervals, so I was wondering whether I should just cherry-pick the corresponding values from my data, or calculate an average over the interval, or do some kind of interpolation....

Does anyone know whether either of the standards prescribes a certain way of doing this? Or does anyone know how Optics does this calculation?

I have data at all of the exact wavelengths that the standard Suns are given at, but it seems almost a waste to throw away data at all the other wavelengths.

Many thanks!

Joe

[Jacob C. Jonsson]

Hi Joe,

WINDOW (and Optics) calculates according to the
standards files selected. Example for how the
solar calculation is defined (W5_NFRC_2003.std):

Name : SOLAR
Description : NFRC 300-2003 Solar
Source Spectrum : ASTM E891 Table 1 Direct AM1_5.ssp
Detector Spectrum : None
Wavelength Set : Source
Integration Rule : Trapezoidal
Minimum Wavelength : 0.3
Maximum Wavelength : 2.5

The "Wavelength Set : Source" means that the
spectral data for the glass layers are interpolated
to the wavelengths defined in the source spectrum.
The "Integration Rule : Trapezoidal" dictates
trapezoidal numerical integration.

This is documented in Chapter 7 of the technical doc:
https://windows.lbl.gov/sites/default/files/software/WINDOW/WINDOW%20Technical%20Documentation.pdf

> Does anyone know whether either of the standards prescribes a certain
> way of doing this? Or does anyone know how Optics does this calculation?

In WINDOW and Optics you can create your own .std files to
modify the behavior. Some standards are strict on how to
do the numerical integration, some use weighted ordinate to
remove the uncertainty of that step.

> I have data at all of the exact wavelengths that the standard Suns are
> given at, but it seems almost a waste to throw away data at all the
> other wavelengths.

It might be, depending on how much is "happening" between the
wavelengths in the spectrum. Also worth considering is that
the energy is not distributed evenly in the whole spectrum so
where you have higher density matters.

I hope that gives some clarity on how Optics does it (i.e. lets
you get very good agreement with Optics if you are doing it
yourself). Let me know if you have more questions.

Best,
Jacob

[Joe Roberts]

Hi Jacob,

Thanks for your detailed reply, the technical doc is also really helpful. My aim was to replicate the calcs Optics does myself in Python, and now I get the same exact results.

Many thanks,

Joe

[Jacob C. Jonsson]

Hi Joe,

If you are interested to do more than just the
spectral integration (multilayer calculations etc)
I recommend you take a look at pyWinCalc
https://github.com/LBNL-ETA/pyWinCalc

Best,
Jacob

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[Joe Roberts]

Hi Jacob,

Thanks for the suggestion about pywincalc.

I've got another question... Do you know how Optics deals with the integration limits, i.e. if they don't match those from the source wavelength set? The technical document doesn't specify a method and just says:  

If they have other values, and particularly if they do not coincide with wavelengths for which tabulated weighting function values are available, special methods may be needed to incorporate the endpoints into the calculation.

I have data starting from 339 nm (then 340.5, 341.9, 343.4.....). I've tried simply creating rows at 300, 305, 310, 315, 320, 325, 330, 335 and giving them the same transmission/reflection as 339 nm, but I don't get the same results. I immagine it's some sort of extrapolation (linear??). Could you give me some details about how Optics does this, and whether it's different for EN 410 and NFRC 300?

Many thanks again,

Joe

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[Jacob C. Jonsson]

The NFRC300 requires that you have measured data in
the whole integration range, no extrapolation allowed.
And I believe EN410, ASTM E903, and ISO 9050 are the same.
I cannot find the answer what WINDOW/Optics is doing it,
but I strongly recommend you do not do it.

Best,
Jacob

On 9/2/2022 3:34 AM, Joe Roberts wrote:
> Hi Jacob,
>

> Thanks for the suggestion about pywincalc.
>
> I've got another question... Do you know how Optics deals with the
> integration limits, i.e. if they don't match those from the source
> wavelength set? The technical document doesn't specify a method and just
> says:

> /
> /
> /If they have other values, and particularly if they do not coincide

> with wavelengths for which tabulated weighting function values are
> available, special methods may be needed to incorporate the endpoints

> into the calculation. /
> /
> /

> <mailto:berkeley-lab-window%2Bunsu...@googlegroups.com>
> > <mailto:berkeley-lab-wi...@googlegroups.com
> <mailto:berkeley-lab-window%2Bunsu...@googlegroups.com>>.

> > To view this discussion on the web visit
> >
> https://groups.google.com/d/msgid/berkeley-lab-window/71acf8bb-68bc-4dda-ba01-44ee954f51ebn%40googlegroups.com
> <https://groups.google.com/d/msgid/berkeley-lab-window/71acf8bb-68bc-4dda-ba01-44ee954f51ebn%40googlegroups.com>
>
> >

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>