[Radiance-general] transmission/transmissivity

[Greg Ward]

Hi Bernhard,

This formula comes from the more general formula you can compute from the normal transmittance and index of refraction.  If you look in ray/src/cal/cal, you will find the file "trans.cal" that contains the more complete definition.  The numbers you quoted are the simplification of the equations with the index of refraction (n) set to 1.52, a typical value for glass.

The formula itself was derived from Fresnel's equations and the infinite series solution for float glass.  You can trust me on that or re-derive it yourself, as I don't have the derivation handy.  I don't know if there is an ISO standard where you can look it up.

For double-paned glazing, there are some good approximations built into the glaze script, which is distributed with Radiance.  It does not provide formulas, however.  The DOE Window program is available from:

	http://windows.lbl.gov/software/window/6/W63_faq.html

Best,
-Greg

> From: Bernhard Sommer <bernhard.sommer at uni-ak.ac.at>
> Date: January 23, 2014 4:02:23 AM PST
> 
> Hi,
> Sorry, for the interrubtion. 
> On radiance-online.org, you are quoting the formula
> tn = (sqrt(.8402528435+.0072522239*Tn*Tn)-.9166530661)/.0036261119/Tn
> to get from transmission to transmissivity; 
> Can you tell me the source of that formula?
> Is there an ISO standard to calculate this?
> 
> Also, you say, there is a software called window, to calculate the transmissivity properly for double glazed panes.
> Can you provide a link to that software.
> 
> I am teaching Energy Design for architects in Vienna and would like to provide the students with some proper information.
> 
> Thanks a lot
> Bernhard Sommer
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