# transmission/transmissivity

Hi everyone

In radiance manual, when you want to create a glass material, we use this function:

void glass id
0
0
3 rtn gtn btn

where tn is the transmissivity which comes from this formula:

tn = (sqrt(.8402528435+.0072522239*Tn*Tn)-.9166530661)/.0036261119/Tn

where Tn is the transmittance.

For example, if i have light transmission for a window equal to 60% (given by a manufacturer), do i have to calculate tn with Tn =0.6

and then put my results in the function, so with this example we'll have: tn = 0.65 with Tn = 0.6?

void glass id
0
0
3 0.65 0.65 0.65

Other type, other question:

for a mirror, what is the default specularity if I define default mirror with this function:

void mirror window
0
0
3 0.965 0.965 0.965

Thank's a lot for your help.

Regards

Alexis

Regarding the glass definition, what you wrote makes sense although I didn't check the math.

Regarding mirror, it is fully specular - all of the 0.965 reflection in your definition below would be reflected in a purely specular manner.

···

Sent: Thursday, November 18, 2010 10:57 AM
To: [email protected]

Hi everyone

In radiance manual, when you want to create a glass material, we use this function:

void glass id
0
0
3 rtn gtn btn

where tn is the transmissivity which comes from this formula:

tn = (sqrt(.8402528435+.0072522239*Tn*Tn)-.9166530661)/.0036261119/Tn

where Tn is the transmittance.

For example, if i have light transmission for a window equal to 60% (given by a manufacturer), do i have to calculate tn with Tn =0.6

and then put my results in the function, so with this example we'll have: tn = 0.65 with Tn = 0.6?

void glass id
0
0
3 0.65 0.65 0.65

Other type, other question:

for a mirror, what is the default specularity if I define default mirror with this function:

void mirror window
0
0
3 0.965 0.965 0.965

Thank's a lot for your help.

Regards

Alexis

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Hi Alexis.

Hi everyone

For example, if i have light transmission for a window equal to 60% (given
by a manufacturer), do i have to calculate tn with Tn =0.6 and then put
my results in the function, so with this example we'll have:
tn = 0.65 with Tn = 0.6?

void glass id
0
0
3 0.65 0.65 0.65

That is correct. The change from transmittance to transmissivity
accounts for internal reflections in the glass. Please note that
strictly speaking this formula should be applied to a single sheet
of glass. If you want to model a double glazing unit the glass
material does not properly represent the angular dependency
of the light transmission. To accurately model this you should
use a tool like Window which allows you to export a description

Other type, other question:

for a mirror, what is the default specularity if I define default mirror
with this function:

void mirror window
0
0
3 0.965 0.965 0.965

The mirror material in Radiance does not represent a totally
reflecting surface as in real world. It is used to accurately represent
a reflection of a light source (like the sun) in a scene.

Regards
Thomas

···

On Thu, Nov 18, 2010 at 10:56 AM, alexis Loubeyre <[email protected]> wrote:

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

···

--
Arch. Dipl.-Ing. Bernhard Sommer
staatl. befugter u. beeideter Ziviltechniker

ED_energiedesign
Forschung & Lehre

Universität für angewandte Kunst Wien
Institut für Architektur
A-1010 Wien, Oskar Kokoschka-Platz 2
T +43 1 711 33 *2372
F +43 1 711 33 *2339
[email protected]
www.dieangewandte.at/energie.design

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:

Best,
-Greg

···

From: Bernhard Sommer <[email protected]>
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