# Absorption for a layer of glazing

Hi Experts,

I am wondering if there is a way to take into account the amount of absorption for a layer of glazing system in Irradiation nodes simulation.

just assume we have 2 different glass panes with the same Tvis but with different SHGC.

as far as I understand, for radiance materials as glass the only parameter is Transmission:

`Void glass id         0         0         3 rtn gtn btn`

Even for dielectric material, there is no absorption parameter:

`Void dielectric id         0         0         5 rtn gtn btn n hc`

Should I define a Trans material to control the absorption of a glass layer?

`Void trans id         0         0         7 red green blue spec rough trans tspec`

Thank you very much in advance.

Best,

Fazel

Tags: absorption, dielectric, glass

Views: 218

### Replies to This Discussion

Hello Fazel,

The trans material is not ideal, because it does not account for the directional incidence of light in terms of its reflection / absorption. The glass material, as far as I am aware, will use the simple Fresnel equations to calculate reflectance, and the rest of the light is absorbed or transmitted as per the definition in order to produce a normalized Tvis of whatever is set. You should be able to simply define a new glass material using the SHGC values (translated into transmissivity) for the irradiation nodes simulation.

You can even, roughly, make the SHGC of the total glazing system for a single glazing surface: SHGC_total = SHGC_1 * SHGC_2. Otherwise, some more advanced tools like Optics are required.

Best,

Alstan

Thank you, Alstan.

The problem is whenever we are using such a selective material with different Tvis and Tsolar we have to be careful about the results of Irradiation. I think your solution is useful.

I actually wanted to compare the Irradiation values with what I've expected based on the optical properties in  LBNL Window. and I find it confusing... :(

Do you have any information about the Sky condition in Radiance. dose it use the visible spectrum or the solar spectrum?

Hello Fazel,

Well Radiance is explicitly putting out a 'fake' solar spectra based on a 179 lumens / watt efficacy for any visible light calculation with the CIE skies, etc. That's not very useful.

Gendaylit; however, can output the explicit solar irradiance:

Usage: gendaylit      month day hour      [...]
or: gendaylit -ang altitude azimuth  [...]
followed by:      -P          epsilon delta [options]
or:      [-W|-L|-G]  direct_value diffuse_value [options]

Description:
-P epsilon delta  (these are the Perez parameters)
-L direct-normal-illuminance diffuse-horizontal-illuminance (lux)

Output specification with option:
-O [0|1|2]  (0=output in W/m^2/sr visible, 1=output in W/m^2/sr solar, 2=output in candela/m^2), default is 0
gendaylit version 2.4 (2013/09/04)

This output isn't available from the DIVA GUI, but it can be done by hacking the source files and running them yourself in C:\DIVA\Temp\ProjectName\ by experts. Once you switch to the solar spectrum for your sources, all of your material properties should also be in terms of solar reflectance.

Best,

Alstan