Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page
Ask Your Question
3

Radiance material for reflective glazing

asked 2018-04-10 13:16:10 -0600

Arch2010 gravatar image

Radiance glass material doesn't take into account the reflectivity of glazing. However, I'm interested in studying the effect of reflective glazing on occupants' visual comfort (glare) in surrounding areas, so reflectance of glass is a required input in this case. How would I define a radiance material for the glazing highlighted in the picture below (the second one, named " Silver 50/30")? it has a reflectance value = 0.39 image description

edit retag flag offensive close merge delete

3 Answers

Sort by » oldest newest most voted
3

answered 2018-04-11 10:26:51 -0600

updated 2018-04-12 11:05:29 -0600

Where'd ya hear that one? Radiance's glass mat does take into account relectance, but if you want to specify a specific product, I'd recommend you use LBL's Optics program to create a glass and optional BRTDfuncs for the front and back sides of the glazing. It's been about ten years since I've done this so I don't have specific rendered examples handy, but I've done this extensively for precisely the kind of study you describe in your post. We'd create IGUs that match a given spec in Optics, and export to Radiance materials, apply, and render.

We also used a methodology where the two BRTDfunc definitions are merged with glazing.cal as discussed in this post, but if you're purely interested in the exterior appearance, you could just use the 'front' BRTDfunc you get out of Optics.

Update 2018.04.12 Using your specific example Optics output, which looks like this:

void  glass       RefliteArcticBlue6mm_glass
0
0
3     0.160     0.179     0.131

void  BRTDfunc    RefliteArcticBlue6mm_front
10
      0.555     0.561     0.478
      0.147     0.164     0.120
      0 0 0
      .
0
9 0 0 0 0 0 0 0 0 0

void  BRTDfunc    RefliteArcticBlue6mm_back
10
      0.121     0.236     0.275
      0.147     0.164     0.120
      0 0 0
      .
0
9 0 0 0 0 0 0 0 0 0

You could use the RefliteArcticBlue6mm_glass definition for a single polygon. If you model both panes of the IGU though, I think you'd use RefliteArcticBlue6mm_front for both polygons; this way, renderings from either vantage point (outside looking at the building, or inside looking at the window) would be accurate. In the BRTDfunc case, you need to ensure the surface normals are facing the right way -- that is, the exterior lite facing out to the exterior, and the inner lite facing toward the interior. In the case of the glass def, the surface orientation does not matter.

Lastly, any of these three materials will work fine with Honeybee as they do not reference any cal files. Pretty sure Honeybee will work with cal files though, as it pretty much uses straight Radiance, and as long as you have a properly installed Radiance setup, it all should work.

edit flag offensive delete link more

Comments

3

If you end up using Optics, you may wish to process the output with the Radiance "optics2rad" script, which cleans it up and eliminates the unnecessary definitions.

GregWard gravatar imageGregWard ( 2018-04-11 11:43:38 -0600 )edit

@GregWard, I could be wrong I'm no expert at this, but isn't exporting from Optics as a rad file good enough? I get few headlines on the top of the file that I delete manually and the rad file is good to go. I looked in Radiance's bin folder and couldn't find this module (optics2rad)!

Arch2010 gravatar imageArch2010 ( 2018-04-12 00:09:33 -0600 )edit

I tried your suggestion, first by modeling one polygon facing the outside and using the RefliteArcticBlue6mm_glass definition but I didn't get the desired output, for example, I don't see the reflection of other nearby buildings on the glass (it's a highly reflective glass). I tried the two-polygon method and I used the RefliteArcticBlue6mm_front for both, and I got this error

rpict: fatal - (glaretest_IMG.oct): truncated octreer
Arch2010 gravatar imageArch2010 ( 2018-04-12 15:32:22 -0600 )edit
1

You will need to bump up some of the rendering parameters to ensure you capture the mirror reflections. How are you running these commands? The truncated ocrtree error suggests an issue with the workflow, so please "show your work". =)

rpg777 gravatar imagerpg777 ( 2018-04-12 15:36:23 -0600 )edit
1

This is awesome, as soon as I pumped the ab to 7 I was able to get the reflections. Also the I just figured out that error was caused by HB; it didn't parse the material correctly from the library. So, now everything is good :)

Arch2010 gravatar imageArch2010 ( 2018-04-12 17:11:49 -0600 )edit
1

answered 2018-04-17 11:31:21 -0600

updated 2018-04-17 17:23:25 -0600

That the glass material is reflective is stated in the Reference Manual:

Glass is similar to dielectric, but it is optimized for thin glass surfaces (n = 1.52). One transmitted ray and one reflected ray is produced.

Since this is a perfect abstract material, I expect the reflectance to be (1 - T), where T is transmittance. The details are in the code in glass.c.

(Hey, Unmet Hours, how 'bout support for equations?)

(They have it!)

edit flag offensive delete link more
1

answered 2018-04-12 00:05:02 -0600

Arch2010 gravatar image

updated 2018-04-12 10:29:53 -0600

@rpg777, thank you so much for the helpful answer. I assumed that Radiance doesn't account for reflectance based on the sample definition of glass material in Radiance's guide, in which only transmissivity is required to complete the material definition

For example, the following glass material definition mentioned the glass's transmissivity of 0.75 and there is no mention of the glass's reflectance

void glass SinglePane
0
0
3 0.75 0.75 0.75

Anyway, I just found a glazing unit that I would like to use, so I exported it to radiance and here is what I got

void  glass       RefliteArcticBlue6mm_glass
0
0
3     0.160     0.179     0.131

void  BRTDfunc    RefliteArcticBlue6mm_front
10
      0.555     0.561     0.478
      0.147     0.164     0.120
      0 0 0
      .
0
9 0 0 0 0 0 0 0 0 0

void  BRTDfunc    RefliteArcticBlue6mm_back
10
      0.121     0.236     0.275
      0.147     0.164     0.120
      0 0 0
      .
0
9 0 0 0 0 0 0 0 0 0

I tried to read the post that you referenced in your answer to combine both BRTDfunc but the post is so messy and hard to follow, and I was wondering if anyone here can direct me to a clear reference to do this?

Alternatively, can I model two surfaces for both glazing layers? first layer's material will include the first three lines and the BRTDfun of the front, and the material of the second layer will have the first three lines and the BRTDfunc of the back? is this the right way to do this?

Finally, a quick note: I'm using Honeybee for Grasshopper. Will I be able to run annual simulations (daysim) with these type of materials? I know that in the past Daysim couldn't handle material definitions that contain a .cal file.

edit flag offensive delete link more

Comments

Yeah, the glass material doesn't have an input for reflectance, but it is considered, in a default way. This is documented... somewhere. Maybe just in the source code? And yeah, the radiance-general mailing list archives are in a bit of disarray, help may be on the way for that soon. I'll answer your other questions here in my previous answer.

rpg777 gravatar imagerpg777 ( 2018-04-12 10:46:31 -0600 )edit

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

 

Question Tools

1 follower

Stats

Asked: 2018-04-10 13:14:31 -0600

Seen: 187 times

Last updated: Apr 17