Determining Specularity Values - Spandrel Panels: Marble vs Fritted Glass

Hello All,

I am trying to compare the difference in glare given off a building
after a cladding retrofit (from the perspective of occupants in
neighboring buildings). The original curtain wall cladding is a
white/grey marble and glass spandrel panels with a white patterned frit
are proposed for the retrofit. This is my first time using the software
and I am using Radiance in IES' Virtual Environment Program.

I have some questions about determining/selecting material properties,
specifically deciding what material type (plastic/metal) and secularity
to use for the two cladding options.

My assumptions:

Marble - Originally white, but panels have become grey with dirt over
time, not polished, dull finish

Type: Plastic

Colour: (0.91, 0.91, 0.83)

Specularity: ??? (I think this may be low because it is not
polished marble)

Roughness: 0.05 (???)

Glass Frit - Composed of: Clear glass, 60% coverage white frit, clear
glass, 100% coverage white frit. From Window 6, I estimate the solar
reflectance of the system to be around 25-35%.

Typle: Metal ??? (Does the frit have specular and diffuse
reflection of light?)

Colour: (1.0,1.0, 1.0)

Specularity: ??? (Clean polished glass)

Roughness: 0.00

If anyone has information from doing a similar simulation or has any
suggestion on what specularity values I should use to model these
materials it would be very much appreciated.

Thanks,

Nastassja

Nastassja,

Marble – Originally white, but panels have become grey with dirt over time, not polished, dull finish
Type: Plastic
Colour: (0.91, 0.91, 0.83)
Specularity: ??? (I think this may be low because it is not polished marble)
Roughness: 0.05 (???)

Probably much darker than that, perhaps as low as 0.5. You can estimate its reflectance with a camera with a telephoto lens (or a spot meter) and an 18% grey card; procedure at <http://www.kodak.com/global/en/consumer/products/techInfo/am105/am105kic.shtml >. Use a tripod and stand far enough off so that your shadow doesn't affect the measurement.

I don't have any good ideas for frit glass--perhaps the manufacturer has some information?

Randolph

I have some questions about determining/selecting material properties, specifically deciding what material type (plastic/metal) and secularity to use for the two cladding options.

My assumptions:

Marble – Originally white, but panels have become grey with dirt over time, not polished, dull finish

Type: Plastic

Colour: (0.91, 0.91, 0.83)

As Randolph wrote this is far too 'white' for a typical white material,
especially if it's a bit dirty.

0.75 throughout would be the colour of white wallpaint, so dirty marble
might be in the range of 0.6 to 0.65 (just a guess).

Specularity: ??? (I think this may be low because it is not polished marble)

Roughness: 0.05 (???)

Specularity and roughness have an impact on the visual appearance
but have no effect on glare in the Radiance model.

Glass Frit – Composed of: Clear glass, 60% coverage white frit, clear glass, 100% coverage white frit. From Window 6, I estimate the solar reflectance of the system to be around 25-35%.

Typle: Metal ??? (Does the frit have specular and diffuse reflection of light?)

Why don't you start with a glass material? If the frit is on the inside
you have at least one surface that will produce specular reflections
and might cause glare. A glass material would simulate that to some
degree. You can simulate the colour and transmittance with a trans
material behind the glass.

Colour: (1.0,1.0, 1.0)

Specularity: ??? (Clean polished glass)

Roughness: 0.00

If you are interested in glare from reflected sunlight you should also
consider a 'mirror' material to trace the sun position more accurately.

Regards,
Thomas

···

On 4 Nov 2008, at 14:49, Pearson, Nastassja wrote:

Hello Nastassja,

Marble – Originally white, but panels have become grey with dirt over time, not polished, dull finish

Type: Plastic

Colour: (0.91, 0.91, 0.83)

Specularity: ??? (I think this may be low because it is not polished marble)

Roughness: 0.05 (???)

I concur with the others that this reflectance is too high for any surface that has been exposed to the exterior for a period of time. My experience is that dirt & grime reduce reflectance values by a significant amount. I would suggest a reflectance of 50-60% for white marble. The specularity is probably around 0.01 and a roughness of 0.08 for an unpolished surface is a reasonable guess.

Glass Frit – Composed of: Clear glass, 60% coverage white frit, clear glass, 100% coverage white frit. From Window 6, I estimate the solar reflectance of the system to be around 25-35%.

Typle: Metal ??? (Does the frit have specular and diffuse reflection of light?)

Colour: (1.0,1.0, 1.0)

Specularity: ??? (Clean polished glass)

Roughness: 0.00

I would start using the "glaze" script, entering the above parameters. This should give you good results. Attached is what I used for your 60% fritted glass and the output produced.

Best,
-Greg

% glaze
Enter the number of panes in the system: 1

Window normal faces interior

   > >
   > >-->
   > >
  s1 s2

Supported surface types are:
    1 - clear glass
    2 - VE1-2M low-E coating
    3 - PVB laminated
    4 - V-175 white frit
    5 - V-933 warm gray frit

What is the type of s1? 1
What is the type of s2? 4
Enter fraction coverage for s2 (0-1): .6

···

############################################
# Glazing produced by Radiance glaze script
# $Revision: 2.7 $
# Material surface normal points to interior
# Number of panes in system: 1
# Exterior surface s1 type: clear glass
# Interior surface s2 type: V-175 white frit
# s2 coating coverage: .6
# Exterior normal hemispherical reflectance: 0.228534
# Interior normal hemispherical reflectance: 0.384534
# Normal hemispherical transmittance: 0.478928
#
void BRTDfunc glaze1_unnamed
10
         sr_frit_r sr_frit_g sr_frit_b
         st_frit_r st_frit_g st_frit_b
         0 0 0
         glaze1.cal
0
11
0.354
0.1536
0.1518
0.1506
0.126
         -1 .6

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P.S. Apologies if these answers are redundant with others, as I am composing this on a plane w/o internet access.