We are trying to do a daylight analysis that includes some frosted glass or “kalwall” style skylights which diffuse the light into the space.  We can get the VLT (Visible Light Transmittance) values easy enough.  However, is there some way to accurately (or semi-accurately) account for the rays being dispersed and spread through the frosted glass? 

Simulating frosted glazing in 3ds Max Design for lighting analysis is doable.  You however need to know how to do it properly.  Here is how:

Some useful background information:

First, as opposed to Radiance, the A&D Material has a few internal “things” going on that you need to be aware.  The most important one is that the A&D Material performs internal energy conservation as follow:

Transmissivity wins over Specular Reflectivity which wins over Diffuse Reflectance.  On top of that, the Transmissivity is weighted against a Specular / Diffuse factor.  This factor is ruled by the Translucent color / weight controls in the interface.

In contrast, in Radiance, one can specify a material that is reflecting 100% diffuse and 100% specular while transmitting 100% of the light, leading to “creating” energy.  This is why the parameters of the Radiance materials cannot be plugged “as-is” in the A&D Material.

Translucent Panels:

We compared translucent glazing simulation in mental ray against radiance and measured data and got convincing results (see image) with the following settings:

Desired Diffuse Transmittance:  0.1621  (16.21% Diffuse – Diffuse Transmittance)

• Treat surface as a single polygon in the model
• A&D Diffuse Level:  0.0
• A&D Diffuse Color:  pitch black  (so no weighting is given to the diffuse reflectance)
• A&D Reflection | Reflectivity Level:  1.0
• A&D Reflection | Reflectivity Color:  pure white (the color is a multiplier, we need it to be 1.0 1.0 1.0)
• A&D Refraction |  Transparency Level:  1.0
• A&D Refraction | Transparency Color:  pure white (the color is a multiplier, we need it to be 1.0 1.0 1.0)
•  A&D Refraction | Translucency Checkbox : ON
• A&D Refraction | Translucency Weight: 1.0  (we want it fully translucent)
• A&D Refraction | Translucency Color:  0.1621 0.1621 0.1621 (the color is a multiplier, we need it to be set to the desied transmissivity level “as-is”, equally for all RGB components)
• BRDF | Custom Reflectivity Function: ON
• BRDF | 0 Deg Refl:  0.0
• BRDF | 90 Deg Refl:  1.0
• BRDF | Curve:  ~5  (we need to approximate a typical Fresnel curve)
• Advanced Rendering Options | Thin-Walled : ON

Some important notes: 

• While the illuminances will carry through properly on light meters, the glazing appearance may not look “natural” in “pretty picture renderings”.   It seems that there is currently a limitation with the appearance of the surface when it is hit by light:  its resulting luminance won’t be correct (AFAIK) so glare analysis based on luminance measurements won’t be convincing.
• The following image shows a graph comparing 3ds max, radiance and measured data.  Ignore the “3ds max 2009 SP1 Initial Submission” curve, this is representing a case where our material settings in 3ds Max where wrong, which we corrected later on – in fact, we forgot to turn off a layer so we had 2 panes of glass on top of each other…).  The green curve is what we need to look at….

This group maintains a database and publishes this data via a program called Optics 5. From Optics 5 you can then export a Radiance Material (*.rad file), which can be interpreted
as mental ray A&D Material parameters.

To convert Optics 5 data into A&D Material suitable for lighting analysis, export a glazing or glass definition as Radiance (*.rad) from Optics 5. You will find this command under File | Export to Radiance File.

Once the file is exported on disk, open it in Notepad and search for a section that looks like this:

void BRTDfunc B530_front
10
0.245 0.281 0.340
0.169 0.197 0.187
0 0 0

The color coefficients (RGB) for the ideal specular reflection corresponds to 0.245 0.281 0.340. The color coefficients for the ideal specular transmission corresponds to 0.169 0.197 0.187. Those values will need to be used as a basis for the mental ray A&D Material.

How to use the provided Microsoft Excel spreadsheet to convert to the mr A&D Material.

To correctly convert specular reflection and transmission from a Radiance material to a mr A&D Material, we need to take into account internal energy conservation methods that are built in the mr A&D Material that are not factored by the Radiance material. In other words, numbers can’t be “plugged-in” as is.

To help you with this task, we developed a Microsoft® Excel® software spreadsheet that will let you do this precisely. The spreadsheet can be downloaded here.

Limitations:

• The Radiance Materials exported from the Optics 5 database does not take into account angular dependency: A Fresnel falloff curve is assumed so metallic-coated glazing systems may be less precisely simulated.
• The Optics 5 database contains optical data measured spectrally. The exported Radiance materials and the A&D materials use RGB colors which are a crude approximation of the visible light spectrum. Therefore, lighting simulations are done within limitations of RGB colors.

The measured color palettes can be exported from the Color Munki software as *.CxF files and read back in 3ds Max with the help of a script that I have developped.

The 3ds Max script can be downloaded here.  Simply run it from within 3ds Max using the Maxscript | Run command, browse to a CxF file and drag and drop colors in your materials from the color swatch of your choice.