[Radiance-general] Modeling the real world for lighting analysis.

[Guglielmetti, Robert]

Hi German,

I have cross-posted this reply to the radiance-openstudio list.

To change the reflectance of a material in OpenStudio, you need to change the *absorptance* to the inverse of the reflectance you want. Since OpenStudio's first allegiance is to EnergyPlus, materials are monochromatic and take absorptance rather than reflectance as a parameter. We need to work within the EnergyPlus worldview for now.

Remember, in OpenStudio parlance, surfaces have *constructions* applied to them, which are made up of one or more materials. So the process is first you need to identify what construction is applied to your surface, then look at the makeup of the construction, to find the material that affects your light reflectance (absorptance). Make sense?

In the future we'd like to add direct entering of Radiance material parameters so you can include specularity and roughness (and color), but for now it's diffuse gray. =)

- Rob

From: Germán Molina Larrain [mailto:gmolina1 at uc.cl]
Sent: Monday, April 29, 2013 2:56 PM
To: Radiance general discussion
Subject: Re: [Radiance-general] Modeling the real world for lighting analysis.

ok! thanks! how do you do it on Open Studio anyway???

2013/4/29 Guglielmetti, Robert <Robert.Guglielmetti at nrel.gov<mailto:Robert.Guglielmetti at nrel.gov>>
Well, it's a good start, and it keeps things simpler, anyway.

- Rob

From: Germán Molina Larrain [mailto:gmolina1 at uc.cl<mailto:gmolina1 at uc.cl>]
Sent: Monday, April 29, 2013 2:28 PM
To: Radiance general discussion
Subject: Re: [Radiance-general] Modeling the real world for lighting analysis.

haha, I know things are not free.

So, basically using grey diffusing materials with relatively correct reflectivity would be great for achieving good numbers?

THANKS FOR THE ANSWER

German

2013/4/29 Guglielmetti, Robert <Robert.Guglielmetti at nrel.gov<mailto:Robert.Guglielmetti at nrel.gov>>
Hi German,

Yeah, modeling materials can be tricky, particularly if you want accuracy. Life is not always fair, you get what you pay for, and there's also no such thing as a free lunch, either. =)

For me, doing relative studies of architectural scenes, it's always made sense to keep things simple which does mean keeping color out of the equation (both for materials and light sources), and using generally diffuse materials (which is really the majority of architectural materials that cover broad expanses of interior spaces). When specularity is a critical component of a surface, such as in light redirecting materials, I resort to BSDFs or other means of describing the light scattering (photometric distribution file resulting from light forwards ray tracing exercise, usually).

Some amazing things can be done with textures and patterns to enhance the realism of a rendering, but I'm generally more interested in numbers so I have not played with those too much. I can tell you that you are automatically introducing more variables to the physical definition of the surface to which you are applying those textures and patterns, so tread carefully.

Material definition has been discussed at length on this list, so I'd search the archives. Also check out Carsten Bauer's amazing textures and patterns from his Workshop talk in 2002 (linked on the Radiance-online.org website) for inspiration as to what's possible with textures, patterns, and the Radiance functional language -- sick stuff!

-Rob



Rob Guglielmetti
NREL Commercial Buildings Research Group
Golden, CO 80401
robert.guglielmetti at nrel.gov<mailto:robert.guglielmetti at nrel.gov>


-----Original Message-----
From: Germán Molina Larrain [gmolina1 at uc.cl<mailto:gmolina1 at uc.cl>]
Sent: Friday, April 26, 2013 09:46 AM Mountain Standard Time
To: Radiance general discussion
Subject: [Radiance-general] Modeling the real world for lighting analysis.
Dear List,

I am a starter on the use of Radiance, and I have some doubts about the modeling (of the real world, as it always is, in engineering at least). I have heard that material reflectivity is a very important parameter that. How important are:

- Color (I am pretty sure I read something in this list about grey-world simulations... I think it was Rob).
- Textures
- Specularities
- Patterns
etc.

My concern is that defining materials is actually not a very easy task, so I just wanted to know what are the most important things here.

THANKS VERY MUCH!

German

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