specularity values

Dear recipients

For a research project using Ecotect and Radiance, me and my colleague are
looking for specularity values of building materials.

Can anybody help us find specularity values?

All help is welcome!


Saskia Gabriël
PhD Candidate in Architectuur

Design Sciences
Artesis University College of Antwerp, Belgium

For a research project using Ecotect and Radiance, me and my colleague
are looking for /specularity/ values of building materials.

Dear Saskia,

I try to help out a bit, even though I will not provide you with any such value here. I hope that I will be able to explain why.

First, the term. I guess you are referring to specular reflectance, maybe also direct transmittance. These define the fraction of light getting reflected / transmitted directly. With a lot of simplification something like incident angle equals outgoing angle for reflection (mirror), or light passing straight through (glass). Be aware that the real world is not that simple, but let us assume it is.

If you want to measure the specular reflectance, you typically do it indirect. A very common approach is to use an integrating sphere with either a photocell (if you do not need color information) or a spectrometer connected. In a first run, you would measure the total reflectance. In the second, you would open a small hole in the sphere so that light reflected according to the "incident=outgoing angle"-rule would simply escape the sphere and thus be skipped in the measurement. This leads to the diffuse reflectance. Substracting both would give you the specular reflectance. For transmissive samples, the procedure works more or less the same. Doing a direct measurement is rather complicated... Measurements on an integrating sphere are a standard measurement.

The procedure seams to be straight forward. However it is not. First, you can never measure only one outgoing direction, and you never have only one incident direction. There is always an angular range, larger then zero. Imagine the device for the measurement described above - there will be a light beam used, and that is not perfectly parallel and has a diameter. And if you open a hole in the sphere, that has more then zero mm^2 area - thus it excludes more then one direction. In other terms, you are always talking about solid angles.

The reason is not only imperfection in making measurement devices. Even a very good mirror reflects light not only into one direction. For each incident direction you will always get a distribution of reflected and, sometimes, transmitted light. There are some labs offering measurement of such distributions (maybe you have seen candela distributions for electrical lighting - it is something like that), I have been working at one until just recently. Using the measured data is not simple, as the data isn't. That is why the observation that for some surfaces, there are predominant directions where most of the light is directed to, led to the use of terms such as specular and diffuse. We use these simplifications to be able to describe the characteristics and, in simulation, we use these to optimize algorithms.

Now, for building materials, you will find very different levels of support from manufacturers. Some will give you detailed information on reflective and transmissive properties (go to the website of Alanod, they let you download the bidirectional reflection distribution data of their frontside mirrors - the distribution I describe in the paragraphe above at a high angular resolution). Quite a lot of manufacturer of materials where lighting is involved will give you some values like direct/specular and diffuse (glazing, sunshades, ...) but not more. And then, a lot of manufacturers may be able to give you color information.

You will also find that, depending on the application, different terms are used when talking about reflective and transmissive characteristics. Gloss, shininess, application standards differ often from what is more established in physics.

The conclusion is that there is no way to simply collect specularity values of building materials. Define what you are interested in and what your application is. If you are talking about appearance, energy transports, ... Are you looking at facades? Glass? Paints? Different polishing techniques?

I hope that this helps to find a starting point...

Cheers, Lars.