[Radiance-general] iso for specularity and roughness

[Sotiris Papantoniou]

Dear list,

 

Hello from Greece. I am trying to find a standard for measuring specularity
or roughness for walls (plastic description), or some values that can lead
to a correct estimation/ description of these values. Is there any standard
for this description? I have read Lars mail from the 24th of June, but I
still can't configure how to find the correct value to describe correctly
these values

 

Lars mail:

 

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 seems 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 than 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 than zero mm^2 area - thus
it excludes more than 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.

 

Any Suggestions???

 

Sotiris Papantoniou

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