[Radiance-general] trans mat

[Axel Jacobs]

Greg,

> Specular is more difficult to measure, but you can usually guess with
> reasonable accuracy.  If a surface is a single dielectric interface
> (plastic or glass), it's specular reflectance will be around 4%. If it
> is a double interface, as in a pane of glass, it will be about twice
> that, unless the glass is very dark, where it will be somewhere between
> 4% and 8% depending on the amount of absorption.

Is this the so-called Fresnel reflection you're talking about? It's 4% at
right angles. I'm with you...

> As for roughness, I have a nice little device with LEDs and a viewport
> that allow me to estimate this value, but I don't know how to offer it
> to anyone as it's a piece of hardware, and requires a lot of experience
> to use effectively.

I wonder if it was possible to just measure the size of the hilight and
derive the roughness from there, given a known light source diametre and
distance, e.g. sun? Well, it's not going to be have sharp edges, but one
could take an HDR image and a luminance profile through it that finds the
50% intensity...

>> Is this only possible through visual comparision?
>> What are the steps to A1..A7 if given:
>> - reflectance (luxmeter + luminance meter)
>> - transmittance (2x luxmeter)
>> (practical scenario, assuming grey colour)
>> if
>> - Lambertion properties or
>> - clearly see-through
>> What is the plastic equivalent to a trans without transmittance?
>
> I'm not sure I understand the question.  If you set A6 to 0, trans is
> the same as plastic.

What I means is almost exactly what you described above:
To measure the reflectance: refl = L * PI / E. Measuring the luminance and
the illuminance for the same spot (even illuminance assumed) gives us a
figure for the reflectance
To measure the transmittance: trans = E_behind_material /
E_in_front_of_material, ideally taken with 2 lux meters, but under
relatively constant illumination and with some averaging over several
readings, it will work.

Question is: how far does this improvisation get us with the trans material?

Axel