[Radiance-general] trans mat

[Axel Jacobs]

Dear all,

I am trying to nail this one down, once and for all.
What is your experience/approach? Feedback welcome.

refman:
"The transmissivity is the fraction of penetrating light that travels all
the way through the material.
The transmitted specular component is the fraction of transmitted light
that is not diffusely scattered.
Transmitted and diffusely reflected light is modified by the material color.
Translucent objects are infinitely thin."

BIG QUESTION:
How do CAPITAL indeces (RADIANCE) relate to
lower case indices (PHYSICS)??? See below. Also for a lot more little
questions...

mod trans id
0
0
7 red green blue spec rough trans tspec
#7  A1   A2   A3  A4    A5    A6    A7

Let's talk about grey objects for now:
A=179*(.265*A1+.65*A2+.065*A3)

[hope you like ASCII art]
!!! View with monospaced font !!!
             _.
 _|          /|
  \         /
 1 \  ---  /
    \/   \/  b \
    /\   /\     > a
   |  \ /  | c /
   |   V   |
----------------  g
   |   \   |
   |    \  | f \
    \    \/     > d
     \   /\  e /
      ---  \
            \|
            -.
Legend
------
Incident light = 1 (is that our assumption?)
Reflected light a
 - specular b
 - diffuse c
Transmitted light d
 - specular e
 - diffuse f
Absorpt light g

What I am quite sure about:
---------------------------
1 = a + d + g
a = b + c
d = e + f

Some Qs:
--------
The specular components (both diff and spec) are hard to measure.
However, this is what the calculation in the radbook is based on.
How would one make estimations?
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?
Would that be _wrong_ or just _stupid_ to use?

References
----------
- http://radsite.lbl.gov/radiance/digests_html/v2n7.html#TRANS_MATERIAL
- radbook 5.2.6, page 325 (new edition)

Cheers

Axel