[Radiance-general] trans mat

[Greg Ward]

Hi Axel,

> 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...

Well, you can take an HDR photo and use a known source and geometry to 
fit the distribution, but it's a fair amount of work.

>>> 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?

Well, it's OK, but you still don't know how much of the transmitted 
light is specular and how much is diffuse.  You could try putting a 
tube on your lux meter (or using a luminance probe) and couple with a 
collimated source (a good quality flashlight will do in a pinch) and 
back the source and probe away from the material.  Subtracting the 
measured specular (direct) transmittance from the previous integrated 
measurement will give you the approximate diffuse value.

This is all very, very rough.

-Greg