[Radiance-general] Rtrace for Radiance & Irradiance Calc.

[Germán Molina Larrain]

Nasa? Awesome... haha.

Anyway, I did not really finished reading, but I have some questions

   1. Radiance already has a virtual photogoniometer... genBSDF program.
   easy yo use and, I would say, very optimized. Why is that you do not want
   to use it?
   2. Radiance also has some programs that allow you to give a BSDF
   "function" into a BSDF matrix or tensor tree: bsdf2klems, bsdf2tensortree,
   or something like that.

Were you aware of this? is there a reason not to use them?

Best,

Germán

2015-04-09 11:38 GMT-03:00 Kolomenski, Andrei (JSC-SF311)[WYLE INTEG. SCI.
& ENG.] <andrei.kolomenski at nasa.gov>:

>  Hello Dear Radiance Users,
>
>             I have some questions about the utilization of rtrace to
> determine radiance & irradiance values within a scene. I greatly appreciate
> your responses and help. To give some context, I’m working on developing a
> virtual gonio-photometer in Radiance in order to obtain BRDFs of Radiance
> primitive materials and then export the BRDF data for analysis with other
> software. To do this I need a clear understanding of how Radiance measures
> radiance and irradiance with rtrace. First, I will explain my current
> methodology and please correct me if I’m wrong at any point.
>
>             A BRDF is a ratio of the reflected radiance of a material over
> the incident irradiance on the material surface, and this value is
> evaluated over four spherical angle ranges that define the orientation of
> the incident light and the orientation of the reflected light as captured
> by a radiance meter.
>
>             The units of a BRDF value is (1/steradian), since the units of
> radiance are (W*m^2/sr.) and the units of irradiance are (W*m^2). Also,
> radiance is independent of measurement distance unlike irradiance. Due to
> this the BRDF geometry is constrained to a unit meter hemisphere, with the
> material sample centered at the origin.
>
>             To compute the incident irradiance on the material sample I
> use rtrace with –I argument. I pass the position (0,0,0), the center of the
> material sample and the direction (x,y,z) that is a unit vector aligned
> with the optical axis of the light source that points to the center of the
> light source (ring surface). I’ll call this irradiance value L1.
>
>             A second way to compute the incident irradiance is to use
> rtrace with the –i argument and pass position (x,y,z), the position of the
> light source, and direction (-x,-y,-z) that points toward the origin, which
> is the center of the sample. I’ll call this irradiance value L2.
>
>             After looking at the irradiance values it seems like L2 is the
> Lambert’s cosine corrected irradiance, meaning L2 = L1*cos(theta), where
> theta is the radial incident angle of the light source. Is this true? If
> not, can someone please explain the difference between these two irradiance
> measurements?
>
>             Also, in the rtrace help page could someone help explain this
> statement that concerns the –i argument: “This only affects the final
> result, substituting a Lambertian surface and multiplying the radiance by
> pi.” I understand that a Lambertian surface is one that is purely diffuse
> and the radiance and luminance is constant in all directions, but I’m
> unsure of how this applies to irradiance.
>
>             Finally, I need to obtain reflected radiance from the
> material, and I want to make sure that my radiance measurement doesn’t take
> into account the radiance emanating from the light source itself. To do
> this, given an arbitrary radiance meter positon along the unit hemisphere
> (x,y,z). I setup rtrace with no input arguments other than position (x,y,z)
> and direction (-x,-y,-z), a unit vector that points to the center of the
> material (0,0,0), from the radiance meter position (x,y,z).
>
>             My problem arises when I evaluate the BRDF at various
> locations. I see a radiance value that is about 3x-4x larger than my
> measured incident irradiance value, at the position of the specular
> reflection (mirror-like reflection). Of course, I will see the highest
> reflected radiance at the specular position, however it seems to be
> unrealistically large, since somehow more energy is reflected than
> emanating from the single light source itself. Is this type of behavior
> common for a metal Radiance primitive that follows the Ward BRDF scattering
> model? With my current observations, the conservation of energy is not
> maintained. Do I need to correct the radiance value by a steradian value to
> be able compare the two as apples to apples? Or am I missing something else.
>
>             I apologize for the long message, but I wanted to be as clear
> as possible with my questions. Thank you in advance for your time and help.
>
>
>
> Best Regards,
>
> Andrei Kolomenski
>
>
>
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