Talk:Bidirectional reflectance distribution function

This article could use more context. Who uses this parameter, and for what?--Srleffler 01:50, 14 April 2006 (UTC)
 * Expanded a bit. `'mikka (t) 02:31, 14 April 2006 (UTC)

Measurement of BRDF
The third paragraph should be stricken. The statement that the BRDF function cannot be measured because it represents infinitessimal solid angles is not logical. The statement that BRDF cannot be measured due to the infinitessimality of the input parameters is equivalent to saying that one cannot measure any physical quantity that is not immediately discreet. One can measure the function with the best possible angular resolution, and then (by applying the sampling function) deconvolve the instrument function and reconstruct the original BRDF.

Instruments that measure BRDF are readily used in the optics community to determine the BRDF of the materials used to shield optical systems. Without measurements of BDF, stray light predictions would not be possible. —The preceding unsigned comment was added by 192.91.172.36 (talk) 21:42, 16 March 2007 (UTC).
 * I agree with you, corrected article pruthvi 17:39, 18 October 2007 (UTC)

transmission and scatter functions?
Does anyone have any content on BTDF and BSDF? should we just fold them into this article?58.247.205.172 (talk) 23:00, 19 May 2009 (UTC)

Irradiance "from a specific direction"?
The following phrase confuses me: "irradiance incident on the surface from direction $$\omega_i$$." The definition of irradiance does not mention an incoming direction. What does it mean for irradiance to be incident on the surface "from a certain direction"?--Singularitarian (talk) 10:12, 2 September 2009 (UTC)

This confused me also. I think it should be "radiance" rather than "irradiance". Irradiance and radosity are directionless quantities representing incoming and outgoing power respectively. The term 'radiance' is used to describe both incoming & outgoing directional power. -- Anonymous, May 2018

BRDF use in remote sensing
It should also be noted that BRDF has some uses in remote sensing of Earth. Namely, the BRDF derived from the satellite sensor (such as MODIS) data via linear least squares (should one have a suitable model), may be used to calculate black- and white-sky albedos, and also to produce images free from any geometry effects, such as.

— Ivan Shmakov (talk) 18:29, 26 January 2010 (UTC)

Light reflection planar?
In saying that both light rays are parameterized in terms of the azimuth angle, it seems like the reflected ray is not in the plane defined by the normal and the incoming ray. Nowhere is it mentioned that the BRDF is a function that applies to a surface and you pick the direction of incoming light (parameterized into the azimuth and zenith angle) and the point it strikes to plug into the function to get the direction light is reflected from.

At least, that is my understanding after reading the article. If that’s not how it works, the article could use some clarification.

- Adam Gulyas — Preceding unsigned comment added by 131.136.242.1 (talk) 16:13, 6 January 2012 (UTC)

Why is Clarification Needed for the Surface Normal Vector, n, in the Opening Paragraph???
Anyone who actually looks up this topic knows what a surface normal vector is, and is used to the standard n notation, so is the "[clarification needed]" there for something else? I didn't write that paragraph, but it made perfect sense to me...

...I guess clarification is needed as to why clarification is needed, lol.

99.116.176.234 (talk) j —Preceding undated comment added 03:46, 8 December 2012 (UTC)


 * I wrote it because it's unclear what it means that two vectors are defined with respect to a third vector. Would it have made the situation different if they wouldn't have been defined with respect to n? —Kri (talk) 10:51, 8 December 2012 (UTC)


 * Lol. —Kri (talk) 19:47, 22 December 2012 (UTC)


 * I agree that clarification is needed. The omegas are unit vectors.  These vectors are not *defined* by their relation to the unit normal vector.  They are defined by where the light is coming from and where the spot is being observed from.  You can describe the angles between these unit vectors and the surface normal.  But the omegas are not defined with respect to that normal.  It is possible to talk about the components of the unit vectors in a coordinate system that is partly defined by the normal vector.  For instance, you could construct an x,y,z coordinate system with the z axis along n.  Then you could express the components of the omega vectors in that coordinate system.  But n itself cannot tell you the direction to the x axis, for example.  Something else has to define that direction on the surface.  We don't actually use a Cartesian coordinate system, we use a spherical polar coordinate system with the zenith angle, theta, and azimuth angle, phi, (and r = 1).  At best, all that n can tell you is the zenith angle to omega.  It can't tell you the azimuth angle.  So given that you can't even describe one of the coordinates of omega with respect to n, how can you even talk about *defining* omega with respect to n.  That statement is actually quite meaningless when you think about it. — Kimaaron (talk) 13:54, 3 April 2014 (UTC)

I changed the wording to something that is accurate, even if it is not perfect.

There is an implicit assumption of rotation invariance here, as follows:

In full generality, f_r depends on all three of ω_i,ω_r, and n, i.e. f_r=f_r( ω_i,ω_r, n). That makes 6 real variables. But it is reasonable to assume that f_r( ω_i,ω_r, n)=f_r( R ω_i,R ω_r,R n) for any rotation matrix R. This would be implied, for example, if both bulk media (the media on either side of the surface) are isotropic. Since the rotation group has 3 dimensions, this leaves 3 true variables for f_r. They can be identified as the angles between each pair of ω_i,ω_r, and n -- making a total of 3 angles. They are free to vary, subject to some common-sense inequalities.

The setup in the article, with 4 variables, is quite reasonable. The extra variable corresponds to a rotation of space that leaves n unchanged, but rotates ω_i and ω_r rigidly around n. Presumably one retains the the requirement that f_r is invariant under this rotation. Then f_r has a 1-dimensional symmetry, and one of the 4 variables is redundant, but we leave all 4 variables in anyway for ease of presentation. These 4 variables can then be identified with ω_i and ω_r, in any coordinate system where n is the unit z-vector.

178.38.105.43 (talk) 15:40, 9 April 2015 (UTC)

diagram
We need a better BRDF diagram -- something lik www.oceanopticsbook.info/view/surfaces/the_brdf --Gondi56 (talk) 03:41, 7 February 2015 (UTC)

External links modified (January 2018)
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wrong link for Lommel-Seeliger
At: models > Some examples, the link for "Lommel-Seeliger" does not go to that topic, but to "Opposition surge"

The Lommel-Seeliger relation, cosine(incidence)/(cosine (incidence) + cosine(emergence)) is appropriate for particulate surfaces of low albedo (multiple scattering not importent) THere is no page for this yet. OldMartian (talk) 17:19, 30 July 2020 (UTC)

Conservation of Energy
Conservation of energy requires that the total energy in equal the total energy out, so the integral over the solid angle should only equal 1, and less than is also forbidden.

(That said, I suspect it is common to call the constraint that on the basis that the material can absorb the light, converting it to a different form, usually heat. However, the opposite can happen in some situations, where the medium emits more than it absorbs - e.g. lasers. It's just less common.) 2600:1700:3D2D:8810:5CCE:ABD5:83C0:8045 (talk) 03:07, 4 November 2023 (UTC)