Talk:Diffuse reflection

Lambertian reflectance

In the article it says: "the reflected light will be evenly spread over the hemisphere surrounding the surface (2π steradians)."

Isn't the reflected light scattered according to uniformly distributed surface normal angle and thus the reflected photons are NOT equally distributed over the hemisphere surface? 80.221.29.165 (talk) 22:05, 9 October 2008 (UTC)

Not necessarily. See Lambertian reflectance and Lambert's cosine law. An ideal diffuse scatterer scatters light equally in all directions, without regard to the angle from the surface normal.--Srleffler (talk) 22:36, 9 October 2008 (UTC)

My point is, that if you shoot or reflect photons into random direction, equally in all directions, then the resulting distribution of photons in the hemisphere (photons/m^2) will not be evenly spread as the article says, but top of the hemisphere will be more densely populated than the sides. With ideal diffuse reflector, photons in a solid angle decreases as the cosine of an angle with the surface normal.80.221.29.165 (talk) 11:15, 10 October 2008 (UTC)

OK, you're right, and the article is poorly worded. What they were probably thinking of is that while the intensity (photons per steradian) falls off as cos(θ), the apparent brightness of the surface to an observer remains constant because the apparent area of the surface also falls off as cos(θ). More technically, the radiant intensity for an ideal Lambertian reflector is proportional to cos(θ), while the radiance is independent of θ. This article should probably describe Lambertian surfaces explicitly. They are a pretty important special case. Note that not all surfaces are Lambertian, however. In particular, for some surfaces the intensity falls off much slower than cos(θ).--Srleffler (talk) 15:52, 10 October 2008 (UTC)

Proposed merge

I oppose the proposed merge with Scattering from rough surfaces. That article is on charged particle scattering. This article is on light. The subject matter is sufficiently distinct for two articles.--Srleffler (talk) 06:01, 13 July 2009 (UTC)

I also oppose it; and the editor who put the tags didn't even bother to put a rationale or start a discussion, so I'd say let's just take them away already. Dicklyon (talk) 02:58, 14 July 2009 (UTC)

Light scattering

Editors of this article might be interested in the new article (possible content fork) Light scattering. It's possible that the two articles could be merged, although there may be merit in keeping them separate. Discussion at Talk:Light scattering.--Srleffler (talk) 16:53, 27 July 2009 (UTC)

It looks a lot broader than diffuse reflection. I don't like to see articles spring to life fully formed like that, and without URLs for the refs, but that's what we got. Dicklyon (talk) 06:12, 28 July 2009 (UTC)

Discussion is at Talk:Light scattering.--Srleffler (talk) 06:11, 29 July 2009 (UTC)

Merge with Lambertian reflectance?

This article seems to cover very similar material to Lambertian reflectance to the point that they both use the same intro image. The difference seems to be in emphasis: this article focuses on the physics whereas Lambertian reflectance is primarily about modeling it in computer graphics. The fact that they are different articles seems to imply that they aren't the same thing. —Ben FrantzDale (talk) 13:41, 24 February 2011 (UTC)

Actually they are quite different things: diffuse reflection is the general phenomenon shown by almost any surface we see. Lambert's cosine law is the idealized angular behaviour we expect to find in an ideal, homegeneous, "featureless" surface, for example a white surface with very small, randomly oriented irregularities. Many (or most) diffusing objects are not Lambertian: see, for example, Oren–Nayar reflectance model, or, for celestial bodies, Geometric albedo. The figure here represents just an example (and includes also specular reflection). --GianniG46 (talk) 00:12, 25 February 2011 (UTC)

I agree with Gianni. This is an important point: one cannot assume all diffuse reflectors are Lambertian. It's an idealized model.--Srleffler (talk) 04:44, 25 February 2011 (UTC)

Who the hell wrote this?

What is the exact mechanism of diffuse reflection? What happens to a photon when it strikes a surface? What causes the photon to reverse direction? Is it the electron, is it a force, is the nucleus?

This article reads like it is aimed at children. Can someone please fix this? — Preceding unsigned comment added by 78.146.81.42 (talk) 19:15, 18 August 2012 (UTC)

Welcome to Wikipedia, the encyclopedia that anyone can edit. This article is written by many volunteers working collaboaratively. If you don't like the way an article is written, you are welcome to edit it and try to improve the style or content.

I think the problem is that you are looking for more details on the mechanism of reflection itself, rather than specifically the mechanism for diffuse reflection. Check out Reflection (physics) and see if it answers some of your questions. Particularly the section Mechanism. This article seems to me to provide pretty good information on the mechanism for diffuse reflection, presuming one already knows how reflection works.

Regarding the "aimed at children" comment: keep in mind that this is an encyclopedia, not a physics textbook. For topics that are not very specialized, a good article will have an introduction that is aimed at about the level of a smart 11 year old. A good article will cover topics of general interest first, and then move into more difficult or technical material further down in the body of the article, if necessary. Obviously, some topics demand a more advanced level of exposition than others. --Srleffler (talk) 16:28, 19 August 2012 (UTC)

Google Ankush9830311981 (talk) 17:39, 13 October 2017 (UTC)

Vector Length in Figure 1

In the first paragraph it is said " An illuminated ideal diffuse reflecting surface will have ***equal luminance from all directions*** in the hemisphere surrounding the surface (...)". However, looking at Figure 1, the red arrows (that indicate diffuse reflection) have varying lengths. It can be confusing reading that and then seeing varying length vectors in Figure 1. I suggest to edit the figure in order to explain what the length of the red vectors mean (I personally do not do that because I am also confused .. :). Capagot (talk) 18:46, 10 October 2012 (UTC)

Done. The rays represent luminous intensity rather than luminance. --Srleffler (talk) 04:42, 11 October 2012 (UTC)

Make it Clear the Light is not "Bouncing"

The wording of this article is technically correct, but does not make it sufficiently clear that photons involved in diffuse reflection are being absorbed and re-emitted. Many pop descriptions of diffuse reflection say incorrectly that light "bounces" off atoms. This is not true. All light involved in diffuse reflection is absorbed and (possibly) re-emitted. The non-absorbed light is scattered by either Thomson Scattering or Resonant Scattered. Either way it is absorbed and re-emitted. It does not "bounce" off the atoms. Thus, for a normal opaque, colored material, like an orange or a leaf, the light we see from the object is composed of newly-created photons, not the photons that originally impinged on the object to illuminate it. This should be made perfectly clear in the article so that there is no confusion about this. John Chamberlain (talk) 10:14, 10 April 2017 (UTC)

Backscattering yields a uniform full moon image

There is no true photo of light scattered from a surface that obeys Lambert's cosine law.

The mean backscattering from the full moon is directed back to the sun because the scattering dipoles on the moon oscillate in a plane perpendicular to the coming sunlight.

Any calculation that does not take the direction of the polarizing dipoles into account will not be correct.

Backscattering yields a uniform moon image, as well as the earth's image and the images of all the planets and their moons.

Back-scattering from any surface will not depend on the surface inclination angle to the coming light, and a curved surface will look uniform. It is, therefore, not surprising that true images, that obey Lambert's cosine law, are difficult to find.

In the calculation of Lambert's cosine law, there is a hidden assumption that the scattering dipoles oscillate in all random directions in space. This is not the case with the moon, and probably with other examples.

Urila (talk) 09:43, 30 October 2018 (UTC)

Wikipedia is not a venue for posting your original research. Stop posting this stuff.--Srleffler (talk) 04:21, 31 October 2018 (UTC)

The Scattered light is considered in the literature as a diffusive light, light that passed a number of scattering events before it left the scattering material. Diffusely scattered light must obey Lambert's Cosine scattering law. In the case of unidirectional light scattered backward from a surface of a sphere, the meaning is maximum scattering intensity in the middle of the sphere, and a decline to zero toward the periphery by the cosine law. The full moon looks uniform and people continue to assume that the light is diffusely scattered from it. More than that. The nearly uniform sphere image is common to all the planets and their moons, including the earth as observed from the moon. Out of thousands upon thousands of true photos, there is no single true photo that obeys Lambert's Cosine law. The only photos that do obey the law are rendered photos, photos that are at least partly simulated.

<Material was deleted here after the discussion below.>

Urila (talk) 09:34, 16 May 2020 (UTC)

The opposition surge, the significant increase of 180 degrees backscattered light, is a well-established and well-documented phenomenon. See "Opposition surge" in Wikipedia, and the references therein. On the other hand, there is no single true photo that obeys Lambert's Cosine law, "Lambertian reflectance" in Wikipedia. The first figure in the article, "Diffuse reflection" in Wikipedia, is an erroneous artist view. Urila (talk) 03:51, 9 October 2020

Lambertian reflectance is an idealization; real diffuse scatterers are not exactly Lambertian, and some diffuse scatterers are quite far from being Lambertian. This is not a surprise. The lead section of the article is explicit about this. The first image in the article illustrates that idealized case, and says in the caption that that is what it shows.--Srleffler (talk) 16:58, 10 October 2020 (UTC)

The first image is an outcome of Lambertian scattering. However, out of the many, many thousands of true photos, there is not a single one that resembles this idealized case, not even vaguely, not even near to that. On the other hand, all such true photos are nearly uniform, in the gas phase, liquid phase, solid phase, and mixtures of them. See the "blue marble" photos by NASA. All this is absurd. Urila (talk) 18:54, 11 October 2020 (UTC)

I presume you're talking about astronomy? This is not an astronomy article; it's an optics article. Diffuse reflection that approximates Lambertian reflection certainly does happen, and is of some theoretical importance. Yes, people do err in assuming Lambertian reflectance in cases where it is not a good approximation.

You're really engaging in the wrong forum here. We don't publish original research. One of our core rules is that everything we publish needs to be verifiable by reference to reliable sources. An analysis that you've done yourself is irrelevant, unless it's been published in a peer-reviewed journal. You can make all the arguments you like, but if you can't cite any reliable sources you're just wasting your time. --Srleffler (talk) 19:40, 11 October 2020 (UTC)

I omitted my work and the reference to it on May 16th. Although I presented an astronomical example, it is valid in general and not limited to astronomy. Again, "Diffuse reflection that approximates Lambertian reflection certainly does happen", but no single true photo in the literature. Urila (talk) 09:16, 12 October 2020 (UTC)

That's a very interesting claim, but unfortunately it's not something we can do anything with here. This is not the right forum for exploring that idea.--Srleffler (talk) 18:43, 12 October 2020 (UTC)

What us irregular reflection

bzbzbz 146.196.34.22 (talk) 13:49, 11 December 2022 (UTC)

diffuse reflection

The figure at the beginning of the page is a misleading and erroneous artist view. Scattering from solids is "Mie" scattering. It has a sharp maximum in the back direction of the incident ray. There is no true scattering that looks like that in the figure. urila — Preceding unsigned comment added by Urila (talk • contribs) Urila (talk) 18:18, 31 March 2023 (UTC)

It's not an "artist view". The diffuse scattering shown is Lambertian reflectance, a commonly used idealization for scattering. Yes, no real material is a perfect Lambertian reflector, and we might be better off using something more reflective of reality for this article.--Srleffler (talk) 16:20, 1 April 2023 (UTC)