Specular and diffuse reflection

[bjomor]

Im trying to dig into the details of specular and diffuse reflection.

Is diffuse reflection always distibuted uniformly in all directions? In other words, when light hits a white paper, will it bounce off equally in all directions +/-90 degrees? If not, does it depend on the angle of incidence of the rays?

Are there materials that act mostly diffuse at low angles of incidence and mostly specular at high angles of incidence? Looking from the side they look like a mirror, and looking straight down they look like a white paper.

Thanks for any help with this.

 

[lumenharold]

A couple hour search will tell you what a complex issue you are looking at. There are many published papers posted if you look for 'gloss measurements of paper' or 'color measurements of paper' (which will include white). It is a relationship between the pulp content, fiber size, filler type and content, wavelength of the light, etc. It might come down to how accurate you need your model to be.

Harold
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http://www.lumenflow.com

 

[IRstuff]

Actually, there's a further complication in that most processed diffuse surfaces hace a specular component, because the physical processes that produce a flat surface tends to flatten the microscopic surface profile, resulting in a slight specularity.

As for diffuse reflections, that also depends on the micro and even nano structure of the surface. There are holographic transparent materians that result in a non-Lambertian diffuse distributions.

Additionally, many physical surfaces will look specular at grazing angles, either because of the material itself or the air just above it. That's the genesis of many mirage effects.

Rather than asking such a general question, it might make more sense to specify what you're trying to achieve.

TTFN

FAQ731-376

 

[DanStro]

Is diffuse reflection always distibuted uniformly in all directions?
- No. Paper is usually referenced as being Lambertian (uniform in all directions) but I don't think there is a material that exists that is really Lambertian, just kind of close to it.

In other words, when light hits a white paper, will it bounce off equally in all directions +/-90 degrees?
- See answer above.

If not, does it depend on the angle of incidence of the rays?
- Yes, for many materials it is very dependent on angle of incidence. See IRstuffs response.

Are there materials that act mostly diffuse at low angles of incidence and mostly specular at high angles of incidence?
- Assuming by 'low' angles of incidence you are talking about angles close to the surface normal then yes. In my experience most materials have this quality.

As others have stated this is not a simple problem, if you need a surface that is Lambertian than paper is sort of the 'back of the envelope' type of solution that works in many applications. But if you need something that is closer there are some surface treatments that are very close in narrow angle/wavelength ranges. LabSphere has one of the I believe.

HTH
Dan

 

[IRstuff]

Of course, it's hard to get a true Lambertian surface, but the point should be that there are materials and surfaces that are "good enough." But, only the end-user can realistically determine that.

TTFN

FAQ731-376

 

[hioptix]

Lambertian is just a simplified model of real surface. A more realistic model is BRDF (bidirectional reflectance distribution function). In this model it clearly shows the relationship among incident light, specularly reflected light and scattered light. If google BRDF, you can get a lot of information.

Even BRDF is still a simplified model of real surfaces. Besides, the polarization of specular reflection and scattering makes things even more complicated. So it's true it really dependss on your application.

HiOptix