View Full Version : fresnel reflection

05 May 2010, 09:15 AM
Why are reflections brighter on edge then straight on, described as the Fresnel reflection effect? From a "scientific" perspective.

Does the amount or quality of this effect change on certain materials in a predictable way? Is there some sort of Fresnel index table?

05 May 2010, 12:39 PM
It's really rather complicated to be honest and has to do with the interaction of photons with the electrons in the atoms at the interface to your material. There's no intuitive explanation that I've come across.

What is important about it is that the fresnel equations tell you the probability of a photon being reflected, transmitted or absorbed at an interface, or to think about it another way, what proportion of the incident light is reflected, transmitted or absorbed.

This proportion is dependent on the angle the light hits the surface at, and the index of refraction of the surface. That is why reflections appear to get brighter at grazing angles - because more of the light is being reflected rather than transmitted, while 'in the middle of an object' the reverse is true.

Another important consequence of the fresnel effect is that glossy reflections tend to get sharper at glancing angles as all reflection directions apart from the mirror direction get cut out.

There's some more explanation of this in the 'science of cg' thread that's stickied on this forum, as well as links to tabulated ior data for different materials.

05 May 2010, 12:29 PM
If I remember correctly the fresnel effect is mainly based on the index of refraction. So you can use tables like the following to adjust your materials.

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05 May 2010, 12:29 PM
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