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all you need to know about mr fresnel

Topics covered:

Fresnel Effects, Surface Properties, Glass, Metals

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All you need to know about Mr Fresnel

Introduction welcomeAugustin Jean Fresnel (1788-1827 - pronounced Frennel ) is one Frenchman that every 3D artist should be grateful to!His life's work was varied as an engineer, mathematician and physicist, but in 1815 he published his first paper on the Wave Theory of Light. Whilst this may not sound like the kind of literature you'd subscribe to Reader's Digest for, it began to fully explain, for the first time, some very important effects observed by scientists and artists alike for many hundreds of years. Because, in the computer age, 3D graphics software often makes use of the formulae Fresnel first described, the visual phenomena that they predict and explain have themselves become termed 'Fresnel Effects'. On the next page you can see a typical example of the 'Fresnel Effect' at work. This tutorial has been created to observe, explain and implement the effect within Lightwave. Enjoy!

Augustin Jean Fresnel A very clever Frenchman!

Notice: The portrait of Augustin Fresnel is believed to be in the public domain. If this is not the case and you believe it is being displayed iimproperly, please contact webmaster@the-worms-of-art.com

2002-2003 The Worms of Art Ltd (9/5/03). All rights reserved

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All you need to know about Mr Fresnel

Fig 1.1 shows two photos of a glass-fronted building. In the first we are looking directly through a pane of glass. Although we can see there are some reflections, we can also clearly make out the people and objects inside. In the second photo we are still looking at the pane of glass, but this time from a much more acute angle. It has now become almost impossible to see inside. All we can see are the reflections from objects on our side of the glass. This is the first important observation to note regarding the 'Fresnel effect': Objects become more reflective the more acute the angle we look at them. In the case of a transparent medium like glass, this also means that the material becomes less transparent at acute angles.

Fig1.1 - Two photos of a glass building. One front on, the other at an acute angle.

2002-2003 The Worms of Art Ltd (9/5/03). All rights reserved

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All you need to know about Mr Fresnel

Simple physicsTransmission and reflectanceThe effect seen in the photos of Fig1.1 is a consequence of the way light behaves when it bumps into glass. Fig2.1 demonstrates graphically what's going on. Shockwave viewers can click and drag the eye.

Fig2.1 - Light hits a transparent surface at different angles. Reflectance and transmission change depending on the viewing angle.

When light hits glass, a percentage of it passes straight through (transmission) and a percentage is bounced back (reflectance). As the angle to the object increases, so less light is transmitted and more is reflected. From inside the building in Fig1.1, as the angle increases less light is transmitted through the glass and so we find it harder to see what's inside. From our side of the glass, more of the exterior objects' light becomes reflected, further obscuring the inside from view. Reflection and transmission are therefore inversely related - as one goes up, the other goes down.

2002-2003 The Worms of Art Ltd (9/5/03). All rights reserved

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All you need to know about Mr Fresnel

Different materials, same effectDifferent materials have different reflective and transmissive properties, but the effect always works in the same general direction. Even totally non-transparent surfaces like stone and wood react according to the Fresnel principle and are in fact more reflective at acute angles (seeFig 2.2). Although these materials are not transparent, light is still being absorbed and scattered within. This brings us to the essence of the 'Fresnel effect': The Fresnel principle.

All real-world materials react to light differently from different viewing anglesGo out there into the real-world and try to observe this for yourselves in everyday materials.Try particularly hard to test the theory in ways that don't get you arrested for suspicious behaviour!

Fig2.2 - In this photo, you can see two of the granite faces of this pillar. You can see how much more reflective one is than the other due to their different viewing angles. This is the Fresnel effect at work!

2002-2003 The Worms of Art Ltd (9/5/03). All rights reserved

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All you need to know about Mr Fresnel

Glass v MetalDifferent materials, different effects!How pronounced the differences in reflectance are at different angles, depends to a great degree on how conductive a material is. Conductive materials include metals - non-conductive materials include plastics, ceramics and glass. Non-conductive materials are also known as dielectrics. Conductive materials exhibit a much more subtle Fresnel effect than non-conductive, dielectric materials. This means there is less difference in reflections at different viewing angles. For polished surfaces, our eyes use this as an important visual cue. We get the sense that something polished is metallic because the Fresnel effect is less pronounced than plastic or glass, which are dielectric in nature. Fig3.1 shows renders of two materials. Can you tell which is meant to be ceramic and which is metal?

Fig3.1 - Apart from altering the Diffuse channels these surfaces are identical except for the differences in their Fresnel effects. Both surfaces are 85% reflective at their edges, but one is only 5% reflective when looking at it directly. Which looks like ceramic and which looks like metal?

In Fig3.1 the non-conductive ceramic - or dielectric - is hardly reflective at all looking straight at it, but is very reflective at acute angles. The metal is slightly more reflective at the edges, but the effect is much more subtle. In the Pshelf metals collection we have actually exaggerated the Fresnel effect of metals slightly because, in the end, it looked nicer! This brings us to our second important idea:

Art is an art, not a science!

2002-2003 The Worms of Art Ltd (9/5/03). All rights reserved

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All you need to know about Mr Fresnel

Fig 3.2 shows the same surfaces as before but as two diagrams, showing the reflectance amounts as the viewing angle changes. It's hard to consciously perceive these differences, because our brains cleverly process the information to produce the idea of "metal" or "plastic" or "ceramic". But as a 3D artist it's important to realise that, subconsciously, it's these reflectance changes that are influencing our judgement.

Fig3.2 - Hopefully, you're thinking that the sphere on the left looks metallic and the sphere on the right looks ceramic! The reflection amounts change as the viewing angle changes. Our eyes use this as a visual cue, telling us what a material is made from. The bigger the difference, the more 'dielectric' the material appears.

folders v saucepansFig 3.3 shows two photos. We'd like you to answer a simple question about them: Q. Which of these two objects is reflective?

Fig3.3 - Which of these two objects is reflective?

2002-2003 The Worms of Art Ltd (9/5/03). All rights reserved

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All you need to know about Mr Fresnel

The natural answer to give is, that the metal saucepan is reflective and the blue folder is not. For everyday life this conclusion is sufficient. We are all happy to observe that a mirror is reflective but a wall isn't. However as an artist this isn't good enough. You must ask yourself more questions, such as the following: Q. If the blue folder isn't reflective, how come we can see it and know that it's blue?! This second question requires a much deeper understanding of how light works in order to answer. Discussions about this very subject raged throughout Augustin Fresnel's own lifetime. A commonly held belief was that people's eyes actually emitted light and illuminated the objects all around them! The true answer to the second question is therefore not as obvious as we might like to think.

What is reflective?OK. Let's eliminate the possibility that our eyes are sending out light particles. This is not true! A more helpful way to think of light bouncing around our environment is as follows: When light hits a surface, that surface can deal with the light in several ways: It can absorb the light and convert it into something else - usually heat. It can ignore certain light frequencies and let them pass straight through - known as transmission or transparency (just as glass or water lets through most visible light frequencies). It can absorb the light but immediately reflect it back out in the general direction of where it came from. This is known as reflection. If, on a microscopic level, the surface is very flat and polished, all the reflected light will come back at uniform and predictable angles. This gives the surface a mirrored appearance, as with the metal saucepan. This is known as SPECULAR reflection. If the surface is random and rough on the microscopic level, the reflected light will be scattered outwards with a degree of randomness. This is a little like pool balls scattering after a break-off shot. This kind of reflection is known as DIFFUSE reflection and is what's happening to the blue folder. The concept of two reflection types - SPECULAR and DIFFUSE - explains the differences between the appearance of the blue folder and the metal saucepan.

2002-2003 The Worms of Art Ltd (9/5/03). All rights reserved

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All you need to know about Mr Fresnel

When we look at the blue folder, the surface is absorbing most of the light frequencie