Confusion, apologies - Glass, IOR, Fresnel...

[Ros. Riha]

On a project, after some quite minor changes of design, renderings started to turn out very different - no changes in lighting etc. Trying to remedy it, I found I am not familiar enough with controls, affecting the final result:

1. What does exactly "Fresnel" do? On glazing, I keep it usually 0.7-0.8, what if I decrease it?
2. How about IOR? I keep it between 1.5-17 on glazing... does 1.7 mean I will see more of the interior than on 1.5 or vice versa?
3. What should be proper color of glass? Should I keep it very light, or not? How will darkening it affect the final result(with conjunction of other pramters?

I apologize again, I remember this must have been discussed already, but just now (deadline approaching and renderings keep looking ugly) I would mor than welcome some comprehensive explanation.

[Marc Chaumier]

In most case, I do the same as Peter (transparency to 1.0) and even more reflectivity ... But don't forget that when no light of the sun hit the glass (north orientation) so the glass lets appear the slightest reflection, because all the environment in front of your glass is in south direction, and so very enlightened (even the sky's elements).

[Peter Milner]

Fresnel is where you get greater reflection at glancing angles. The higher the number the greater the effect.

IOR for architectural glass is typically 1.5 to 2.0. However if you model your glass as thin faces then you need to tag the glass as Thin and IOR will make no difference at all. Higher numbers for thick glass will result in less transparency.

For clear glass I tend to use a very light grey colour. For toughened glass I add a smidgen of green and blue to the mix.

[Roy Hirshkowitz]

You're pretty close.  In the Path Tracer:

 

The IOR setting controls two aspects of light transport through transparent media.  

 

1. The refraction, or direction change, as light enters and exits a thick medium.  If the object is tagged as thin, this does not occur.
2. The fresnel effect, technically called fresnel reflection of dieletrics, which causes reflection as a ray enters a transparent medium (and reduces transmission by an equal amount.)  This occurs even when an object is tagged as thin.  The Fresnel setting affects only reflective materials.  It controls an effect known as fresnel reflection of conductors.  If your material is not 100% transparent, then this setting may come into play-- otherwise the IOR controls this.

 

The reflection sharpness slider does still alter the reflections of transparent materials. 

 

I don't quite remember how all of this goes in the packet tracer-- but it should be similar.

[Ros. Riha]

As most materials are NOT 100% transparent and somehow reflective, I would like to know, where to find proper Fresnel settings - could you please point me to where find them?

There are two approaches I am not able to chose from:

1. Roy says: keep it simple, use preset...
2. Peter says: Add a little bit of this and a little bit of that

My approach is more towards No2 - but I am being frequently reminded about its weaknesses.

 

PS: Do not worry about packet tracer, pathtacer seems to be THE choice.

[Roy Hirshkowitz]

Leave it alone.

 

Set the transparency to 100%.  Use the IOR and the color to control the transparency.  This will model a pure glass material as closely as possible.

 

For coated glass or laminated glass you may want to back the transparency off a bit.  If you haven't touched anything it will automatically add the right kind of reflection for this type of material.

 

The fresnel setting is useful for glossy materials, not transparent ones.

 

 

 

 

 

[Ros. Riha]

"Set the transparency to 100%.  Use the IOR and the color to control the transparency." - good advice, simple and working.

"The fresnel setting is useful for glossy materials, not transparent ones" one is always learning - thanks again - glossy: wall paint, concrete, metal railing, roof shingles, exposed brickwork, plastic...etc,etc - what is proper fresnel setting for these?

[Roy Hirshkowitz]

In general, leave it alone.  

Fresnel reflectivity of conductors models the tendency of many materials to become more specular (mirror-like) at glancing angles, while retaining more matte properties at perpendicular viewing angles.  

You can reduce it somewhat for very dark materials, to prevent too much reflection.  You can increase it for materials like varnished wood, where the fresnel reflectivity is more pronounced.  

The angular blend complex pattern can provide more control over this effect, if you require it.  It is not as physically sound a model though, so I wouldn't use it unless necessary.

 

[Peter Milner]

A question on Fresnel:

When Fresnel is applied, does the Reflection value set the minimum or maximum or mid amount?

[Ros. Riha]

Many thanks to all of you for valuable advices.