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 Post subject: FACET DIAGRAM ANGLES
PostPosted: Sun Jun 26, 2022 9:41 am 
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When I look at the diagrams, they always state that they are for specific RI's. What is the effect of using gem material of a significantly different RI? Any references I could review?


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 Post subject: Re: FACET DIAGRAM ANGLES
PostPosted: Sun Jun 26, 2022 1:54 pm 
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You can load the diagrams into a program such as GemCutStudio. Then try different materials with different RIs on that design. You can also optimize a design for any RI in the program.

In general I have found that as long as you keep the minimum angle on the pavilion a degree or so above the critical angle for a material designs work well in regard to brilliance. Then the biggest affect of changing RI will be on pattern and extinction.

RI is just a proportional number based on the difference in the speed of light in air and in the material. So as long as the angle that light strikes a surface doesn't violated the CA, the path the light follows doesn't change because of the RI of a material.

The angle of reflection equals the angle of incidence just like a pool ball.


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 Post subject: Re: FACET DIAGRAM ANGLES
PostPosted: Sun Jun 26, 2022 5:12 pm 
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Here is a good explaination of the difference in Snells Laws for Refraction, and Reflection:

https://www.optics4kids.org/what-is-opt ... wo%20media.


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 Post subject: Re: FACET DIAGRAM ANGLES
PostPosted: Mon Jun 27, 2022 12:26 am 
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firedrake wrote:
When I look at the diagrams, they always state that they are for specific RI's. What is the effect of using gem material of a significantly different RI? Any references I could review?
I would have to ask to define "significantly". The majority of designs out in public domain are in three major indices, 1.54, 1.76 and 2.16 respectively. As Steve would probably agree, the majority of gem rough in these groups tend to dictate the angles appropriate to the size of rough L/W ratios to maximize the best return where optimal performance is of a lesser concern. A matter of opinion. Generally I have found that shape, design styles play a significant role in performance, even when a design is called out for a specific RI.
But to answer your direct question as to the effect is, windowing, leakage where light entering is not returning to the viewer. Basically what Steve said as well. I personally find Gem Ray gives me a better performance return to GCS, again a matter of opinion.

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 Post subject: Re: FACET DIAGRAM ANGLES
PostPosted: Mon Jun 27, 2022 2:40 pm 
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Thanks everyone...


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 Post subject: Re: FACET DIAGRAM ANGLES
PostPosted: Tue Aug 09, 2022 8:44 pm 
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1bwana1 wrote:
So as long as the angle that light strikes a surface doesn't violated the CA, the path the light follows doesn't change because of the RI of a material.

This is just so, so wrong...

Even small changes in RI can have a massive impact in the way that light moves through a stone. These kinds of differences are typically seen with brilliant ovals/pears/marquises, or with odd-symmetry designs like trillions, pentagons, and heptagons.

For example - I have a 3-fold design that works in very low RI materials, from 1.43 to 1.61. The stone looks great up to about 1.61, 1.62 - then as soon as you get to 1.63, 1.65 it completely changes the reflection pattern, and new windows appear because of the way light is redirected. Then again around 1.90, there's a sudden inflection point and then the appearance changes rapidly from brilliant with tiny windows lateral to the table, to crushed-ice.

The path that light follows can change substantially with tiny changes in the RI of a material.


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 Post subject: Re: FACET DIAGRAM ANGLES
PostPosted: Wed Aug 10, 2022 12:03 am 
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Faceting 101 by Arya wrote:
1bwana1 wrote:
So as long as the angle that light strikes a surface doesn't violated the CA, the path the light follows doesn't change because of the RI of a material.

This is just so, so wrong...

Even small changes in RI can have a massive impact in the way that light moves through a stone. These kinds of differences are typically seen with brilliant ovals/pears/marquises, or with odd-symmetry designs like trillions, pentagons, and heptagons.

For example - I have a 3-fold design that works in very low RI materials, from 1.43 to 1.61. The stone looks great up to about 1.61, 1.62 - then as soon as you get to 1.63, 1.65 it completely changes the reflection pattern, and new windows appear because of the way light is redirected. Then again around 1.90, there's a sudden inflection point and then the appearance changes rapidly from brilliant with tiny windows lateral to the table, to crushed-ice.

The path that light follows can change substantially with tiny changes in the RI of a material.


As I said, the RI of the material has a huge effect on the path the light follows when it enters the stone. The angle of light changes because of the difference in how fast the light travels in air and the material. Which of course is the mathmatical definition of R.I.. At the boundries the path light will follow can chaange drastically. This sets up the path that it will follow through the stone and can affect the design strongly.

https://en.wikipedia.org/wiki/Snell%27s_law

But once inside the stone as long as the path is not inside the cone of the criticil angle of the material, the light travels like a pool ball. The angle of reflection is equal to the angle of incidence. This is true no matter what the R.I. of the material is.

Many devices take advantage of the total internal reflection, including optical waveguides (like optical fiber). Rays that intersect the interface between the waveguide material and the surrounding material at angles equal to or larger than the critical angle are trapped in the waveguide and travel losslessly along it.
Rays can be trapped in a waveguide through total internal reflection.

So, the differences you see in your designs is cause by the refraction at the boundry causing the original light path to be different as it initialy enters the stone, and the light that leaks out of the stone because of the cone of the critical angle. In certain designs this can result in very different looks.

However on most of the traditional cutting designs if you keep your angles and proportions within a fairly flexible range of angles and proportions then the result will still be excellent over a broad range of RI materials.

This is the whole basis of classic gem cutting as practiced by the best cutting houses in the World. If you ask a fine gem cutter from Idar, Bangkok, etc. what angles he used, he won't be able to tell you. He doesn't know and he doesn't care. His cutting machines don't even have the ability to measure angles. This will be the case with the vast majority of stones that win the Spectrum awards each year. These are excelent stones and the crowns almost always have very good and precise meetpoints and symmetry. The pavilions sometimes less so in order to save weight.

As I have said, it is the difference in approach between being a facetor and a gem cutter. As in all things there are crossovers and this line is fuzzy and sometimes very wide.


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 Post subject: Re: FACET DIAGRAM ANGLES
PostPosted: Wed Feb 01, 2023 3:04 am 
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The appearance of a gemstone as you would see, for example, in GemRay, depends on a complex interaction of several variables. At a minimum these are:

The orientation of the stone to the source(s) of illumination.
The refractive index of the material, which will determine the path light rays travel through the stone, particularly on entering and leaving the stone.
The orientation of the stone to the viewer.
The geometry of the stone's crown and pavilion.

Happily for us, the incredibly complex mathematics of this interaction can be solved by ridiculously affordable ray tracing programs like GemRay or Gem Cut Studio.

There is a very informative article that Barbra V. posted in this forum on November 15th, 2018 - highly recommended, but a fairly long read.


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