Calling all experts...question

[Laila619]

Hi there,

I have tried to do some Google searches but nothing really comes up. I don't see anything on Pricescope's tutorial about this either. So what I am wondering is this: what is the relationship between the stars/lower girdle facets and crown/pavilion angles? In other words, if you have a 'steep deep' stone, is it best to have short LGF or long LGF? And if you have a shallow stone, say crown angle of 34, pavilion 40.5, what would be the best star/LGF percentages for that stone?

I recall someone mentioned that longer LGF can sometimes 'save' a steep deep' stone. Is that true? Or maybe I'm not remembering correctly.

Thanks for the insight and education!

 

[Karl_K]

I wrote an article on this:

http://journal.pricescope.com/Articles/68/1/Do-the-pavilion-mains-drive-light-return-in-the-modern-round-brilliant.aspx

That covers the deep side.

On the shallow pavilion side longer lgf% can help minimize the effects of obstruction caused by a shallow pavilion.
The arrows are smaller so when they are showing obstruction they have less of an effect on the overall brightness of the stone.

 

[stone-cold11]

You have to understand that lower half is represented as a percentage of the main pavilion facet size. So a longer lower half will have angles closer to that of the pavilion angles and vice versa. In a steep/deep, the main pavilion facets are still reflecting, you can see the arrow shafts/contrast pattern in the IS, so if the lower half is closer to that angle it will still reflect, but if it is smaller, has a steeper angle it will leak light. But that is all assuming perfect symm, no digging/painting. So an IS image is still preferred.

 

[Laila619]

Date: 11/3/2009 10:45:20 AM
Author: Karl_K
I wrote an article on this:

http://journal.pricescope.com/Articles/68/1/Do-the-pavilion-mains-drive-light-return-in-the-modern-round-brilliant.aspx

That covers the deep side.

On the shallow pavilion side longer lgf% can help minimize the effects of obstruction caused by a shallow pavilion.
The arrows are smaller so when they are showing obstruction they have less of an effect on the overall brightness of the stone.

Ahh, thank you Karl!

So I must have misread, longer LGF can sometimes ''save'' a stone with obstruction, not a steep deep. Interesting.

 

[Karl_K]

Date: 11/3/2009 10:50:32 AM
Author: Laila619

Date: 11/3/2009 10:45:20 AM

Author: Karl_K

I wrote an article on this:


http://journal.pricescope.com/Articles/68/1/Do-the-pavilion-mains-drive-light-return-in-the-modern-round-brilliant.aspx


That covers the deep side.


On the shallow pavilion side longer lgf% can help minimize the effects of obstruction caused by a shallow pavilion.

The arrows are smaller so when they are showing obstruction they have less of an effect on the overall brightness of the stone.

Ahh, thank you Karl!


So I must have misread, longer LGF can sometimes ''save'' a stone with obstruction, not a steep deep. Interesting.

You read it right it can also save a steep deep stone.

 

[Laila619]

Date: 11/3/2009 10:50:02 AM
Author: Stone-cold11
You have to understand that lower half is represented as a percentage of the main pavilion facet size. So a longer lower half will have angles closer to that of the pavilion angles and vice versa. In a steep/deep, the main pavilion facets are still reflecting, you can see the arrow shafts/contrast pattern in the IS, so if the lower half is closer to that angle it will still reflect, but if it is smaller, has a steeper angle it will leak light. But that is all assuming perfect symm, no digging/painting. So an IS image is still preferred.

Hmm ok I think I get it Stone-cold! Have to re-read again...

 

[Karl_K]

Here is the relevant section:

"How can the cutter remove the under-table leakage?



3 ways:

A: Remove the lower girdle facets from under the table by shortening them to 40%, image A.

B: Bring the angles of the lower girdles down by lengthening them out to 83% and an angle of 42.24 degrees. Image B.

C: Paint the lower girdles to an angle of 42.24 degrees. Image C"

 

[Laila619]

Now my question is...what is considered ''longer'' LGF? Anything 80 or over?

 

[stone-cold11]

Karl, how does a Morse/Wade cut look like in normal lighting condition?

Can you generate a normal lighting image/ gem file?

thanks.

 

[Karl_K]

Date: 11/3/2009 10:57:02 AM
Author: Laila619
Now my question is...what is considered ''longer'' LGF? Anything 80 or over?

77-80 is middle long,
80+ is long
75-76 is middle short
74 and under is short

 

[stone-cold11]

Actually, sliding scale again, depending on the pavilion angle.

 

[Lorelei]

Date: 11/3/2009 11:02:36 AM
Author: Karl_K

Date: 11/3/2009 10:57:02 AM
Author: Laila619
Now my question is...what is considered ''longer'' LGF? Anything 80 or over?

77-80 is middle long,
80+ is long
75-76 is middle short
74 and under is short

Thats how I categorize them too.

 

[Karl_K]

Date: 11/3/2009 11:00:14 AM
Author: Stone-cold11
Karl, how does a Morse/Wade cut look like in normal lighting condition?


Can you generate a normal lighting image/ gem file?


thanks.

The GOG OEC is very similar in performance to Morse except for the open culet and smaller table on Morse.
They were designed with the same thing in mind, chunky light return and not to dark from obstruction.

 

[Karl_K]

Date: 11/3/2009 11:03:05 AM
Author: Stone-cold11
Actually, sliding scale again, depending on the pavilion angle.

That is true when using the words "to short" which is a sliding scale.
But I would consider them to still fall into the classes I put them in.

 

[stone-cold11]

Ok, nice to know.

 

[Laila619]

Thanks for all the replies, I love learning all this fascinating stuff on PS!

 

[Karl_K]

Date: 11/3/2009 11:24:23 AM
Author: Laila619
Thanks for all the replies, I love learning all this fascinating stuff on PS!

Your welcome!!!
It is a lot of fun talking about this kind of stuff when someone is interested in it.