LGF Depth/Length, Star Facet, and Lower half relationships

[KobiD]

I've been doing some more light reading on the finer details, flavours, and characteristics of what can be found in a diamond.

I've identified the differences between AGS reported LGF Depth vs GIA LGF Length, their typical ranges, and how their variances impact on the pavillion size, seen as the arrow in certain lighting environments.

I've read that ranges for the star generally vary between 45-55% and also work in conjunction with the upper girdle. Variances not only change the window through the star, but also the angles of the facets (which are also dependant on the crown angle).

I'm not sure how the lower half % or even total depth of the diamond influences the relationship. IE, will a 75% LGF present as thicker/thinner arrows on a 43% lower half, vs a 43.5% lower half?

I've read certain combinations can also result in obscuration issues. Do we have any examples of this?

I've also come across the topic of some ideal cut diamonds appearing dark in full light environments. Is this more likely to occur with shorter LGFs rather than longer?

 

[KobiD]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Anyone?

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

lgf and lhf are the same thing.
lower girdle facets, lower half facets are the same thing.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

I've read that ranges for the star generally vary between 45-55% and also work in conjunction with the upper girdle. Variances not only change the window through the star, but also the angles of the facets (which are also dependent on the crown angle).

45-55 is common but there is nothing wrong with 60 or even 65 with the right crown angle and table size and 35 was not unusual in old cuts.

The upper girdle angles are not only dependent on the the crown angle and star length but also to what degree painting or digging is present.

 

[darkfury18]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Some light reading on the topic if you haven't ran across this site before http://www.proposalring.com/tutCutPropMinor.htm

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

basically it goes like this:
With a steep crown you can go with lower angled uppers which gives you short stars so that the upper girdle angle isn't overly steep and you lose light return.
With a shallow crown you can go with steeper upper girdles and longer stars to increase the life of the stone.
Table size also makes a difference and is another factor.

That is assuming you balance the crown angle/table size with the right pavilion angles and lower girdle/half angles.

 

[Serg]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

I've been doing some more light reading on the finer details, flavours, and characteristics of what can be found in a diamond.

I've identified the differences between AGS reported LGF Depth vs GIA LGF Length, their typical ranges, and how their variances impact on the pavillion size, seen as the arrow in certain lighting environments.

I've read that ranges for the star generally vary between 45-55% and also work in conjunction with the upper girdle. Variances not only change the window through the star, but also the angles of the facets (which are also dependant on the crown angle).

I'm not sure how the lower half % or even total depth of the diamond influences the relationship. IE, will a 75% LGF present as thicker/thinner arrows on a 43% lower half, vs a 43.5% lower half?

I've read certain combinations can also result in obscuration issues. Do we have any examples of this?

I've also come across the topic of some ideal cut diamonds appearing dark in full light environments. Is this more likely to occur with shorter LGFs rather than longer?

see here http://www.octonus.com/oct/products/helium/polish/parameter05.phtml explanation of difference between LGF Depth and LGF Length

 

[KobiD]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Thanks for the info everyone.

Was aware of differences between lgf depth vs length. In regards to lower half I may have used the incorrect terminology. I was refering to the actual height of the lower part of the diamond. IE steeper pavs = deeper stones. I mean, a shorter/shallower stone showing a 75% lgf look thicker than a 75% lgf on a deeper stone, given the crown angles and table are the same. Is that correct?

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Thanks for the info everyone.

Was aware of differences between lgf depth vs length. In regards to lower half I may have used the incorrect terminology. I was refering to the actual height of the lower part of the diamond. IE steeper pavs = deeper stones. I mean, a shorter/shallower stone showing a 75% lgf look thicker than a 75% lgf on a deeper stone, given the crown angles and table are the same. Is that correct?

Given the same table and crown configuration and a 75% lgf% a 41 degree pavilion will result in slightly smaller looking arrows face up compared to a 40.6 pavilion.
Is that what you were asking?

 

[Rhino]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Greetings Kobi,

I've been doing some more light reading on the finer details, flavours, and characteristics of what can be found in a diamond.

I've identified the differences between AGS reported LGF Depth vs GIA LGF Length, their typical ranges, and how their variances impact on the pavillion size, seen as the arrow in certain lighting environments.

Correct. The length of the lower halves will impact the width of the pavilion mains ... ie. the arrows.

I've read that ranges for the star generally vary between 45-55% and also work in conjunction with the upper girdle. Variances not only change the window through the star, but also the angles of the facets (which are also dependant on the crown angle).

I'm not sure how the lower half % or even total depth of the diamond influences the relationship. IE, will a 75% LGF present as thicker/thinner arrows on a 43% lower half, vs a 43.5% lower half?

Where you may be confusing readers here is I believe you are confusing pavilion depth (43%, 43.5%) with lower half facet length.
43-43.5 lower half facet length is EXTREMELY SHORT. Even most OEC's don't have lower half facet lengths this short. Assuming your question is ... "I'm not sure how the lower half % or even total depth of the diamond influences the relationship. IE, will a 75% LGF present as thicker/thinner arrows on a 43% lower half, vs a 43.5% pavilion depth?

The answer to your question is no. A 75% lower half facet length will not appear thicker/thinner whether the pavilion depth is 43% or 43.5%. Total Depth or Pavilion Depth will not impact the thickness of the arrow (pavilion main). That is strictly a function of lower half facet length.

I've read certain combinations can also result in obscuration issues. Do we have any examples of this?

The article you are linking to was originally authored by myself and published with permission on the Singapore site. And the answer is yes. When you lengthen lower half facet length there are small points of obscuration reflecting off the pavilion facets under the table. The small points of obscuration however are nothing so notable that it would impact the optical grade via AGS or GIA Laboratories and in spot lighting environments can add additional pin point scintillation. The star facets in particular amplify those reflections of obscuration and is an observation I've made when I was first utilizing reflector based technologies in our analysis of cut.

I've also come across the topic of some ideal cut diamonds appearing dark in full light environments. Is this more likely to occur with shorter LGFs rather than longer?

Not necessarily. All types of ideal cut diamonds tend to take on a dark body appearance in spot lighting environments. Their fire and sparkle events will be more frequent and intense however.

Hope that helps.

Rhino

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

43.0% and 43.5% pavilion depth with the same lgf% crown and table.
At 43.5 the arrows are shorter and slightly thinner at the widest point.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

To make the difference more apparent here is 42% vs 44%

 

[Texas Leaguer]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

To make the difference more apparent here is 42% vs 44%

Interesting what a difference that 1% and even .5% pavilion depth makes in terms of leakage.

 

[darkfury18]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Well the depth is directly related to the pavilion angle, so the higher the angle the deeper the pavilion. So what are the pavilion angles from those examples?

 

[KobiD]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Spot on Rhino. I confused the terms, and meant pavilion depth rather than lower half.

Thanks Karl for taking the time to crunch some numbers and give visual representations of the differences, and confirming my beliefs/understanding. The differences are subtle, but still occur. What is more noticable though is the leakage that is introduced as the depth/steepness of the pavilions increase. Would you mind sharing the figures that you used for the simulation? and if I posted a couple of configs would you be willing to run them and post the outputs?

Many thanks again for all of the input.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Well the depth is directly related to the pavilion angle, so the higher the angle the deeper the pavilion. So what are the pavilion angles from those examples?

43.0% 40.7 degrees
43.5% 41.02 degrees
42% 40.03 degrees
44% 41.35 degrees

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Would you mind sharing the figures that you used for the simulation? and if I posted a couple of configs would you be willing to run them and post the outputs?

Many thanks again for all of the input.

57T
34.5c
46.1 star
lfl% 75
lfd% 76.9
Pavilion as posted above.
Sure ask away.
There are several of us that can do these and I will help as time allows but be patient if someone don't post them right away.
Make sure and state what you are looking for as there are several different images we can produce and one may show it better than another.

welcome.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

re leakage
It is not the pavilion mains that are leaking it is the lower girdle angles that are too steep and drawing light from the opposite side of the pavilion instead of the crown not leakage like a window.
This explains a lot of it:
https://www.pricescope.com/journal/do_pavilion_mains_drive_light_return_modern_round_brilliant

If I was redoing it today I would clarify the difference between leakage like a window and drawing light from the opposite pavilion side which is what is commonly called "leakage".

 

[KobiD]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Thanks once again Karl, both for the clarification and the further resources.

I went down this route a little in a previous thread, as its something I've read here in several threads about peoples personal cut offs/thresholds for steep deep stones. Obviously they are just opinions, but they are the kind of range where people will say "it'll be nice, but you could find better".

T: 55.5%
CA: 35
CH: 15.6%
PA: 40.9
PD: 43.3%
TD: 62.4%
Girdle: Thin - Med

Is it possible to see say 4 different simulated IS images to highlight the variances between different star facets, and then different lgfs.

1. Star 49%, LGF Depth 75% (length approx 73%)
2. Star 49%, LGF Depth 82%
3. Star 55%, LGF Depth 75%
4. Star 55%, LGF Depth 82%

That should give a decent spread of the average range I've seen them cut in. I'm interested in seeing if/how the stars and girdles effect a stone with such proportions.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

no problem
I will post some images tomorrow if someone else doesn't I have a sinus headache tonight.
One thing to be aware of is how badly gia messes with the numbers on the report.
Have you read up on it yet?
Maybe someone can help with a link if not a google search with "pricscope gia rounding" should bring up some threads.

 

[KobiD]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Roger that Karl.

Have done lots of reading into rounding and tolerances, both with AGS and GIA, but with GIA being considerably more crude in their approach. Then you also have to take into account variances between individual facets and optical symmetry, which also have an effect on the overall performance.

 

[Texas Leaguer]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Roger that Karl.

Have done lots of reading into rounding and tolerances, both with AGS and GIA, but with GIA being considerably more crude in their approach. Then you also have to take into account variances between individual facets and optical symmetry, which also have an effect on the overall performance.

Most definitely. For example the first IS simulation Karl posted with 43 vs 43.5% pavilion depth is within the GIA rounding (nearest .5)
And averages can be quite misleading when optical precision is not high. This is where reflector images really fill some gaps in understanding the cut quality of GIA stones.

The primary reason AGS reports are superior to GIA reports is the fact that the AGSL cut grading methodology takes into account the contribution of every facet to light performance by ray tracing a 3D model of the diamond. AGS reports with ASET light maps take it one step further in that they give you a graphical view of optical precision.

That is why many prosumers here advise posters that they should ask their vendors for reflector images on GIA stones, or just look for AGS 0 stones.

 

[Rhino]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Well the depth is directly related to the pavilion angle, so the higher the angle the deeper the pavilion. So what are the pavilion angles from those examples?

Correct. One thing I do note however and addressing Kobi's question is that the lower half length remains the same and the thickness/thinness of the arrows also remains the same although leakage becomes more prominent as you increase pavilion depth/angles.

 

[Rhino]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Spot on Rhino. I confused the terms, and meant pavilion depth rather than lower half.

Thanks Karl for taking the time to crunch some numbers and give visual representations of the differences, and confirming my beliefs/understanding. The differences are subtle, but still occur. What is more noticable though is the leakage that is introduced as the depth/steepness of the pavilions increase. Would you mind sharing the figures that you used for the simulation? and if I posted a couple of configs would you be willing to run them and post the outputs?

Many thanks again for all of the input.

No prob. Karl, Texas and I love this geek talk.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Well the depth is directly related to the pavilion angle, so the higher the angle the deeper the pavilion. So what are the pavilion angles from those examples?

Correct. One thing I do note however and addressing Kobi's question is that the lower half length remains the same and the thickness/thinness of the arrows also remains the same although leakage becomes more prominent as you increase pavilion depth/angles.

Hi Jon, Hope your day is going well.
Actual the arrow shafts do change width slightly and length more so with changes in pavilion depth see images above.
They also change how much they "pop" or stand out in real world viewing.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Thanks once again Karl, both for the clarification and the further resources.

I went down this route a little in a previous thread, as its something I've read here in several threads about peoples personal cut offs/thresholds for steep deep stones. Obviously they are just opinions, but they are the kind of range where people will say "it'll be nice, but you could find better".

T: 55.5%
CA: 35
CH: 15.6%
PA: 40.9
PD: 43.3%
TD: 62.4%
Girdle: Thin - Med

Is it possible to see say 4 different simulated IS images to highlight the variances between different star facets, and then different lgfs.

1. Star 49%, LGF Depth 75% (length approx 73%)
2. Star 49%, LGF Depth 82%
3. Star 55%, LGF Depth 75%
4. Star 55%, LGF Depth 82%

That should give a decent spread of the average range I've seen them cut in. I'm interested in seeing if/how the stars and girdles effect a stone with such proportions.

75 with 49 and 55 stars as you can see a 75% lgf depth is not a good match for this combo.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

As you can see a 82 lgf% depth is a better match but with 55 stars the upper girdle angles are too steep.
This is a combo where the upper and lower girdles make or break the combo.

 

[KobiD]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Thanks Karl. Some very clear examples of how the LGF and stars can impact on the performance of a stone.

Tying this back to my other thread ([URL='https://www.pricescope.com/community/threads/diamond-proportions-ags0-ideal-and-leakage.209671/']https://www.pricescope.com/community/threads/diamond-proportions-ags0-ideal-and-leakage.209671/[/URL]), all of the above combo's actually come in at AGS0. You can see very clearly why a cutter may opt not to pay the premium for a platinium report if their simulations show that they could end up with one of the less desirable ASET images.

 

[KobiD]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Karl, would you mind punching out the same images in plain red and black as your first set were for comparisons sake, please.

 

[Karl_K]

Re: LGF Depth/Length, Star Facet, and Lower half relationshi

Karl, would you mind punching out the same images in plain red and black as your first set were for comparisons sake, please.

IS doesn't show the difference as well as ASET which is why I picked it.