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Obstruction
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I have to disagree from an educational standpoint David. Obstruction and contrast are taught in gemological coursework and in diamond grader training. Moreover, I suggest that many consumers may have an interest in the associated effects.
With respect, I also feel a need to point out that leading gemologists and researchers are far past the "acceptance" stage of the importance of head obstruction & the viewer's effect on a diamond. Here are some references with snipped excerpts. They have been extremely helpful to me and I hope you can enjoy them too.
Bruce Harding
http://en.wikipedia.org/wiki/Bruce_Harding
>
Faceting Limits, 1975
http://www.gemology.ru/cut/english/faceting/f1.htm#viewer
>
GIA: Cut Uncovered, 2005
http://www.gia.edu/diamondcut/pdf/cut_uncovered_winter_05_loupe.pdf
>
AGS: Definition of Contrast, 2005
http://www.americangemsociety.org/theperformancebasedc.htm
>
AGSL: Landmark Paper (published by SPIE), 2007
http://agslab.com/spie/spie_lo_res.pdf
>
AGSL is not alone in their use of obstruction-assessment as part of a grading system. GIA also teaches visual cut estimation in diamond grading labs and contrast pattern judgments constitute 1/3 of the total estimation.
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True enough, but the presence of your hand absolutely influences the light paths - unless you are the invisible woman (again) - just as your presence as a viewer has influence on the diamond.
One of the first articles I got excited about when researching obstruction was Michael Cowing's "Describing Diamond Beauty." In my opinion it is a seminal work on the subject, brilliantly presented. Below is an excerpt and graphic. Like the above links, however, I feel one should absorb the entire work for full context and appreciation.
Readers who have stuck with this thread this long may want to bookmark this article.
Michael Cowing: Describing Diamond Beauty
http://www.acagemlab.com/articles/Describing.htm
b] that introduces contrast to otherwise diffuse illumination. the dark appearance of the viewer in the prong is the factor providing the primary contrast in illumination. it is the synergy of the diamond's cut taking advantage of this contrast in illumination that produces both the contrast and fire present in the diamonds in figures 7,9 and 10. without contrast in its illumination the best diamond cut will lack both contrast brilliance and fire, as figure 5 demonstrates. in typical viewing circumstances that always include the viewer's presence, the superior beauty of the best-cut diamonds becomes apparent. the factors that characterize the contrast quality of brilliance are the sharpness, number, sizes, and uniformity of the distribution of the diamond's mosaic-appearing pattern of reflections.>>
ACA Gem Lab Image
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Hi All,
Again, it''s a pleasure to exchange ideas John.
I have tremendous respect for you, and others who research the physical aspects of diamonds.
I have looked at all the links- I did not read the 25 page one- but I did read all the others.
I appreciate the links John, but to me, they shed no additional light on the actual subject- to me that being- in a practical sense, what should we look for to find effects of "head obstruction" when judging a diamond for cut?
Do diamond shoppers, or diamond lovers need to be concerned about the effects of "Head Obstruction"?
Bruce Harding''s assertion:
" Rays which are reflected to the viewer`s eye must come from directions which missed his head. Figure 2 shows that at a viewing distance of one foot, as when examining a stone prior to purchase, the angle (or divergence) between incident and reflected directions of the same ray must be at least 10°; otherwise the viewer will see reflections of himself."
The above statement assumes the position of lighting.
In the AGS Landmark Paper example again- there''s the assumption that the light arrives from above or behind.
The easiest example to refute these assumptions is where you hold the diamond under a grading lamp. It''s a physical impossibility for the head to obstruct the main source of lighting. There''s many other lighting situations where the head does not obstruct the primary lighting source.
A diamond will show contrast even if the light is not above or behind the viewer- or in positions that simply can not allow the head to obscure the light. If it''s a very deep diamond, that contrast may be an unpleasant darkness in the center of the diamond.
This darkness will be visible even if the head has no possibility of obstructing the light hitting the diamond.
Therefore a deep diamond does not look dark due to head obstruction. It looks dark due to bad geometry of angles.
We agree that the diamond WILL reflect back things from the environment: your head, people, walls, ceiling , a bird flying by the window.
That''s part of the reason we love them. The reflections of light picked up from the environment create the contrast we love. Well cut diamonds have lovely contrast even in dim lighting. They have attractive contrast even if the head does happen to be obstructing the light
Using the angles to predict how the diamond will perform in those rare situations where the head does actually obstruct the main source of lighting is surely an interesting subject to some.
The practical application is far less clear.
If it''s possible to calculate "obstruction issues" by PA/CA/ Table Depth, and those issues are real, photographs would be able to show them, in my experience.
My experience is that if a diamond has a dark center, this shows up in photos. The GIA "Cut Uncovered" article had photos depicting badly cut diamonds, including a dark centered stone.
FB- my main concern in the example you posted would be the spread.
What''s the stone expected spread, at 64% depth?
Again, it''s a pleasure to exchange ideas John.
I have tremendous respect for you, and others who research the physical aspects of diamonds.
I have looked at all the links- I did not read the 25 page one- but I did read all the others.
I appreciate the links John, but to me, they shed no additional light on the actual subject- to me that being- in a practical sense, what should we look for to find effects of "head obstruction" when judging a diamond for cut?
Do diamond shoppers, or diamond lovers need to be concerned about the effects of "Head Obstruction"?
Bruce Harding''s assertion:
" Rays which are reflected to the viewer`s eye must come from directions which missed his head. Figure 2 shows that at a viewing distance of one foot, as when examining a stone prior to purchase, the angle (or divergence) between incident and reflected directions of the same ray must be at least 10°; otherwise the viewer will see reflections of himself."
The above statement assumes the position of lighting.
In the AGS Landmark Paper example again- there''s the assumption that the light arrives from above or behind.
The easiest example to refute these assumptions is where you hold the diamond under a grading lamp. It''s a physical impossibility for the head to obstruct the main source of lighting. There''s many other lighting situations where the head does not obstruct the primary lighting source.
A diamond will show contrast even if the light is not above or behind the viewer- or in positions that simply can not allow the head to obscure the light. If it''s a very deep diamond, that contrast may be an unpleasant darkness in the center of the diamond.
This darkness will be visible even if the head has no possibility of obstructing the light hitting the diamond.
Therefore a deep diamond does not look dark due to head obstruction. It looks dark due to bad geometry of angles.
We agree that the diamond WILL reflect back things from the environment: your head, people, walls, ceiling , a bird flying by the window.
That''s part of the reason we love them. The reflections of light picked up from the environment create the contrast we love. Well cut diamonds have lovely contrast even in dim lighting. They have attractive contrast even if the head does happen to be obstructing the light
Using the angles to predict how the diamond will perform in those rare situations where the head does actually obstruct the main source of lighting is surely an interesting subject to some.
The practical application is far less clear.
If it''s possible to calculate "obstruction issues" by PA/CA/ Table Depth, and those issues are real, photographs would be able to show them, in my experience.
My experience is that if a diamond has a dark center, this shows up in photos. The GIA "Cut Uncovered" article had photos depicting badly cut diamonds, including a dark centered stone.
FB- my main concern in the example you posted would be the spread.
What''s the stone expected spread, at 64% depth?
FB.
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outatouch0
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A picture is really worth a thousand words.
RD, assuming your confusion is genuine (I''ve been up for 36 hours and even I get the concept) the yellow line there that is most relevant to obstruction is the line from the eye to the diamond. This might actually be better represented as 2 separate lines.
The eye is either looking at a mirror or at a flashlight.
If it is looking at a mirror (which is what a diamond is) it will reflect the person looking at the mirror (obstruction i.e. dark portions). This goes FROM eye TO diamond BACK TO eye.
The non dark part of the line (flashlight) travels FROM light source (above head or under a grading lamp is irrelevant), bounces in the prism of the diamond, and TO the eye. Call this what you will: fire, scintillation, etc. This goes FROM light source, TO diamond, then TO the eye.
The combination of these two = contrast.
This may not be the most technical explanation but it really is as simple as that.
As to why people here warn people of potential problems with dimensionally challenged diamonds. I already answered that for you in your steep deep thread.
arjunajane
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There are none so blind as those who will not see.
Later edit after appropriate resaearch
The most deluded people are those who choose to ignore what they already know. The proverb has been traced back in English to 1546 (John Heywood)... In 1738, it was used by Jonathan Swift in his 'Polite Conversation,' and is first attested in the United States in the 1713 'Works of Thomas Chalkley' http://www.funtrivia.com/askft/Question41522.html
Later edit after appropriate resaearch
The most deluded people are those who choose to ignore what they already know. The proverb has been traced back in English to 1546 (John Heywood)... In 1738, it was used by Jonathan Swift in his 'Polite Conversation,' and is first attested in the United States in the 1713 'Works of Thomas Chalkley' http://www.funtrivia.com/askft/Question41522.html
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Clearly, if the lighting sources are as illustrated in Garry''s diagram, the head would obstruct some of the light.
Would the head obstruct any light in the light/diamond/viewer position as in my diagram?
The viewer is in a darkened room, the main source of lighting is the gem lamp. The way a diamond would be looked at in a grading lab for portions of the examination.
Although parts of the sparkle we love is when a diamond''s facet is acting as a mirror, contrasting other facets that are reflecting the light the other way, the object does not need to be either a flashlight or a mirror- the illumination is provided by the light.
In my example the eye obscures the light coming off the diamond- as well as every other bit of light reflecting off anything.
This part is simple- it''s called seeing something- the light bounces off the thing, and back to your eye.
True, you''d need to be "the invisible woman ( or man) not to obstruct any light on it''s way to the diamond in Garry''s example. But the mere act of seeing something obstructs the light from going past you head, or eye.
Discussion on physics, or subjects such as "If a tree falls in the woods and no one can hear it, is there a sound?" are interesting enough, but what is the practical application?
What is not all that simple is determining what pleasant contrast is versus detrimental contrast.
That''s why I feel photos are an important tool in this conversation.
There have been cases, in the other thread about steep deep- where some people felt the photos showed a problem that others may not have.
That, in itself shows value to the conversation.
We all agree that deep diamonds can have dark centers- and that many shoppers would want to avoid that.
The important point I keep attempting to illustrate is that what some view as a detrimental dark area, others view as pleasant contrast.
This is similar to the way a shopper asking for a J color may be told "Oh, J colors are junk, you don;t want one of those."
Followed by "Let me show you a 35% more costly G color"
Many people LOVE J colors precisely for the reasons others do not like them
Would the head obstruct any light in the light/diamond/viewer position as in my diagram?
The viewer is in a darkened room, the main source of lighting is the gem lamp. The way a diamond would be looked at in a grading lab for portions of the examination.
Although parts of the sparkle we love is when a diamond''s facet is acting as a mirror, contrasting other facets that are reflecting the light the other way, the object does not need to be either a flashlight or a mirror- the illumination is provided by the light.
In my example the eye obscures the light coming off the diamond- as well as every other bit of light reflecting off anything.
This part is simple- it''s called seeing something- the light bounces off the thing, and back to your eye.
True, you''d need to be "the invisible woman ( or man) not to obstruct any light on it''s way to the diamond in Garry''s example. But the mere act of seeing something obstructs the light from going past you head, or eye.
Discussion on physics, or subjects such as "If a tree falls in the woods and no one can hear it, is there a sound?" are interesting enough, but what is the practical application?
What is not all that simple is determining what pleasant contrast is versus detrimental contrast.
That''s why I feel photos are an important tool in this conversation.
There have been cases, in the other thread about steep deep- where some people felt the photos showed a problem that others may not have.
That, in itself shows value to the conversation.
We all agree that deep diamonds can have dark centers- and that many shoppers would want to avoid that.
The important point I keep attempting to illustrate is that what some view as a detrimental dark area, others view as pleasant contrast.
This is similar to the way a shopper asking for a J color may be told "Oh, J colors are junk, you don;t want one of those."
Followed by "Let me show you a 35% more costly G color"
Many people LOVE J colors precisely for the reasons others do not like them
outatouch0
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uhhh, yea RD WTH are you talking about?
Me is starting to think this is a game you play. Opposite of bringing knowledge in to the maul to mess w a SA to play w their ignorance. This is just bringing feigned idiocy to mess w knowledgeable tradespeople and consumers. I would not rate myself as the most knowledgeable of consumers but I can see BS when the signs are so evident.
Brown.Eyed.Girl
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I''ve been following this thread, and I think there have been some great explanations. Really, as someone who hasn''t taken a science class in years, this still makes sense thanks to the time that our experts and prosumers have taken to explain obstruction in this thread.
At this point though, I think TGal''s av sums it up:
(TGal, I couldn''t find your old one! So Merry Christmas all
)
At this point though, I think TGal''s av sums it up:
(TGal, I couldn''t find your old one! So Merry Christmas all
risingsun
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I really appreciate the experts who have taken the time to provide education about head obstruction and head obscuration. It has been very helpful. Perhaps one of you [or more in collaborative effort] would be willing to write a journal article for PS on this topic. I think it would be a valuable addition to our knowledge base.
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I could not agree more with all those who feel frustrated that no simple explanation of how all this has any practical effects has been presented by any of the people who feel obstruction ( or obscuration) is a valid issue to shoppers, and diamond lovers.
IN fact, those "in the know" just keep moving the bar.
Is it obstruction, as Garry has said, or Obscuration, as Sergey has said? Does the head reflect, or the eye?
Sergey, I'm NOT speaking about Head Obstruction( or obscuration) I'm asking about how it affect people looking at diamonds in practical terms.
Maybe we could solve this by having people with no heads looking at diamonds.
IN fact, those "in the know" just keep moving the bar.
Is it obstruction, as Garry has said, or Obscuration, as Sergey has said? Does the head reflect, or the eye?
Sergey, I'm NOT speaking about Head Obstruction( or obscuration) I'm asking about how it affect people looking at diamonds in practical terms.
Maybe we could solve this by having people with no heads looking at diamonds.
RD,
Ok. It is good challenge , I will try again
Please take candela( in glass jar for safety reasons ) , diamond , your head and go to the dark room . When candela is behind your head you see shadow of your head instead diamond.
is it Head Obstruction . is it clear enough?
then check all other places for candela between your and diamond, except when candela is between your head and diamond( in cone where diamond is vertex of the cone and your head is the cone basis ) . during this tests you will see flashes in diamond. it could be only reflections from one single candela( I hope your are still in dark room)
if you see at least one flashes for EACH virtual facet( do you know what is Virtual facet) , it mean your diamond has not head Obscuration
but if for same facets you can not find position of candela when these facets have bright flashes, your diamonds has head Obscuration.
it is DEAD Virtual facets because you can see in this facets only reflection of your head. same diamonds have few such Dead Virtual facets, but some diamonds have a lot such virtual facets.
there are semi-dead virtual facets, when you can see candela reflection only if you put candela behind your diamond.
Good luck
Dreamer_D
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Sergey, I appreciate your efforts- honestly- I do.
The basic properties of how a solid object in front of a light will obstruct the light are so simple that it seems ridiculous even to have to describe them.
The candle test sounds interesting enough- however it''s not possible for me to make such a test practically. Is there way to demonstrate this problem in normal viewing environment?
Now Karl, we need to to wear a red ski mask and vest when looking at a diamond to see the problem?
If we need to wear a red ski mask and jacket to see this problem, is it relevant for the non skiiers out there?
Karl, you are clearly referring to obstruction- not the same thing Sergey''s talking about right?
That''s "obscuration"- is this two totally different problems?
The head banging icon would be well placed here.
Does the head or eye reflect?
Is it obstruction or obscuration?
What is the problem we''re supposed to see in actual diamonds the way normal people look at them???
The basic properties of how a solid object in front of a light will obstruct the light are so simple that it seems ridiculous even to have to describe them.
The candle test sounds interesting enough- however it''s not possible for me to make such a test practically. Is there way to demonstrate this problem in normal viewing environment?
Now Karl, we need to to wear a red ski mask and vest when looking at a diamond to see the problem?
If we need to wear a red ski mask and jacket to see this problem, is it relevant for the non skiiers out there?
Karl, you are clearly referring to obstruction- not the same thing Sergey''s talking about right?
That''s "obscuration"- is this two totally different problems?
The head banging icon would be well placed here.
Does the head or eye reflect?
Is it obstruction or obscuration?
What is the problem we''re supposed to see in actual diamonds the way normal people look at them???
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I''d be extremely happy to answer any question raised by my post.
I feel it''s very clear- but if you''re not sure what I''m asking about, please let me know the specific points you feel are confusing, and I will clarify them.
What I''ve been doing here is asking for specific answers, yet not getting any by and large.
If there were a more balanced representation of tradespeople involved in this conversation, many would ask exactly the same questions I have asked.
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I believe the simplest explanation is that some diamonds darken more quickly than others as the viewer's head or (for PS) the camera lens gets close. It is most-seen in diamonds with shallow pavilion configurations.
You actually showed simple example in the steep/deep thread, where it appears your camera's proximity to the 63%T diamond caused tremendous darkness as a result of obstruction.
I don't think anyone is moving the bar; it's just not simple math. Starting a thread called "Color" implies a lot more than just saying "D is white, Z is yeller." There are significant explanations and examples required for even a foundation of understanding. Higher levels of knowledge are built on that foundation. Threads do wander, and if you were to study head-shadow, obstruction and obscuration in proper taxonomical context I suggest it would make more sense. Instead we're jumping from basic math to trig and back.
For fun, imagine someone asking how to finger G on clarinet. Simple, right? I show them G above the staff. Oh but wait, if playing in the key of concert Ab there's an alternate fingering which more in tune, unless you're performing the 7th scale step in Europe (M7 is tuned differently there in which case you must lip-tune). Oh, and there's a third fingering which depends on which part of the chord you have, as well as sustain/vibrato.
With evolving answers the supplicant may think the bar is being moved. In fact it is not; musicians who have had decades of schooling and experience know that environment and situation dictates what should be fingered... The goal is the same though: We want the note to be in-tune for the end-listener - whether or not he/she understands how much study went into the decision.
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Interesting reading and yet I have not seen one of the questions that has been asked by RockDiamond addressed and if his supposition is correct then I worry that many have been mislead if they believe that obscuration or obstruction is a bad thing.
Too much, or too little or the wrong patterns can be bad as shown by these examples as shown in the original presentation by the AGS about the ASET tool.
As successful hunter gatherers, our ancestors needed to be able to detect edges and contrasts and thus we are hard wired genetically to be able to see these things and to appreciate them. hence, the bright areas next to a dark area will seem brighter than an area that is only bright. In fact it is the on/off sparkle effect that makes diamonds and other gems exciting as even the beating of our heart is enough to make the diamonds move enough to show this effect. It is also why static pictures of diamonds are rarely appealing, they simply can not sparkle the way a live viewing or video of the same gem will.
Obscuration, or obstruction, or what ever you want to call it is an essential part of the recipe for beauty in a diamond. Without it we might as well put small mirrors in our rings and call it boring!
Wink
P.S. Images used with permission, although I did photoshop two images together without specific permission, but I bet it is okay...
Too much, or too little or the wrong patterns can be bad as shown by these examples as shown in the original presentation by the AGS about the ASET tool.
As successful hunter gatherers, our ancestors needed to be able to detect edges and contrasts and thus we are hard wired genetically to be able to see these things and to appreciate them. hence, the bright areas next to a dark area will seem brighter than an area that is only bright. In fact it is the on/off sparkle effect that makes diamonds and other gems exciting as even the beating of our heart is enough to make the diamonds move enough to show this effect. It is also why static pictures of diamonds are rarely appealing, they simply can not sparkle the way a live viewing or video of the same gem will.
Obscuration, or obstruction, or what ever you want to call it is an essential part of the recipe for beauty in a diamond. Without it we might as well put small mirrors in our rings and call it boring!
Wink
P.S. Images used with permission, although I did photoshop two images together without specific permission, but I bet it is okay...
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Here''s a different perspective. There is no obscuration or obstruction. Everything is reflection, because that is what a diamond is...a fancy mirror. A diamond is surrounded by a spherical lighting environment in which some items are bright and some are dark. Any light from behind the girdle plane will never make it out the pavilion to your eyes and can be ignored. Any light from in front of the girdle plane will have more or less of an effect on what you see depending on where it''s located and how the stone is cut. What you are calling obscuration or obstruction is just the reflection of your head and upper body and so is darker than most of the surrounding light environment. Want to try a different experiment ? Hold a diamond in front of you in a darkened room and shine a light on your face and upper body. What you see now is obviously not obscuration because your head and upper body are the brightest light source and the diamond will now show the brightest reflection from those areas which were previously dark and "obscured". Showing images which have small light sources and how you''re obscuring the light are not a very good way to look at this since the light which you''re seeing in a diamonds reflection is not coming from a single light source, but from a half spherical lighting environment which all plays a part in how the diamond looks.
What''s the problem ? Your reflection is, as is the cut of the stone. The reflection of the person holding the diamond, how much of the lighting environment they occupy, (how close they are), and how they relate to the CUT OF THE STONE are the problems. Deep pavilions and tall crowns, (if they aren''t so out of whack that they cause most of the light to leave the diamond out of the pavilion), tend to cause light to leave the stone at angles which don''t extend to your eyes, causing the stones to look darker. I''ve attached an interesting picture of light entering a stone from a laser at about 45° and exiting to hit a frosted hemisphere. There''s a lot of stuff going on here some of which, such as diffractive effects are more apparent when you''re doing this in real life, since the camera and the setup cause many of the details that can be seen in person to become too fuzzy in the picture. In any case the idea here is to imagine replacing the bright spots from the laser with dark spots which represent your reflection which is dark and then understanding that there are some combination''s of crown and pavilion angles which cause light, (or dark), areas to be concentrated in areas where your eyes will be looking MOST of the time, (perpendicular to the table in most analytical cases) . I think that the words obscuration and obstruction confuse what''s being seen and are probably poor choices of descriptive words.
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Thank you Wink.
John, the science of optics, combined with diamond cutting is a tremendous boon to our industry. Technology has allowed many advances in diamond cutting.
In no way would I ever want to question the need for technical advances.
The complexity of this subject, while crucial to diamond cutters and those studying these effects creates a unique situation.
PS is all about education.
Well, sometimes a "little knowledge can be very dangerous"
People have heard these terms, and don;t understand the real issues. This leads to people being warned of "obstruction issues" at times when there are no "issues" whatsoever regarding that diamond''s cut
If there''s ANYONE not confused by how to apply knowledge of Obscuration, obstruction, and all the other different terms being thrown, I''d be very surprised.
Michael E''s explanation , while interesting- is exactly like much of the other stuff written in answer to my very simple questions.
Do I need to have PHD to understand diamonds?
Do I have to like to Ski in a red mask?
re:Obscuration, or obstruction, or what ever you want to call it is an essential part of the recipe for beauty in a diamond. Without it we might as well put small mirrors in our rings and call it boring!
Contrast( and specially dynamical contrast) is very important for cut performance, but obscuration is worst method (for cut performance) to create contrast.
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There are maybe 4 people on ps who understand what you just said but I am finding that to be the case with chunky VF diamonds.
The AGS method of analyzing contrast leaves them to dark and doesn't show the true contrast.
It does however work with diamonds with a ton of small VF's and AGS0 RB's with mixed VF's in diffused lighting.
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But you aren''t "creating contrast", you are just using whatever contrast exists in the environment by choosing which parts of the environment in front of the diamond will be reflected to an observers eyes. Since the most dramatic and constant part of the environment, (since you always know where they are looking from), is the observer of the diamond, they are the only thing that you can count on to provide really strong contrast in a consistent manner. What else would you use ? Trees, cars, buildings and so forth change constantly. The horizon changes in elevation as well, so how do you get consistent contrast without using the reflection of the observer ? This question is not an objection, but a question of how you see this.
Just for some clarity on my position. I have no horse in this race other than that I find it interesting and that it helps in the cutting of colored stones by understanding what''s happening with diamonds since the same things are occurring in all faceted gems.
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