What A Scanner Sees
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Sergey:
. We understand each other exactly. Your illustration is excellent.
. Your illustration shows the same as my Fig.8 - that the profile seen is a bent line, giving two false ''slope'' indications if the line of sight is not exactly parallel to the surface of the facet.
. My discussion assumed that the device rotated the stone to what it thought should be the correct direction. I was suggesting that it would have to turn slightly until it ''sensed'' the straight profile if the facet was ''twisted'' (at the wrong azimuth).
. I did not realize that your device steps through hundreds of rotation increments; that is great and should give excellent readings, especially with the greater number of steps. I would like to know how it recognizes the ''best'' one for each facet, but I am sure that is private information.
. As you imply, if the correct direction is between steps of rotation it will see ''incorrect'' bent lines in both views and will never see the straight line profile which is the true ''slope''. In such case the imaginary straight line from tip-to-tip gives the closest reading (note 40.956° vs 40.960° in my example).
. Nevertheless, I think that any reading which is accurate to one decimal place is good enough for any report. To offer more is to invite pointless comparisons. In the 400- vs 200-step scans by Rhino, above, I would accept the 200-step data and omit the second decimal place.
. We understand each other exactly. Your illustration is excellent.
. Your illustration shows the same as my Fig.8 - that the profile seen is a bent line, giving two false ''slope'' indications if the line of sight is not exactly parallel to the surface of the facet.
. My discussion assumed that the device rotated the stone to what it thought should be the correct direction. I was suggesting that it would have to turn slightly until it ''sensed'' the straight profile if the facet was ''twisted'' (at the wrong azimuth).
. I did not realize that your device steps through hundreds of rotation increments; that is great and should give excellent readings, especially with the greater number of steps. I would like to know how it recognizes the ''best'' one for each facet, but I am sure that is private information.
. As you imply, if the correct direction is between steps of rotation it will see ''incorrect'' bent lines in both views and will never see the straight line profile which is the true ''slope''. In such case the imaginary straight line from tip-to-tip gives the closest reading (note 40.956° vs 40.960° in my example).
. Nevertheless, I think that any reading which is accurate to one decimal place is good enough for any report. To offer more is to invite pointless comparisons. In the 400- vs 200-step scans by Rhino, above, I would accept the 200-step data and omit the second decimal place.
To all:
. The previous thread 'Azimuth Shift & Yaw' was written because Rhino was in doubt about the difference, in another thread. Summary: 'azimuth shift' is just rotation (see Fig.2, which was illustrative only - nobody does this because it creates a double facet); 'yaw' is a combination of azimuth shift and slope increase so that there is only one facet but in the wrong place (Fig.3).
. This thread 'What a Scanner Sees' was written because Paul and Brian had asked me how scanners saw 'yaw' - do they give the correct slope? It should not have appeared in the other thread.
. Now Paul asks about reflection from a single facet, which has nothing to do with slope - reflection is an optical thing, slope is a geometric thing, as Paul suggested. I will discuss that here or in a separate thread.
. Let's not get into reflections between opposite facets - that's another subject, already documented. Read "Faceting Limits" at Octonus' website www.cutstudy.com for a good 2-dimensional discussion of principles. 3-dimensional reflections is the ultimate complexity of the real world, which 'DiamCalc' and 'FacetDesigner' do beautifully - too complex to discuss in simple terms - you need software to study it.
. A problem in these threads is people NOT reading what IS there and/or discussing what is NOT there.
. The previous thread 'Azimuth Shift & Yaw' was written because Rhino was in doubt about the difference, in another thread. Summary: 'azimuth shift' is just rotation (see Fig.2, which was illustrative only - nobody does this because it creates a double facet); 'yaw' is a combination of azimuth shift and slope increase so that there is only one facet but in the wrong place (Fig.3).
. This thread 'What a Scanner Sees' was written because Paul and Brian had asked me how scanners saw 'yaw' - do they give the correct slope? It should not have appeared in the other thread.
. Now Paul asks about reflection from a single facet, which has nothing to do with slope - reflection is an optical thing, slope is a geometric thing, as Paul suggested. I will discuss that here or in a separate thread.
. Let's not get into reflections between opposite facets - that's another subject, already documented. Read "Faceting Limits" at Octonus' website www.cutstudy.com for a good 2-dimensional discussion of principles. 3-dimensional reflections is the ultimate complexity of the real world, which 'DiamCalc' and 'FacetDesigner' do beautifully - too complex to discuss in simple terms - you need software to study it.
. A problem in these threads is people NOT reading what IS there and/or discussing what is NOT there.
re:I would like to know how it recognizes the 'best' one for each facet, but I am sure that is private information.
Bruce, We do not need recognize position of nearest photos for each facet. We use other much more complex and precise method. But you right such information is private yet.
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Thank you for this Serg and for extending my education on the algorithms of the Helium. I don''t think I realized (nor do I fully) just how much goes into the concept of "what a scanner sees". Your explanation and illustration really help although I want to study this further.Date: 9/14/2005 3:29:37 AM
Author: Serg
Date: 9/14/2005 12:03:14 AM
Author: Rhino
Bruce will correct me if I''m mistaken but I think he''s trying to continue a conversation that had begun in another thread which has kinda went back now around 3-5 pages ago on the minor facets started by Midnight. The thread eventually became a very hot topic on the subject of azimuth angles and yaw. Brian and John (along with Bruce''s initial graphic demonstrating the technics of it) did what is perhaps is the best presentation of it that could ever be given. Only it didn''t sink into the heads of some fully (including myself) until reading and thinking about it for some time. When John and I had first discussed the topic I thought I was understanding his view, especially with regards to yaw east/west shift in a main. If you take note of John''s graphic''s in that thread he actually (whether he realizes this or not) discusses two elements regarding yaw.
One being the physical shift in the facet ... pushing the main if you will, and in another graphic he demonstrates a yaw in light path ... if I am understanding him correctly ... a yawed or skewed light path that results from the physical yaw. Correct me if I''m wrong John but I am really trying to comprehend this stuff and its easy to understand how one could confuse the two or even the issue.
In any case, the reason I was excited about this on a personal level is because it relates directly to my studies on the optical signature of a diamond. This is my life and passion. I observe and photograph east west shifts every day, I just never had the means by which to explain the phenomena I''ve been photographing. That''s all. I admit I had trouble grasping the physical yawed concept ... cutters would understand that 100x more than me, but when we''re talking about yawed light path ... I can relate to that 100%. This relates directly to ''direct assessment of light reflections'' through the crown. My speciality!
I *think* Bruces point (and by all means correct me again if I''m wrong), perhaps may have been to suggest that scanners, based on measurements taken from a silhouette (like the old GIA Scope, Helium/Sarin/OGI) can not accurately see azimuth deviation from ideal or yaw or perhaps both. I realize this takes a little reading between the lines and also that I may be totally off here (LOL ... me kicks myself in the ass if I am
) which brings us exactly where we left off in the other thread ... burden of proof on me ... and the ability of Helium to do exactly what we are claiming.
I realize this conclusion sounds very selfish and self centered and I apologize if I am being perceived in this light but Bruce ... am I even close?
I see I''ve misunderstood Paul''s point about a facet being examined from many angles (I was thinking angles in relation to girdle plane as opposed to angles in relation to each other ...) but from what I''m putting together from recent threads this is what I am extracting. A kind challenge?
Ok ... everyone get their tomatoes ready.
Bruce, if I am correct give me one of these ..., if I''m close one of these ...
(for effort), if not, one of these...
. If the first I''ll present the proof you seek and we can continue the discussion here in this thread.
Edited to add: I think I realize that you are attempting to bring closure to the conversations that were taking place on that thread. If so, I thank you.
Kind regards,
re:I *think* Bruces point (and by all means correct me again if I''m wrong ), perhaps may have been to suggest that scanners, based on measurements taken from a silhouette (like the old GIA Scope, Helium/Sarin/OGI) can not accurately see azimuth deviation from ideal or yaw or perhaps both.
No.
Bruce Idea:
1) Shadow scanners have fixed step.
2) For example for step 2 degree, distance between nearest contour and “right” position for measurement slope by shadow method could be 1 degree. ‘Right” position is if projection of facet is one segment ( all facet edges are on one line)
3) In “wrong position ” you can measurement “wrong” slope angle only. (
4) Variation of measurement for slope main pavilion facets round diamond for 2 degree step( 180 photos) could be 0.05 degree
5) Because variation of angle is 0.05 degree, records like 40.75, 40.96 are not correct. It should be rounded to 40.8 and 41.0
See illustration below. I had rotated red model from right direction on 5 degree.
You can see difference of ‘slopes main pavilion facet in projection’ for red and blue model
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Date: 9/14/2005 4:56:54 AM
Author: strmrdr
This what your talking about Rhino with the opposite sides?
again excuse the bad art work.
edit: that should say sloped e-w/yawed not sloped/yawed.
excuse the 4am slip of the fingers
Hopping back and forth here today but yes strm... you understand exactly what I''m talking about, however this isn''t what Paul was talking about. Your former illusstration I believe is what Paul is referring to.
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Re: Summary: ''azimuth shift'' is just rotation ; ''yaw'' is a combination of azimuth shift and slope increase so that there is only one facet but in the wrong place. Bruce Harding
For those like myself, coming in late, some explanation and definitions would be helpful.
The proliferation of names uniquely defining the plane of the facet is at best confusing. I wonder if it is necessary.
A point on the facet and two angles uniquely define the plane of the facet in space. In faceting the most convenient two angles to use, because they are the settings on a faceting machine or a ''dop and tang'', are the perpendicular slope relative to the gem''s girdle and the rotation angle or index setting. These are the best angles and terms to use because they directly relate to the settings on machinery used to facet a gemstone.
Because artillery is aimed with two similar angles, elevation and azimuth, it appears that azimuth has been adopted as a synonym for rotation angle or index. Index = rotation angle = azimuth, so a shift in index, as in ''cheating'' is a change in azimuth.
IMO this is where the complexity of terminology should have ended, but someone had to introduce ''yaw''. This seems unnecessary, redundant and of little added value unless confusion is the goal.
Here are definitions of movements of an object such as a plane or rocket: Pitch is up and down like a box lid. Yaw is left and right like a door on hinges, and roll is rotation.
So Pitch = slope. Azimuth or rotation angle could be viewed either as yaw or roll.
Any change in the facet plane from standard slopes and indexes during gemstone cutting can be uniquely described as a combination of a slope and or azimuth change. These angle changes correspond to the two angle settings on a facetting machine. Would someone please explain to the audience why they need to further confuse the issue with the term yaw?
Enquiring minds want to know.
Michael Cowing
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This is what lead me to think what I was about Paul's comments.
I understand. The focus of my studies revolves in this arena to some extent. Not just with ray trace but with photography through reflector based technologies and how they correlate to the results of both ray trace and the optical images produced through the DiamCalc software as a result. If this does not interest you I will not bore you with the details of my research. I am sorry if it seemed I was headed there as in my mind it relates to things you are discussing here (east/west shift in facet construction) as this is phenomena we note and observe but approach from an entirely different angle. You are approaching it from the geometric standpoint while I am approaching it from the optical standpoint. My most current studies are revolving around the area of azimuth shift and how they impact the optical design through the crown thanks to Sergey and the Helium scanner.
I meant no offense Bruce. There were alot of discussions going on in the other thread and while I was trying to follow along in the multiple threads that had resulted I guess I got a little confused. My sincerest apologies.
Thank you for clarifying and we can close this discussion now if you like.
Regards,
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Since the topic of this thread is entitled "What a scanner sees", Mike, Sergey, if you like I will share with you some of the discoveries I am making here with regards to optical design on real diamonds we've photographed and how they correlate to azimuth angles and the models produced by the Helium scanner. To me this is really fascinating but if not that's cool too. I think this research reveals, to an even deeper extent through its modeling capabilities, the amazing accuracy of the Helium and *what this scanner sees*. You see ... while I am a strong proponent of direct assessment through the crown I make it my business to translate, to the best of my ability, how those numbers translate to optics. I never stress numbers OVER optics however if an accurate knowledge of the numbers can be attained on the various facet sets, we can, to a degree know how this stone will appear. If we can determine this to a certain degree of accuracy, people can be helped and that, in the end is the ultimate goal. Helping people.
It's funny you should post Mike. You and I are so much on the same frequency it's scary. I hear what you're saying 100% and is where I began when this conversation first started except I *thought* I had understood JohnQ. when he first told me about this.
Let me also clarify that Bruce's intentions, I believe, was not to confuse. He was posting this for my benefit although admittedly its not the easiest thing to grasp and for that I have to thank him. I have no reason to believe that Bruce and/or Paul would try to come here and purposely try to confuse me.
Bruce, I do believe, thanks to you and others that I have the *beef* of the teaching.
What's funny about this Mike is your question asking for clarification on terms and definitions kinda brings this entire discussion full circle.
The reason why this is important to folk like Brian (and me too for that matter) is because this geometric shift alters the optical signature as observed through the pavilion in an H&A viewer.
Peace out,
It's funny you should post Mike. You and I are so much on the same frequency it's scary. I hear what you're saying 100% and is where I began when this conversation first started except I *thought* I had understood JohnQ. when he first told me about this.
Let me also clarify that Bruce's intentions, I believe, was not to confuse. He was posting this for my benefit although admittedly its not the easiest thing to grasp and for that I have to thank him. I have no reason to believe that Bruce and/or Paul would try to come here and purposely try to confuse me.
Bruce, I do believe, thanks to you and others that I have the *beef* of the teaching.
What's funny about this Mike is your question asking for clarification on terms and definitions kinda brings this entire discussion full circle.
The reason why this is important to folk like Brian (and me too for that matter) is because this geometric shift alters the optical signature as observed through the pavilion in an H&A viewer.
Peace out,
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Thankyou Michael.
I have avoided Yaw threads because yaw does not mean anything to me that is not able to be expressed in terms of slope and azimuth.
I still do not understand why we are discussing slope profiles. Sure, the slope will vary if the azimuth is measured incorrectly.
But Beryl the algorithms calculate the slope angle - it is not taken solely from the shadow profile. This would result in miss read angles - but surely no-one would imagine Sergey (and other scanner companies) would ignore that?
It would seem more instructive to post a picture of a Helium scan of a diamond with the azimuth showen.
I have avoided Yaw threads because yaw does not mean anything to me that is not able to be expressed in terms of slope and azimuth.
I still do not understand why we are discussing slope profiles. Sure, the slope will vary if the azimuth is measured incorrectly.
But Beryl the algorithms calculate the slope angle - it is not taken solely from the shadow profile. This would result in miss read angles - but surely no-one would imagine Sergey (and other scanner companies) would ignore that?
It would seem more instructive to post a picture of a Helium scan of a diamond with the azimuth showen.
Michael & Garry & some others:
. If you will read back, starting with Midnight's thread about minor facets, you will see why I tried to explain Brian's terminology in 'Azimuth Shift & Yaw'. This led logically to 'What a Scanner Sees' because it is what Brian and Paul asked me many months ago, with regard to Brian's term 'yaw'. These efforts were intended to help others understand the factors that concerned them. If you noticed, Brian and Paul were discussing this even in Moscow.
. I will conduct any further discussion offline unless many people show interest.
. If you will read back, starting with Midnight's thread about minor facets, you will see why I tried to explain Brian's terminology in 'Azimuth Shift & Yaw'. This led logically to 'What a Scanner Sees' because it is what Brian and Paul asked me many months ago, with regard to Brian's term 'yaw'. These efforts were intended to help others understand the factors that concerned them. If you noticed, Brian and Paul were discussing this even in Moscow.
. I will conduct any further discussion offline unless many people show interest.
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Re: Summary: ''azimuth shift'' is just rotation; ''yaw'' is a combination of azimuth shift and slope increase so that there is only one facet but in the wrong place. Bruce Harding
Let me also clarify that Bruce''s intentions, I believe, was not to confuse. He was posting this for my benefit although admittedly it’s not the easiest thing to grasp and for that I have to thank him. I have no reason to believe that Bruce and/or Paul would try to come here and purposely try to confuse me. Jonathan aka Rhino
Jonathan,
My post was not a criticism of Bruce. His words, which I sited, were clarifying, not confusing. I was simply questioning the need for the term yaw, which appears to be unnecessary when gemstone and diamond cutters already have the sufficient terms, slope and rotational angle (index, or azimuth).
I am still waiting to hear from someone who can explain why yaw, which seems an odd word for the stone cutting industries, is necessary.
Bruce''s posts and the points he is making are ''crystal'' clear as always.
Michael Cowing
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Re: If you will read back, starting with Midnight''s thread about minor facets, you will see why I tried to explain Brian''s terminology in ''Azimuth Shift & Yaw''. This led logically to ''What a Scanner Sees'' because it is what Brian and Paul asked me many months ago, with regard to Brian''s term ''yaw''. These efforts were intended to help others understand the factors that concerned them. If you noticed, Brian and Paul were discussing this even in Moscow.
. I will conduct any further discussion offline unless many people show interest. Bruce L. Harding
Michael Cowing
. I will conduct any further discussion offline unless many people show interest. Bruce L. Harding
I am following these discussions with interest, and support their continuation on line. As Sergey has said in the past, free interchange of ideas such as those discussed here, is how advances in knowledge and understanding are made. Contributions by Bruce and Sergey are high on the list of those advancing knowledge and understanding in this area of diamond and gemstone cutting analysis.
Sergey''s posts indicate that their Helium scanner and associated analysis software do not suffer from inaccuracies of Sarin-like, optical comparators that have practical limitations due, among other things, to discrete rotational angles.
As Bruce and Sergey indicate, discrete steps of rotation will result in inaccuracies due to measuring facets that are seldom perfectly on edge. It would take some smart software to accurately calculate the true facet angles from noisy measurements of the slope of shadows projected from facets not perfectly on edge. This is a practical limitation of this type device.
From the indications I have seen, I believe Sergey’s team of scientists has overcome or minimized this limitation by advances in both hardware and software with the Helium machine. I am jealous of Jonathan who has the Helium, and would have one myself if I could only justify the expense.
In a world where the best product prevails, I believe the Helium scanner would be the hands-down winner. I do not believe that currently available software that predicts light performance from the output of Sarin and OGI are sufficiently predictive of a diamond’s actual light performance. IMO, the Helium machine and its processing software will enable, along with the DiamCalc software, sufficiently accurate predictions of actual diamond light performance.
IMO, you do not get sufficiently accurate predictions of actual diamond light performance from idealized, mathematically symmetric, computer models constructed from average pavilion, crown, lower half, etc. data from Sarin or OGI.
I note a comment by Al Gilbertson to the GIA Alumni Association last night. He indicated that GIA, in its research, employed metrologists to get accurate measurements of a their master set of diamond cuts, because of the concern over the accuracy limitations of the Sarin Machines. He said that GIA now uses these known diamond cuts to check their Sarin machines on a regular basis for calibration.
I also note that both they and AGS have Helium technology.
Michael Cowing
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Hey Mike!
Just got back from listening to Al Gilbertson tonight here on Long Island. Interesting stuff about the digging and painting eh?
I know your comment wasn''t a criticism of Bruce. I just didn''t want him to get the wrong impression. I LIKE BRUCE!
BTW just wanted to say what an excellent time it was being there in VA with ya at the symposium and meeting Rob. My wife really enjoyed talking with you and getting to know you too.
I''ll try and hop on here tomorrow with some interesting stuff dealing with Helium scans.
Just got back from listening to Al Gilbertson tonight here on Long Island. Interesting stuff about the digging and painting eh?
I know your comment wasn''t a criticism of Bruce. I just didn''t want him to get the wrong impression. I LIKE BRUCE!
BTW just wanted to say what an excellent time it was being there in VA with ya at the symposium and meeting Rob. My wife really enjoyed talking with you and getting to know you too.
I''ll try and hop on here tomorrow with some interesting stuff dealing with Helium scans.
re:My post was not a criticism of Bruce. His words, which I sited, were clarifying, not confusing. I was simply questioning the need for the term yaw, which appears to be unnecessary when gemstone and diamond cutters already have the sufficient terms, slope and rotational angle (index, or azimuth).
I am still waiting to hear from someone who can explain why yaw, which seems an odd word for the stone cutting industries, is necessary.
Michael,
It is slang just. Sometimes slang is very popular, sometimes no.
We( I and You) do not need such type of slang, but may be for some cutters such slang is convenient.
Time is best judge for convenience slang. We can not do it better .
I am still waiting to hear from someone who can explain why yaw, which seems an odd word for the stone cutting industries, is necessary.
Michael,
It is slang just. Sometimes slang is very popular, sometimes no.
We( I and You) do not need such type of slang, but may be for some cutters such slang is convenient.
Time is best judge for convenience slang. We can not do it better .
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Re: BTW just wanted to say what an excellent time it was being there in VA with ya at the symposium and meeting Rob. My wife really enjoyed talking with you and getting to know you too. I''ll try and hop on here tomorrow with some interesting stuff dealing with Helium scans. Rhino
Jonathan,
I regret that time goes by so fast in our busy lives that it is hard to keep up with everyone''s work and progress. So it was fortunate that we were able to catch up at the symposium. It was a pleasure.
We both agree on the importance of ''direct assessment'' of light performance from face-up and tilted ''snapshots'' of the diamond in typical illumination and viewing circumstances.
Johathan''s unique perspective comes from his tenacious, thorough evaluation of each diamond using almost all available performance assessment tools, which now include the Helium machine. As a sort of ''Consumer Reports'' of diamond cut grading technology, Jonathan will be able to compare and contrast light performance results from all these tools, and let us know how they correlate to his experienced, visual observations.
His processing of a large number of diamonds with all these performance evaluation tools should prove to be a valuable addition to knowledge in this area. He is in a good position to report on the ability of each of these tools, especially the promise of Helium, to adequately predict the diamond''s ''light performance''.
Michael Cowing
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Any change in the facet plane from standard slopes and indexes during gemstone cutting can be uniquely described as a combination of a slope and or azimuth change. These angle changes correspond to the two angle settings on a facetting machine. Would someone please explain to the audience why they need to further confuse the issue with the term yaw?
Here are definitions of movements of an object such as a plane or rocket: Pitch is up and down like a box lid. Yaw is left and right like a door on hinges, and roll is rotation.
So Pitch = slope. Azimuth or rotation angle could be viewed either as yaw or roll. Michael Cowing
On a facetting machine the ''cheater'' is a screw adjustment allowing deviations from standard indexes. This rotational adjustment would be roll, not yaw. Unlike a pitch or slope change, yaw, defined above as left and right movement, on a faceting machine would appear to not change the facet plane at all.
So what do folks who use the term yaw mean by it? What adjustment on a facetting machine or the dop and tang, causing a deviation from standard slopes and indexes, is considered yaw?
Do you see why I find introduction of this odd term, which seems to have no connection to diamond cutting machinery, confusiing? If Brian is the first to use this term, perhaps he is the one best able clear up my confusion.
Michael Cowing
Here are definitions of movements of an object such as a plane or rocket: Pitch is up and down like a box lid. Yaw is left and right like a door on hinges, and roll is rotation.
So Pitch = slope. Azimuth or rotation angle could be viewed either as yaw or roll. Michael Cowing
On a facetting machine the ''cheater'' is a screw adjustment allowing deviations from standard indexes. This rotational adjustment would be roll, not yaw. Unlike a pitch or slope change, yaw, defined above as left and right movement, on a faceting machine would appear to not change the facet plane at all.
So what do folks who use the term yaw mean by it? What adjustment on a facetting machine or the dop and tang, causing a deviation from standard slopes and indexes, is considered yaw?
Do you see why I find introduction of this odd term, which seems to have no connection to diamond cutting machinery, confusiing? If Brian is the first to use this term, perhaps he is the one best able clear up my confusion.
Michael Cowing
JohnQuixote
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I do not mean to intrude in the ''scanner'' discussion with these comments so it will be one post and out.
Michael - I was smiling as I watched this thread bring yaw references full-circle. A few attempts to describe this in 2 dimensions resulted in divergence about (1) what was being observed and (2) the analysis thereof, so Brian and I retreated from the discourse to complete other projects. I told him today that you inquired about it on PS. We''ve spent all afternoon giving the tour de WF to some VIPs
The ''yaw'' question is compelling. On one side are scientific and technical theorists and gemologists saying if you have a flat facet all is measurable. On the other side are hands-on-the-wheel diamond cutters saying that some adjustments are made and disguised that are not picked up by (at least some of) the machines. We know not all indented naturals are picked up - so there is a good case to be made about this phenomenon not being picked up.
Though they resulted in diverging speculations, here is one set of prior illustrations: minor facet thread page 4
Another set, explaining how his/Paul''s observations relate to analysis: minor facet thread page 5
One thing Brian expressed today is that it''s wonderful that Bruce, Michael, Serg, Garry & he will finally come together to discuss these kinds of technicalities - as all you guys said you should back in Moscow in 2004
That said, maybe we can focus the conversation. This thread is entitled ''what a scanner sees'' and may not be the best place for discussing what Brian and Paul know as the yaw phenomenon. Michael, perhaps if you have comments you will bump either the minor facet thread - or Beryl''s azimuth shift/yaw thread?
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Shadow profiling can not see into concavities, but rough scanners can by using lights shining from oblique angles and smart algorithms.
At the bottom of this discusiion there are images showing how it works.
http://journal.pricescope.com/Articles/24/1/Lasers-used-in-planning-the-cutting-of-diamonds.aspx
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