OBSTRUCTION vs SHADOW

[beryl]

. Many folks persist in using the word 'shadow' to describe blockage of light rays that could be reflected by the gem to the viewer's eye; indeed, I have done this myself on a few occasions. I think that this why some people miss the point.
. There is always a shadow, but this is not seen by the viewer; it has nothing to do with rays that are reflected to the viewer by the gem - they pass right through the shadow from a source to the gem.
. Here is an illustration based on the analogy provided by Derek Louey - as the head moves nearer to the stone the shadow decreases but the obstruction increases, and vice-versa.
. Please don't use the word 'shadow' for 'obstruction', 'blockage', or whatever you want to call it; this causes some people to miss the point.
. Would you believe that we just had an article rejected because the reviewer said that close-up view of a diamond is not important; it is only how it looks at a distance that matters! Perhaps he is right! Think about that.

 

[beryl]

. Here is the illustration. I had already uploaded it when my server crashed, and was unable to re-link it with the message above.

 

[pricescope]

Thank you, Bruce, for taking time to post this here. It is a privilege to have you on board.

For those who don't know, Bruce (aka Beryl) is an author of Faceting limits written in 1975 and "Diamond Design" Revisited.

 

[strmrdr]

Iv been saying the same thing for a while.
It does not match the real world on how people actualy look at diamonds once on the finger.
As an experment I ask several people that I know to look at their rings or even just asked them about them.
Not one brought it up and looked strait down on it.
They tilted there hand back towards them with the arm either all the way out or slighly bent with the forarm roughly parellel to the ground and the hand bent back at various angles.
Most then moved their hands around to best catch the light.

The most common comment was "I need to clean my ring"

 

[valeria101]

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On 4/14/2004 8:12:29 AM strmrdr wrote:



It does not match the real world on how people actualy look at diamonds once on the finger.

They [tilt] there hand back towards them with the arm either all the way out or slighly bent with the forarm roughly parellel to the ground and the hand bent back at various angles.

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True. The picture above does show only the worst scenario (making a proof by contradiction, I guess) - and it is a good thing it does. After all, any RBC looks good if care is taken to present the stone in favorable conditions: remember the fabled jewelry store lights?

It may be that no one stares into the table of a diamond (as the picture suggests) but light is not always coming on diamonds from above either. It's great if one does not have to turn and tilt around a stone (as your behavioral experiment shows people do) toooo much to catch the light just right, I would think.

 

[strmrdr]

Val,
I can agree that a well-cut diamond they would have to do less positioning to catch the light and make it look great but I rather suspect they would no matter how well cut it was.

The problem is that if the experts get too hung up on the looks of gemstones in the above illustrated positions the cuts will move away from looking the best in real world positions.

For example playing around with gemcad and various cutting diagrams combined with ray tracing software I found several cuts that strait up have better light return than a RB but tilted at a 20-30 degree angle fall off much quicker than an RB.
In the real world despite there better performance in the above-illustrated position would be worse looking as actually viewed by the people that wear them.

 

[valeria101]

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On 4/14/2004 8:48:00 AM strmrdr wrote:




The problem is that if the experts get too hung up on the looks of gemstones in the above illustrated positions the cuts will move away from looking the best in real world positions.

The position above is meant to show one property, not all... It is good for the particular argment, not every other, I suppose.





[...] several cuts strait up have better light return than a RB but tilted at a 20-30 degree angle fall off much quicker than an RB. In the real world despite there better performance in the above-illustrated position would be worse looking as actually viewed by the people that wear them.

This issue is not speciffically addressed by the scheme above (as far as I understand). But I do remember it to be part of the argument in favor of the "ideal" proportions among the range of proportiosn with good light return but not much else going for them. Also, this failure is mentioned for 'near fish-eye' proportions.

I was wandering myself wether it makes sense to set one 'standard' position for evaluating optical 'performance' (with all components... not just white light return). But the issue has been addressed in more than one way before this PS thread

First, the selection of the 'ideal' range does treat the problem even if with a different approach than replicating 'real life' situations (the discussion on scintillation and 'fish eye' and probably more given this is my hobby, not my PhD thesis). Second comes the need for a standard evaluating procedure for a diamond's optical properties. It is easy to see why making cut grading a simmilar exercise with the rest of grading procedures is essential. You may want to inspect GIA's comments/instructions on standrad grading conditions, esp. the 'geometry'.

At least to me it seem that the current approach - [to find the static properties equivalent (sufficient, really) to the desired (dynamic)visual effect, if I get it right] - is more than justified.


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[glitterata]

Those diagrams confuse me. They seem to depend on there being two light sources, don't they?

Is the point that the head can block one light source (cast a shadow from one light source) without blocking (casting a shadow from) a second? But why are we assuming there's a second light source? Is the point that the direct light source isn't the only source of light--ambient light is coming from everywhere? Or is there some other point instead?

As I said, I'm confused.

 

[Nicrez]

Beryl, so would it be more correct to say that when viewing a stone head on, that there is no shadow, there is a obstruction of light that appears in the form of a bow-tie? is that more accurate?

I had always assumed that which was dark within the stone would be a shadow, I guess it never dawned on me that it would simply be an obstruction of light that didn't allow that particular area of the stone to have light reach it. Is that correct? Thanks! Truly a pleasure to have you on board!

 

[beryl]

Nicrez and others:
. There is always a shadow if the head (and body) is between a light source and the stone.
. Sparkles in a stone often come from many different light sources; they are redirections of light from these sources. A spot where you see no light is reflecting 'light' from a direction where there is no illumination.
. The article mentioned above, "Influence of the Viewer on a Gem's Appearance", explains this beautifully. It was written by Anton Vasiliev, gem cutter and optical physicist, in the Russian Gemological Journal, Vol.(6), Fall, 2002. It has been translated and improved. I was looking forward to seeing it in English. We hope the British Journal's reviewer will relent; if not, I will seek another publisher.
. I will try to show and explain one of the illustrations here. This will explain some of the other questions.
. Sorry, I keep 'booting' myself out of the system by hitting the TAB key to indent. I have been typing manuscripts for far more years than computer forums, and find it hard to remember not to indent via TAB. Note that I use a period to simulate indent - when I remember to.

 

[Nicrez]

Bruce, I didn't get a message on that last post...

 

[Rank Amateur]

I think Bruce is saying that BLOCKAGE/OBSTRUCTION refers to ALL sources of light whereas a SHADOW is cast from ONE particular source. The closer the head is to source "A" the larger the shadow, while at the same time the head blocks fewer sources, one example of which is shown as "B", allowing more possible relections to reach the viewer.

I could be wrong about this as I rarely understand Bruce's meditations.

 

[beryl]

. Here is part of an illustration from Anton's article, as translated. It shows the environment of a gem as though it were painted onto a sphere with the gem at its center.
. See the dots? Those are the spots that are re-reflected to the viewer's eye. These are found by reverse-ray analysis, as though light from the viewer's eye was traced into the gem and out to the sphere.
. The viewer will see what is at each of these 'source' spots - green if grass, black if tree, blue if sky, pink if face - at a corresponding spot on the surface of the gem.

 

[beryl]

. This subject has been discussed before - a few years ago in "DiamondTalk", yet I saw the term 'shadow' used in here by one of the folks from that era. I thought it would be nice to show them the concept that Derek Louey offered to explain the difference. So far, none of them has commented.
. I first realized the significance of this problem when I read the term 'shadow' used by GIA, which explained why they had missed the point. They have been severely criticized for not considering the viewer's head in their computer simulations. Strickland added it to his, I understand. MSU had it in theirs from the beginning (Anton was on the team).
. For those who care to know about it, read 'Faceting Limits', re-published at www.cutstudy.com - go to 'News & Articles' for index.
. Some folks cannot read charts or diagrams. That's OK; they're wired differently. There are many things I can't do, either.

 

[glitterata]

Very interesting.

I think the answer to my question is Yes: the diagram shows two light sources because the direct light source, the one that's casting a shadow, isn't the only source of light that reaches the diamond. Light also reaches the diamond from all around, after bouncing off objects in the environment. The second light source pictured in the diagram represents one possible spot that light might be bouncing off and heading toward the diamond. Right?

 

[beryl]

I feared that I made it too complicated instead of easy.
Originally, I had just the one light source, A, to show the two ranges - shadow and obstruction - one increasing and the other decreasing as the head moved. That was what I was trying to show; the rest was unnecessary for this message.
Then I showed light source B, to illustrate how it is obstructed in the pic on the left, marginal in the middle, and unobstructed on the right.
The 10° angle in the middle is the borderline angle we cite for a viewer looking at a gem from a foot away, with a normal size head/coiffure. It was the key angle in "Faceting Limits", and in Anton's extension of it in "Optimizing Faceting for Beauty", recently published, in English, in the British Journal of Gemmology, January 2004.
We try to choose facet angles so that no rays are re-reflected within this angle; this requires that certain combinations of pavilion and crown main slopes must be avoided. Tolkowsky's 40.75° pavilion and 34.5° crown slopes cause re-reflections just greater than this 10° limit.
Garry Holloway observed that the 'best' cuts are just outside this limit because it causes reflection from the pavilion mains to be bright and from the pavilion breaks to be dark, producing a light/dark contrast which is most pleasing to the eye. This is what causes the 'arrows' in a hearts-and-arrows viewer or Firescope or IdealScope image.
The Russian software, DiamCalc, let's you vary this angle - as though you were viewing the gem from different distances; you can change it and the facet slopes to see the difference in appearance of the diamonds as they are rotated in various lighting environments.
All this has been discussed at length before and is too much to go through again. It will be a factor in discussions at the International Diamond Cut Conference in Moscow, April 23-26, which will try to redefine cut-grading criteria. AGS will be there, GIA will not -too bad we're not all working together.

 

[beryl]

GEMS or REFLECTORS ?
I once cut a gem which reflected a great anount of light from one source to the eye in one position, then 'died' when tilted. I call that a 'reflector'. I didn't like it.
I would rather cut a stone which reflects light to the eye from many sources in many positions. I call this a 'gem'.
Studying reflections with my own crude software circa 1987, I cut a blue-white sappphire which was incredibly beautiful.
It was sold before I ever had a chance to see it set!

 

[beryl]

FIZZIES or BANG-BANGs

I once made a Portuguese cut (150-plus facets). The facets were so small and so close to the same angles that the little sparkles went on and off continuously, producing a 'fizzy' effect which did not stimulate the eye.
The SRB is best for snappy on-off reflections.
My favorite is the cushion cut, or modern 'old-miner' with its 4 oversize mains which give an exaggerated on-off contrast that I call 'bang-bang' - it really gets your attention.

What is 'best'? It's a matter of preferene.