Fire and dispersion techy help please?

[michaelgem]

Here is an asscher producing fire in DC gemcad iso no head.



It is more obvious in step cuts because they produce more internal contrast than an RB.
They produce fire/scintillation in very flat lighting that makes RB's look dead.
I have confirmed this both in DC and actual observations.

Karl,

In this DC simulation and your actual observations there is still contrast in the sphere of illumination. In the DC simulation there is the contrast edge at the boundary between the upper hemisphere of white and the lower hemisphere of black background. Try making both hemispheres white and see what you get.

I am wondering if the fire in the ascher might be coming from the contrast edge due to the dark background.

Michael

 

[beryl]

. I plead ignorance. If we are to communicate, I must understand the words you use.
. What does everyone mean by the term 'virtual facet'? It is new and meaningless to me.

 

[michaelgem]

Post by beryl » 19 Nov 2010 18:14
. I plead ignorance. If we are to communicate, I must understand the words you use.
. What does everyone mean by the term 'virtual facet'? It is new and meaningless to me.

Bruce,

This is a term attributed to the GIA and their early diamond course. Back when I was working for AGS via 8*, I tried to convince Peter to use more descriptive words, and even got him to consider "compound mirrors". But once a term like this one is established, it is fruitless to use any other term.

I address this issue in a section in my unpublished work on diamond optics. Briefly:

A diamond’s appearance is of a pattern of internal mirror reflections, which resemble the tiles of a mosaic artwork.

At any moment, each individual “mirror-tile” reflects a small portion of the panorama of light surrounding the diamond to the observer’s eye.

This pattern is seen as contrasting bright, dark and colored reflections in the face-up diamond photograph. A diamond’s reflections are constantly changing with even slight movement of the diamond, observer or illumination.

A diamond acts as an arrangement of many of these mirror-like tiles. We can think of a diamond as a miniature mosaic consisting of many “mirror-tiles”. Each mirror-tile is actually made up of several mirrors acting together as a “compound mirror”. These several mirrors, which act in unison, are the facets from which a ray/column of light internally reflects along its path inside the diamond. Each mirror-tile reflects to the viewer a small portion of the light from some angle and direction around the diamond. See the illustration of the path of a beam of light reflected to the eye from one of 8 "compound mirror tiles" that comprise the entire table reflection in a H&A Ideal cut diamond.

These mirror-tile reflections have come to be called “virtual facets”, because they give a gemstone the appearance of having more facets than are actually present.

I would welcome a consensus for any other term, since these compound-mirror-tile like reflections are not virtual and they are not facets.

Michael D Cowing

 

[Karl_K]

I find the term virtual facet to be a fairly easy way to convey a complex subject to those that are interested.
I think we should just stick with it rather than confusing people with another term.

 

[michaelgem]

I find the term virtual facet to be a fairly easy way to convey a complex subject to those that are interested.
I think we should just stick with it rather than confusing people with another term.

You are right, of course, Karl,

Just wishful thinking on my part.

As I said: "once a term like this one is established, it is fruitless to use any other term."

 

[Karl_K]

Karl,

In this DC simulation and your actual observations there is still contrast in the sphere of illumination. In the DC simulation there is the contrast edge at the boundary between the upper hemisphere of white and the lower hemisphere of black background. Try making both hemispheres white and see what you get.

I am wondering if the fire in the ascher might be coming from the contrast edge due to the dark background.

Michael

I will look into that and report back

 

[Karl_K]

With a white background I still get fire within 2 degrees tilt in DC
Actual observations will follow in a few days.

 

[michaelgem]

With a white background I still get fire within 2 degrees tilt in DC
Actual observations will follow in a few days.

Karl,

You not only still have fire, you still have a small amount of contrast brilliance. I wonder if both upper and lower hemispheres are uniformly white. If you email me the dmc file of this diamond at [email protected] I will run it with the series of illumination panoramas used in the following experiment and exposition:

Most, although not all, of the contrast brilliance and fire observed in a diamond in usual illumination circumstances is due to contrast in its surrounding panorama of illumination, as illustrated in the first image of a step cut diamond. This fact is demonstrated in the second image where the upper hemisphere of illumination is uniformly white, and most of the contrast brilliance and fire is gone.

The remaining dark contrast and fire comes from the darkness below and the contrast at the light/dark edge. In the third image where darkness only exists at low angles below the diamond the fire is gone as is most all contrast.

And by the time I simulate my photographic experiment with the diamond on a white surface inside a white hemisphere, the same results are obtained: the diamond almost disappears and displays no fire. So this example simulation supports my key assertion:

"No light/dark contrast edges in the illumination environment, no fire, period. And this sweeping statement includes the concept of fractioning. Over ten years ago I demonstrated this by photographing a diamond inside a white hemisphere. The diamond almost disappeared and displayed no fire."

There likely is some conservation of energy principle at work here. If no light is absorbed in a diamond that is uniformly illuminated from all angles then that diamond will essentially be uniformly white virtually disappearing against its white background.

Think of it this way, each virtual facet making up the viewed surface of the diamond is reflecting the same white light that completely surrounds the diamond.

This step cut diamond example also makes the case that in a colorless diamond having negligible absorption, any dark appearing virtual facets cannot be due to what is called extinction. Those virtual facets must instead be dark because they reflect from some relatively dark point in the diamond's surrounding such as the relatively dark points below the diamond and the usually relatively dark observer obstruction, or the relatively dark areas between indoor overhead lighting. And as has been shown fire often results from reflections from the edges of all those dark areas.

Michael

 

[Garry H (Cut Nut)]

I do agree that virtual facets on the edge of dark zones produce observable fire but that contrast can be the lighting, head shadow, environment, and virtual facet light draw direction or leakage.
The smaller the angular size of the light source the more likely fire will be observed from the light coming from that light source returned from the diamond.
This is one reason that Garry's patented florescent tube trick increases fire and scintillation it makes the effective angular size of the light source smaller.
The tubes are interesting in that it has a smaller angular size one direction than the other and a diamond will react differently depending on which way you tilt the diamond or move your head.

I am not convinced however that no fire will exist with no external contrast because the diamond can create its own contrast based on the virtual facets, in particular leakage and virtual facet angle draw.
Even in an evenly lighted dome you will still have contrast.

Karl you have leakage areas causing the darkness that allows us to see the fire.

Please re-read my description a page before- Both times I include leakage and dark from obstruction.
If the diamond was hovering in the center of an igloo with a tiny tiny viewing hole, then there will be no fire no matter how much leakage.

 

[michaelgem]

. Here is a quote from “Rainbow ...”, p.4 (slightly modified for easier reading; I edited it in 2002) ...
”... consider the observer’s eye focused at the surface of a prism nearest it; that is, looking at a facet. The image of that facet is formed on the eye’s retina, each point on the facet’s surface being mapped at a retina point. ... The beam leaving the prism forms a fan-shaped bundle of rays of various colors.
“... Should ALL of this fan enter the pupil of the eye, the eye’s lens will focus it into a white dot on the retina.
“... if the fan width exceeds pupil size at the eye, then only part of the fan cut off by the pupil would be focused on a dot on the retina.
“The greater the dispersion fan angle, and the smaller the pupil, the narrower is the wavelength band cut out from the complete spectrum; this results in higher saturation of the colors observed.”

. The subject of spectrum segments is discussed in detail at Fig.2 and shows why green does not appear if seeing wide sectors of the spectrum.
. The beam angle vs pupil size is nicely illustrated/discussed in later works by Sergey Sivovolenko.

Bruce,

This is the ice-cream from Anton that we were talking about. A study of these quotes will enable an understanding of the initially, somewhat-enigmatic Fig. 2 below that is the heart of his article.

Michael

 

[beryl]

[quote="michaelgem|1290219762|2773650
"No light/dark contrast edges in the illumination environment, no fire, period. And this sweeping statement includes the concept of fractioning.]
. I disagree; dark edges have nothing to do with 'fractioning'. See illustration in my "Fractioning" article (repeated earlier in this thread: page 4, 03 Nov 14:12). That effect comes from a single white light source approaching at a certain angle. In your example there are many such sources and many such fractionings, but the results of the infinite other white light sources overpower these individual results, rendering them unnoticeable - as Garry says "brilliance is the enemy of fire", except I would substutute 'brightness' for 'brilliance' in this case since contrast is a part of 'brilliance'.

PS: Thanks for your explanation of 'virtual facets'. I knew what it was but not by that name = non-communication. I took GIA Diamond Course - 35 years ago!
. Haven't you confused 'GIA' and 'Peter' (Yantzer of AGS)? (in your post re 'virtual facets').
. In addition, newbies don't recognize first-name-dropping stuff; I confess that I have done the same thing (e.g. 'Anton', 'Sergey'). It must seem as smugness to them; it IS rude. We have formed sort of an 'old boy' club here'; sorry, folks, I apologize. Note that no one else asks what we're talking about; I wonder if anyone else is even reading this thread.

 

[beryl]

Michael:
. Your post of 4:47 does not indicate that it is a quote of my prior entry (page 5, 19 Nov 17:51). It sounds like you edited Anton's article in 2002. Everything down to 'Bruce' is my quote and commentary of Anton's article.
. In your illustrations, the upper one is Fig.3. The lower one is Fig.2, which shows why we can't see green if we see a wide section of the spectrum.

 

[diagem]

We have formed sort of an 'old boy' club here'; sorry, folks, I apologize. Note that no one else asks what we're talking about; I wonder if anyone else is even reading this thread.

Reading with great interest

 

[beryl]

. Thank you, DiaGem.

 

[beryl]

. I had written this but could not reproduce Anton’s illustrations. Thankfully, Michael has now done that.
. For the sake of readers, here is an illustration (Fig.2) from “Rainbow in a Colorless Gem”, by Anton Vasiliev, which shows the effect of spectrum width on color intensity; it is a bit difficult to understand.
. The wavy lines indicated the width of spectrum band you can see. When you can see only 20nm, then all colors are saturated – you can see each one distinctly. As the width that you can see increases to 150 nm, the saturation of red and blue is still 100% (but faint) - and you will see them distinctly at their respective ends of the spectrum, but green is reduced to 40% if you see only 150nm wide in the middle of the spectrum. When the width you see reaches 200nm (half of the visible spectrum), then none of the green is visible.

 

[beryl]

. From “Rainbow in a Colorless Gem” (middle of page 4) I quote the following almost exactly; there is very little editing:

. ”As gems are usually viewed in good illumination conditions and from convenient distances, the pupil diameter can be assumed 3mm and the distance to the gem being 25cm, in which case the pupil angular size will be 0.7°. In this case, to see only a wavelength interval of 50nm (to see pure colors) requires angular dispersion from the gem of at least 14°/nm. Angular dispersion less than 3.5° leads to complete loss of green, and the other colors being seen rather rarely. It should be emphasized that fire improves with increasing distance between the gem and the observer, but diminishes at lower illumination, which makes the pupil expand.
. “The above is true only if the prism is illuminated with a single light wave, as generated by a point source or one at a distance much greater than its size; in other words, one of small angular size to the gem. For an expanded source of illumination, the rays emitted from its different points, thus entering the prism from different directions, would mix their colors on the retina, degrading the purity of color. Hence, good fire also requires illumination of the gem from a light source of small angular size, as well as small angular size of the observer’s eye pupil.
. “Since the angular size of the sun, a lamp(?), or a candle does not exceed 0.5°, the fire depends primarily on the pupil angular size. The fire of colorless gems is perfect in sunlight and even better in illumination coming from multiple point sources ... then there will be as many color fans from the gem as there are light sources. The fire from a faceted gem will be completely lost in the light of a dull day under a cloudy sky.”

. Sound familiar, men? Anton published this 15 years ago! It agrees with and/or explains a lot of the things said in this thread.

 

[beryl]

. A funny thing happened while reviewing “Rainbow ...”. Recently I had reviewed my file of things from Anton and found a handwritten table of data with lower case script. I had never translated Russian script. It appeared to be a table of refractive indices and dispersion. From these I recognized the minerals involved and I knew them in Russian, so it became a sort of ‘Rosetta stone’ and I was able to discern the script symbols in the mineral names. When I reviewed the article I found this table on page 3. It confirmed the items I had translated!

. When my son is in Asia next spring, buying gems, I hope he will discover the whereabouts of Anton. I suspect he is in Nepal; he had a cutting shop there and was forced to close LAL in Moscow. I last 'talked' with him in January 2010. I can't find Yuri Shelementiev, of MSU, either. Can anyone help?

 

[stone-cold11]

We have formed sort of an 'old boy' club here'; sorry, folks, I apologize. Note that no one else asks what we're talking about; I wonder if anyone else is even reading this thread.

Reading with great interest

Ditto.

 

[michaelgem]

"No light/dark contrast edges in the illumination environment, no fire, period. And this sweeping statement includes the concept of fractioning." MDC

. I disagree; dark edges have nothing to do with 'fractioning'. See illustration in my "Fractioning" article (repeated earlier in this thread: page 4, 03 Nov 14:12). That effect comes from a single white light source approaching at a certain angle. BLH

Bruce,

The reason I included fractioning in the sweeping statement: "No light/dark contrast edges in the illumination environment, no fire, period. " was to make the point that without the contrast needed for spectral fanning there can be no fire even by the mechanism of fractioning, (since it requires spectral fanning).

I have reason to believe that In normal diamond viewing and illumination circumstances fractioning is at best a minor contributor to the play of fire seen by the observer. But to whatever degree it does or does not occur, fractioning first requires spectral fanning .

Spectral fanning occurs from contrast edges in the illumination. These edges do not have to be the special case edge of your single white light source and its relatively dark immediate surroundings. They could as easily be the edges between the bright sky light and the observer's ear. Both result in spectral fanning from dispersion upon entry at an angle to the gem's facets. The fanned out spectra in both cases, through your mechanism of fractioning, can conceivably then be split diverting one spectral component in a different direction from the other.

So my point was: Without some form of contrast, whether it be your single white light source or not, there is no spectral fanning, no fractioning, and no fire.

Michael

 

[michaelgem]

BLH> Thanks for your explanation of 'virtual facets'. I knew what it was but not by that name = non-communication. I took GIA Diamond Course - 35 years ago!
. Haven't you confused 'GIA' and 'Peter' (Yantzer of AGS)? (in your post re 'virtual facets').
. In addition, newbies don't recognize first-name-dropping stuff; I confess that I have done the same thing (e.g. 'Anton', 'Sergey'). It must seem as smugness to them; it IS rude. We have formed sort of an 'old boy' club here'; sorry, folks, I apologize. Note that no one else asks what we're talking about; I wonder if anyone else is even reading this thread.

Bruce,

Re: "'Virtual facet' is a term attributed to the GIA and their early diamond course. Back when I was working for AGS via 8*, I tried to convince Peter to use more descriptive words, and even got him to consider "compound mirrors". But once a term like this one is established, it is fruitless to use any other term." MDC

There is no confusion on my part, although I see what you mean that newbies might be confused. However, I suspect few that have an interest in this thread, are not well acquainted with Peter Yantzer, Sergey Sivovolenko, and now Anton Vasiliev.

Folks are likely unaware of my involvement with AGS and long time association with Peter, so to clear up the confusion: I was the research engineer performing the work on the contract with AGS, which was won by 8*, and which brought the cut evaluation technology to AGS that began as Al Gilbertson's invention, modified and improved by me, and finally bought by AGS and modified into the ASET technology currently used to determine the AGS cut grade.

Michael

 

[Garry H (Cut Nut)]

I would like to add to my earlier reasons why contrast from observer or leakage is needed to see fire.

If the diamond is in an evenly difuse igloo, and you drill a small hole in the side so the sun can shine on the diamond, and if you look from the correct position you should see fire from the specific facet (or if you are too far away then you will see a blinding bright zone).

 

[beryl]

Gentlemen:
. Read my article on "Fractioning of Light by a Gem" again and look at the illustrations. There is a single source of collimated light. There is no 'spectral fanning' prior to entry, anymore than there is of light from the sun passing through a plain prism.
. Golly gee, Batman - a new buzzword: 'spectral fanning'! I know the phenomenon but not this name. Technical reference, please, Michael.
. The special case of 'fractioning' is that the prism is tipped at such an angle that only part of the spectrum leaves the gem. That's what I thought was interesting about it. I had never before seen the term 'fractioning' applied to light, but am familiar with it in crystal formation (as in pegmatite) or in petroleum cracking where, in each case, different components are separated at different temperatures and pressures. So I am guilty of applying this term to light; blame me.

 

[Garry H (Cut Nut)]

Gentlemen:
. Read my article on "Fractioning of Light by a Gem" again and look at the illustrations. There is a single source of collimated light. There is no 'spectral fanning' prior to entry, anymore than there is of light from the sun passing through a plain prism.
. Golly gee, Batman - a new buzzword: 'spectral fanning'! I know the phenomenon but not this name. Technical reference, please, Michael.
. The special case of 'fractioning' is that the prism is tipped at such an angle that only part of the spectrum leaves the gem. That's what I thought was interesting about it. I had never before seen the term 'fractioning' applied to light, but am familiar with it in crystal formation (as in pegmatite) or in petroleum cracking where, in each case, different components are separated at different temperatures and pressures. So I am guilty of applying this term to light; blame me.

Beryl we can fraction off either end of the spectrum, but we cant do the entire distillation

 

[beryl]

Garry:
. You are correct. You can only split off either end of the spectrum - red by taking it away, or blue by having only that left (in the prism, to exit somewhere else).
. But notice, in the illustration*, where I split off one end from red to green = a new partial spectrum; I could then split that spectrum with another prism to leave only green remaining in that prism to exit somewhere else. An academic thought, perhaps of no value; I think I may try it someday. Too bad we can't save each color in a bottle as we can when fractioning liquids.
* In that example I used only RGB spectrum; the effect would be less noticeable if I included yellow; I'll try it.

. I am still intrigued by the tale of the Japanese man who grew super tomatoes in his office by bringing the sunlight from above the building. By what method? What did he do to the light, in the process, to cause this?

PS: You mentioned fluorescent light bulbs somewhere. I'm sure you know they are available with various spectra (Anton discusses these in a Russian article). I use certain ones to start my plants indoors; note that most plants reflect (thus appear) green, which possibly means they don't want it. Is this a different form of fractioning? I don't think so, but they do separate green from the other colors somehow. Food for thought but nothing to do with gems; sorry to digress.

 

[michaelgem]

BLH> . Read my article on "Fractioning of Light by a Gem" again and look at the illustrations. There is a single source of collimated light. There is no 'spectral fanning' prior to entry, anymore than there is of light from the sun passing through a plain prism.
Golly gee, Batman - a new buzzword: 'spectral fanning'! I know the phenomenon but not this name. Technical reference, please, Michael.

The term "spectral fanning" I formed from your translation of Anton's Rainbow article where he uses the shorthand term fan to describe the "fan-shaped bundle of rays" resulting from dispersion of a beam of white light. Reference:
"The beam leaving the prism forms a fan-shaped bundle of rays of various colors. ... Should all this 'fan' enter the pupil of the eye, the eye's lens will focus all of it into a white-colored dot on the retina. However, if the fan area exceeds pupil size at the eye location, then only a part of the 'fan' cut-off with the pupil would be focused on the retina. … If the angular 'fan' size exceeds the angular pupil diameter, …"

So, when one sees the term "spectral fanning" read "white light dispersed into the colors of its spectrum".

Sorry you got the impression I was talking about spectral fanning prior to entry. In the following statement what I said was: "Both result in spectral fanning from dispersion upon entry at an angle to the gem's facets"

"Spectral fanning occurs from contrast edges in the illumination. These edges do not have to be the special case edge of your single white collimated light source and its relatively dark immediate surroundings. They could as easily be the edges between the bright sky light and the observer's ear.

Both result in spectral fanning from dispersion upon entry at an angle to the gem's facets. The fanned out spectra in both cases, through your mechanism of fractioning, can conceivably then be split diverting one spectral component in a different direction from the other. "

I like your term "fractioning" applied to the fanned out spectrum of white light. I immediately began thinking of the term as perfect for communicating more than: "only part of the spectrum leaves the gem. "

In this more general usage, both parts of the spectrum eventually leave the gem, just in different directions. So I guess your definition of fractioning is more specific than the splitting of the spectrum diverting one segment in a different direction than the other. If you don't object, I'd like fractioning to include this general meaning of "spectral splitting" i.e. diverting one segment of the fanned out spectrum of white light in a direction different from the other. If you are unhappy with this expansion of meaning I will substitute "spectral splitting" for "fractioning".

In light of these definitions perhaps the following points will be better appreciated.

Bruce,

The reason I included fractioning (spectral splitting) in the sweeping statement: "No light/dark contrast edges in the illumination environment, no fire, period. " was to make the point that without the contrast in illumination needed for spectral fanning upon entry there can be no fire even by the mechanism of fractioning, (since it requires spectral fanning).

To whatever degree fractioning/"spectral splitting" does or does not occur, it first requires spectral fanning .

Spectral fanning occurs from contrast edges in the illumination. These edges do not have to be the special case edge of your single, collimated, beam of white light and its dark immediate surroundings. They could as easily be the edges between the bright sky light and the observer's ear. Both result in spectral fanning from dispersion upon entry at an angle to the gem's facets. The fanned out spectra in both cases, through the mechanism of "spectral splitting"/fractioning, can conceivably then be split diverting one spectral component in a different direction from the other.

So my point was: Without some form of contrast, whether it be your single, collimated, white light source or not, there is no spectral fanning upon entry, there is no fractioning/"spectral splitting", and there is no fire.

Michael

PS Sorry if this appears tedious.

 

[beryl]

. I disagree.
. I will stick with the term 'fractioning' because it is analogous to that term in crystallization and petroleum cracking. Your term 'spectral splitting', however, is a good definition of the term.
. There is another phenomenon of diffraction of light flowing over an edge, like water flowing over a weir or through an orifice; I thought that was what you were referring to as 'spectral dispersion'. I think of what you are describing by the simple term 'dispersion'
we have always used; we don't need a new one.
. Let's get back to my original question to Garry: "Why do you think the fire is more pronounced if the stone is cut on the edge of 'Bruce's dark zone'?" Your words do not explain it to me.

 

[michaelgem]

. I will stick with the term 'fractioning' because it is analogous to that term in crystallization and petroleum cracking. Your term 'spectral splitting', however, is a good definition of the term. BLH

I agree Bruce, and will continue to use your shorter, more elegant term fractioning in the context of spectral splitting.

As well, I will also continue using the term spectral fanning as a better and more descriptive term than dispersion to describe the fan of spectral colors resulting from the dispersion of white light upon entry or exit at an angle to or from the gem's facets.

Michael

 

[michaelgem]

. Let's get back to my original question to Garry: "Why do you think the fire is more pronounced if the stone is cut on the edge of 'Bruce's dark zone'?" Your words do not explain it to me. BLH

Hopefully others find my words more adequate, since they contain the explanation. They also explain the more encompassing general principles of light performance in a diamond, those about contrast brilliance and fire.

In Anton's Rainbow article he says: "The fire from a faceted gem will be completely lost in the light of a dull day under cloudy sky. " This statement is true if the ground is covered in snow and there is no observer causing contrast in the illumination by obstructing part of the cloudy sky's illumination.

His words miss the key point that even in the light of a dull day under cloudy sky I still observe and you can see in these photographs fire in an Ideal cut diamond whose virtual facets pick up fire at the contrast edge in the illumination environment due to the ever-present observer obstruction.

From my article: http://www.acagemlab.com/articles/FirePower.htm

Here is a partial analysis of the mosaic pattern of reflections including the blue fire in the attached two and a quarter carat round brilliant cut diamond.

We are viewing this diamond at a slight tilt from the normal, face-up position. The mosaic pattern of reflections from the octagon shaped table contains slender, needle like reflections from the eight pavilion main facets, which meet at the center of the pavilion at the 'culet'.

The two reflections at two and seven o'clock display blue fire, the two at one and five o'clock are dark, the one at nine-thirty is bright and those at four, eight and eleven are partially bright and partially obscured by other reflections. On both sides of the dark main reflection at one o'clock are triangular reflections of blue fire from the halves.

Looking at the photograph of one of the diamond ring's four prongs, you will see mirrored in that prong the panorama of illumination surrounding this diamond. Using our knowledge of both ray tracing and the interaction of the contrast in illumination with the diamond's dispersion properties, we can point out, from the illumination environment reflected in this prong, where each pavilion main reflection originates in the diamond's surrounding .

The two dark main reflections at one and five o'clock are coming from high angles, where the silhouette of my head and outstretched hand is obscuring the bright sky light.

The bright reflections are coming from the bright sky light between my silhouette and the dark silhouette of the surrounding trees.

The reflections of blue fire are coming from high angles right at the edge of the dark-bright transition from my dark silhouette to the bright skylight.

Following is a useful way to think of the fire producing mechanism, which involves dispersion and contrast (like that from observer obstruction). Another form of fractioning/spectral splitting is involved that retains one end of the spectrum or the other :

Using back-tracing from the aperture of my pupil or the camera's lens, we find that due to dispersion a virtual facet reflects long wavelengths from slightly different spots than shorter wavelengths. For instance, virtual facets emanating blue-fire are reflecting/picking up the red-to-yellow end of spectral wavelengths from slightly higher angles due to less bending or refraction of longer waves. But that is where there is darkness from my silhouette, so that end of the spectrum is actually clipped off (fractioned) leaving the blue end of the spectrum whose shorter wavelengths are refracted and dispersed to a greater degree. Since the virtual facets are reflecting/picking up the blue end of the spectrum from this greater angle where there is white light the blue end of the spectrum comes to our eye as spectral fire.

This is why simple head obstruction most often produces blue fire. The reverse situation can occur around an ear or shirt collar where instead of a dark-bright transition there is a bright-dark one. Then the blue end of the spectrum is clipped leaving yellow to red fire to be observed.

As Bruce, Garry and others have noted, this simple dark/bright or bright/dark contrast transition can only result in the fractioning of one or the other end of the spectrum, which is why contrast of this sort from mechanisms such as head obstruction only result in blue and yellow through red fire, but no green. Dark-bright-dark contrast patterns where the bright area has small angular size as in Bruce's collimated beam of white light are necessary for dispersion in the gem to result in the fractioning out of both ends of the spectrum leaving the more elusive green fire to be observed.

With all this detail I hope it is clearer why I said at the beginning:

Re: can you suggest why dispersion is most noticeable here?

The short answer is: Fire emanates from "virtual facets" that reflect from a spot in the surrounding illumination where there is light/dark contrast. With slight movement a reflection from a virtual facet transitions from bright to dark and in between exhibits fire. So reflections from high contrast areas such as the boundary created by observer obstruction result in the display of fire in virtual facets reflecting from those areas. So, not only is the contrast in the illumination from observer obstruction one key to the Ideal's contrast quality of brilliance, it is a factor in the Ideal's superior fire as well.

That is one reason I say "the same diamond exhibiting superior brilliance in lighting conducive to brilliance will exhibit superior fire in lighting conducive to fire." Observer obstruction is a built in producer of contrast which is important in lighting that is otherwise diffuse like the sky in my silhouette image.

So, one key property of the Ideal cut is that reflections from virtual facets from its mains come from the area where there is contrast in illumination at the outer edge of the observer's head. That fact may be one important reason that first Morse and then subsequent cutters like Tolkowsky and his father empirically found a 41 pavilion and close to a 34 crown to be best.

Michael D Cowing

 

[Garry H (Cut Nut)]

Would it help you 2 if I set up a dummy thread for you both to practice using the quoting system?
It would certainly help all of the readers.

Michael you writting style is repetitive enough, without forcing us to the exact same passage over and over in your re-quotes that include you earlier quotes.

BTW it is not hard to do.

BLH>

Spectral fanning occurs from contrast edges in the illumination. These edges do not have to be the special case edge of your single, collimated, beam of white light and its dark immediate surroundings. They could as easily be the edges between the bright sky light and the observer's ear. Both result in spectral fanning from dispersion upon entry at an angle to the gem's facets. The fanned out spectra in both cases, through the mechanism of "spectral splitting"/fractioning, can conceivably then be split diverting one spectral component in a different direction from the other.

Michael

PS Sorry if this appears tedious.

Michael you are correct that there can be colors created at the intersections of bright sources and less bright ones in the environment.
I am sure you all know Alan Hodgsons refaction technique of holding a stone to the eyeball.
Interestingly ao this with a CZ and as you draw the stone away from your eye you can see that there does not seem to be much dilution of the green.
You will need to do this very simple experiment yourself to know what I mean - tried to photgraph it and its hard.

(fractional distillation will need to come from a different source Beryl)

But I doubt there would be any suitable intensity of these rays leaving the stone to be evident to an observer unless the person was in a black igloo with a small hole to allow the rainbow in.

(hahah - if the hole was small enough it will create its own diffraction rainbow and then you could possibly position the stone so that only one fractionated color fell on the stone)

Question: is the fringing color we see an attribut of the gemstone (or my eyelashes) or the light source?

 

[beryl]

. Garry: I am unaware of this Hodgson thing. Reference, please.