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Quote this post and reply to it Post#1 @ 12-05-02 , 04:56 PM


To the computer scientists and physicists out there, a technical question:

The firescope, ideal scope, lightscope all basically work on the same principle:

Red light (coming from above) represents light return reflected back; and that white or desaturated red (pink) represents light leakage.

Now this is not strictly true. Red light does indeed represent crown lighting which returns to the viewer, but white or pink light represents pavilion lighting (light from behind the diamond) which leaks from the other direction. It does not actually represent crown light leaking.

Now this seems pedantic, but we have assumed that the amount of light transmitted from behind the pavilion to crown would be the same as from the crown to behind the pavilion.

It may appear intuitively correct but is it mathematically/optically so?

I have modelled pairs of single ray white light paths on diamcalc in which the ray paths enter and exit in exactly opposite directions. The results suggest that in this single ray model, exiting light intensities are not equivalent in both directions.

(I have noted a thread where monochrome light does not behave the same way but enter and exits at the same intensities)

Of course this may not be the case if we consider the total sum of light exiting from opposite sides in a diffuse light, multi-ray model.

Let us consider a hypothetical situation in which the leakage from crown lighting is different from pavilion lighting.

Let us say the diamond reflects crown lighting extremely well but hardly leaks any light. However, white light from behind the diamond can easily leak out the crown. This renders the otherwise red image, a pale pink. Here we have an example of a diamond which we know should have minimal light leakage but due to light being able to enter from behind, prejudices the appearance of the stone.

Is this scenario possible? If it is, this may cause the firescope to unfairly evaluate otherwise well-cut stones.

Can we model diamonds with diamcalc and quantitate light leakage in both directions - one with the crown lit and one with the pavilion lit?

Are these two always equivalent?

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Quote this post and reply to it Post#2 @ 12-05-02 , 05:59 PM


Try a search on this. I think the pros do not consider light reversible.

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Quote this post and reply to it Post#3 @ 12-06-02 , 04:06 AM


. A light ray which enters a gem at one point and direction will leave by another; another ray would follow the same path in reverse, exiting in the reverse direction if shined from the reverse direction . In that sense light is reversible in a gem.
. Consider intensity, however. The light entering one way is partially reflected as it enters the gem and partially reflected back in as it leaves the gem. So in either direction the percent of light soutce that exits is less than that which entered (the percentage is the same in both directions). So in this sense the amount of light at a given point and direction is not the same as in the other direction.
. Futhermore, the partial reflections at entry and exit are usually a different amount and direction in each case, so this is not the same in the reverse direction.
. This applies 'simply' to monochromatic light. If the light ray consists of several colors, that which enters is dispersed as it enters the gem and even more as it exits from a broader area. To make this ray reversible you would need to shine all the components in the appropriate directions at the appropriate points to reconstruct an exit ray like the original entry ray.
. So light IS and IS NOT reversible, depending on the context of your discussion. None is lost: it all comes out somewhere, sooner or later.
. All of this has been discussed and argued in this forum. I have offered illustrations of specific points, and so has Sergey; I think we agree. I will withhold further elsboration until I see what Sergey offers.

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Quote this post and reply to it Post#4 @ 12-06-02 , 05:14 AM


. Replying to your specific point regarding FireScope and IdealScope images:
- red or pink light is that from above which is reflected to the viewer's eye in the same position above the stone;
- black appears where no light from above is reflected to the viewer's eye in the same position above the stone, and where there is no light transmitted from below;
- white light is light from below that enters the pavilion and exits the crown to the viewer's eve in the same position above the stone. Light from above in the same direction at the same point of the crown will exit the pavilion at the same point and direction = reverse PATH.
. 'Leakage' is common in most other gem materials; it is only in stones of high RI, such as diamond, where it can be totally eliminated, and then only in a very narrow range of viewing. Leakage is not necessarily a 'bad' thing with regard to beauty.
. The functions of the instruments you mention are ...
1) to show how much leakage there is, and where, and
2) to show the geometrical perfection of the cutting (by the geometrical perfection of the pattern).
Neither of these says if the stone is pretty.

Last edited by beryl : 12-06-02 at 06:04 AM.
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Quote this post and reply to it Post#5 @ 12-06-02 , 07:21 AM


Dymonite,
See http://www.diamondring.com/forums/s...=&threadid=9915
http://www.gemology.ru/cut/english/grading1/ll_mono.htm

It can help you.
Ö.
It is necessary to distinguish energy and intensity.
Energy of a beam is reversibility like ray track
Intensity is not reversibility
For a beam with zero diameter there is no difference between intensity and energy.
You tested energy or intensity in DC?

Hi, Bruce
Is it start again?

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Quote this post and reply to it Post#6 @ 12-06-02 , 07:44 AM


GIA opinion:

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Post 105 - #127927 04-02-02 06:17 AM
--------------------------------------------------------------------------------
GIA Comments on Reverse Ray Tracing
Dear Bruce,
. You asked about comments on reverse ray tracing-I can tell you that our conclusion is very straightforward-- If forward and reverse ray-tracing doesn't give the same results for each ray of a given wavelength, then something is wrong with the program.
. Now, it is very critical that when you are reversing that you are thorough and include all of the stuff--exit direction, amplitude, polarization state, etc. This means that what you
would get out in one direction is exactly what you would put back in the other. However, there can be huge differences in terms of how long the computation takes in the computer.
. Some geometries can have convergence problems tracing in a particular direction and end up taking lots of extra time.
. Even though we may not have said this in the article, we are tracingforward for fire-why? Itís easier on the computing time (this is actuallybecause we have a point source and a huge detector). When we first wrotethe paper on WLR, we were tracing backward for WLR -- it was faster in that
mode. By the time we had published the paper, we had found a faster way todo it forward (obviously with identical results). We've continued tracing forward as we explored variations in the brightness metric (in other words, we did it both ways and checked the results). These are programming
considerations, not matters of the underlying physics. If we had infinite computational resources, we could have assembled the fire problem to trace backward, and it still would have yielded the same VFI plots. That would have been the hard way to do it.

. I guess the underlying problem is that people are trying to guess at what we have done or might do in many areas. They are guessing without asking us and this doesn't make sense. However, in some areas, even if they ask us, we might not be able to answer. Why? It might be premature to provide an
answer. Also, we canít spend all of our time answering everyoneís questions and comments-we wouldn't get anything done. So we donít answer very many questions or comments, since that would start a deluge of questions. Unfortunately, we canít anticipate all the questions or assumptions people
make when we write an article.
. For example, just because we didn't include surface reflections in the WLR metric, doesnít mean we havenít considered them at many stages. An aspect not mentioned in our fire article is that we did look at surface reflections for it--On page 182 of the fire article there is a VFI diagram that has the normal spread of colored spots, but also has small black crosses on it. These are the points of surface reflections
from the diamond. This is an example of the fact that we donít talk about all aspects of everything we explore. To go down the path of telling all the details of everything we do is just impractical.

. I hope this clarifies some things for you. Yes, we trace rays both ways and when we get into lots of data crunching we choose the most practical route for time. Yes, you can pass this on to others, or post on DiamondTalk.
. Regards, Al. (Al Gilbertson, GIA)

PS by Beryl: This public response was unsolicited by me, but offered by GIA.

[Edited by beryl on 04-02-02 at 06:28 AM]

http://www.diamondring.com/forums/s...le&pagenumber=5

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Quote this post and reply to it Post#7 @ 12-06-02 , 08:50 AM


Quote:
Originally posted by Serg
Dymonite,
See http://www.diamondring.com/forums/s...=&threadid=9915
http://www.gemology.ru/cut/english/grading1/ll_mono.htm

It can help you.
Ö.
It is necessary to distinguish energy and intensity.
Energy of a beam is reversibility like ray track
Intensity is not reversibility
For a beam with zero diameter there is no difference between intensity and energy.
You tested energy or intensity in DC?

Hi, Bruce
Is it start again?


Serg,

I presume that you are saying that intensity is a function of energy of light beam divided by the area through which it passes.

i.e. same energy; wide beam = low intensity, narrow beam = high intensity

Therefore those percentages % shown on diamcalc ray traces must be representing energy?

Therefore, my question would be better phrased with 'energy' substituting for 'intensity'?

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Quote this post and reply to it Post#8 @ 12-06-02 , 09:12 AM


Quote:
Originally posted by beryl
. Replying to your specific point regarding FireScope and IdealScope images:
- red or pink light is that from above which is reflected to the viewer's eye in the same position above the stone;
- black appears where no light from above is reflected to the viewer's eye in the same position above the stone, and where there is no light transmitted from below;


No problems with this statement


Quote:


- white light is light from below that enters the pavilion and exits the crown to the viewer's eve in the same position above the stone. Light from above in the same direction at the same point of the crown will exit the pavilion at the same point and direction = reverse PATH.



I agree that paths are the same, but from what you have stated previously, is that the energy of the light may not be. In which case, it is possible for white leaking out crown to be greater than red light leaking out pavilion i.e. white light may OVERREPRESENT the actual degree of crown leakage.

Quote:


. 'Leakage' is common in most other gem materials; it is only in stones of high RI, such as diamond, where it can be totally eliminated, and then only in a very narrow range of viewing. Leakage is not necessarily a 'bad' thing with regard to beauty.



What is the point of eliminating light leakage in a very narrow range of viewing, when stones are viewed from a wide range of angles? What are the distinguishing features of bad and good light leakage?


Quote:

. The functions of the instruments you mention are ...
1) to show how much leakage there is, and where, and
2) to show the geometrical perfection of the cutting (by the geometrical perfection of the pattern).
Neither of these says if the stone is pretty.



In summary, how much value has the firescope for evaluating the beauty of a stone? What other factors should you consider? How does one actually make a COMPLETE assessment of the beauty of a stone without having to view dozens of them side by side? The seeming advantage of a firescope is it does not seem to rely on comparisons but an evaluation can be made by examining a stone in isolation.

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Quote this post and reply to it Post#9 @ 12-06-02 , 09:21 AM


Quote:
Originally posted by beryl
. So light IS and IS NOT reversible, depending on the context of your discussion. None is lost: it all comes out somewhere, sooner or later.


Yes, but this my point - it all comes out eventually, but depending on your light source, in different directions and of different energies.

Now to answer my original question, is the proportion of light that leaks backwards from crown lighting exactly equal to the proportion of energy that leaks forwards from pavilion lighting?

OR

Using the firescope example - is the fraction of red light leaking out the pavilion equal to the fraction of white light leaking out the crown?


Quote:

. All of this has been discussed and argued in this forum. I have offered illustrations of specific points, and so has Sergey; I think we agree. I will withhold further elsboration until I see what Sergey offers.


Yes, and does this support my above concerns?

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Re: Single ray paths
Quote this post and reply to it Post#10 @ 12-06-02 , 09:39 AM


Quote:
Originally posted by Garry Holloway
Dym you seek to over simplify.
Sorry I could not respond to your email earlier this week. I had a presentation at a Diamond conference in Perth and was very busy all week.



I understand and not offended I thought I would expand the discussion on the board. Yes, I agree about oversimplification but I wanted to confirm my first concern regarding single rays and white light.

Quote:



if you choose to follow single ray paths each way for each frequency of light you will find full reversability - but you must also account for reflections.



yes, and as I gather from Serg's post - accounting for polarization, amplitutde etc

Quote:


But life is not so simple. When you look with an Ideal-scope most areas of the diamond are returning rays from many hundreds and thousnads of directions. In a section that is entirely white - most of the light has come from behind. If you shone a ray in that direction from the viewing area then you would expect most of the light to be refracted out the pavilion - however there will still be at least a 17% reflection off the crown of the diamond that will actually most probably be sourced from the red reflector.


Is this a potential problem if the backlighting overwhelms the crown lighting?

This also returns to the issue from the other thread re peripheral light return.

I am still convinced that variations in crown lighting situation must be influencing the effect of black areas on the diamond as the head retreats from the diamonds. Put simply the head shadow gets smaller.

Now, I am not saying that the fixed black star pattern will be changed. You have convinced me that these are generated by perpendicular rays incident and reflecting from the diamond, and are irrespective of head distance/viewing distance. However, rays entering and exiting from other angles must have some net influence on the image perceived by the viewer, shouldn't they?

Is this not so? I still need to be convinced otherwise.

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Quote this post and reply to it Post#11 @ 12-06-02 , 09:40 AM


Quote:
Originally posted by dymonite
Serg,





Therefore those percentages % shown on diamcalc ray traces must be representing energy?

Therefore, my question would be better phrased with 'energy' substituting for 'intensity'?


1)diamcalc ray traces can representing both energy AND intensity, FOR YOUR
CHOICE
2) I do not know what do you want?

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Quote this post and reply to it Post#12 @ 12-06-02 , 09:49 AM


Finally,

Just to illustrate my point about what I am mean by reversibility so that we are not talking at crossed purposes.

See the following attachments:

(I have only included one set of views for demonstration purposes)

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Quote this post and reply to it Post#13 @ 12-06-02 , 10:45 AM


. Slightly off the subject, but needing to correct a point of common misunderstanding ...
. As the head moves away from the stone, its 'SHADOW', by light sources behind the head gets BIGGER. however, the angular range of rays 'OBSTRUCTED' by the viewer's head gets SMALLER.
. Head 'SHADOW' and head 'OBSTRUCTION' are two different things. "SHADOW" is blockage of light incident on the surface of the gem, while "OBSTRUCTION" is blockage of rays which would be returned to the viewer's eye if his head weren't in the way (e.g.: a light-bulb on his nose).
. This was illustrated and discussed in DT. Many still do not see the difference. See http://www.diamondring.com/forums/s...ead.php?t=10173
post 79191, 9-28-01 6:36 PM

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Quote this post and reply to it Post#14 @ 12-06-02 , 12:31 PM


Quote:
Originally posted by dymonite
Now to answer my original question, is the proportion of light that leaks backwards from crown lighting exactly equal to the proportion of energy that leaks forwards from pavilion lighting?

. I will only say that a single ray (zero width of single color wavelength) will follow the same path in both directions and will exit at the same percentage of intensity in either direction - commonly about 65% (= 81% at each surface).
. However, the partial reflections are opposite = away from the gem at the surface of entry and back into the gem at the surface of exit.
. That is, if entering the crown, the ray is partially reflected away from the crown, but it is partially reflected back into the gem when it exits the pavilion; in the reverse direction, it is reflected outward as it enters the pavilion and inward as it exits the crown. In this sense it is NOT reversible.
. All this can be seen nicely with DiamCalc.

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Quote this post and reply to it Post#15 @ 12-06-02 , 07:46 PM


Shades of, Peripheral Light Return.




Kirk

www.diamondoptics.com
Exclusive SuperbCert tm Specialists

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Quote this post and reply to it Post#16 @ 12-06-02 , 07:50 PM


Quote:
Originally posted by dymonite
What is the point of eliminating light leakage in a very narrow range of viewing, when stones are viewed from a wide range of angles?

. I guess 'they' assume that if you eliminate it in that one direction, you have minimized it in all other directions, and thus have a generally brighter stone.
Quote:
What are the distinguishing features of bad and good light leakage?

. I think there is not 'good' leakage, just that leakage is not necessarily as 'bad' as is generally implied. In colored-stone-cutting, where 'leakage' is unavoidable and greater in any direction, we seek scintillation and pretty geometric patterns (no one will pay extra for precision cutting). Furthermore, color is the major beauty factor in colored-stones. Diamonds are generally colorless and so reflections are far more important for them.
. A humorous consideration is that, a few hours after it is cleaned, a diamond has lost much of its brightness if it is worn on the hand. Another is that the dark areas in the stone seen by the bald man who buys it are not as numerous as those seen by his lady-friend with a large hairdo (ray obstruction by viewer's head).
Quote:
In summary, how much value has the firescope for evaluating the beauty of a stone? What other factors should you consider? How does one actually make a COMPLETE assessment of the beauty of a stone without having to view dozens of them side by side? The seeming advantage of a firescope is it does not seem to rely on comparisons but an evaluation can be made by examining a stone in isolation.

. Beauty is subjective, not quantitative; what is most beautiful to one person is not to another. An instrument will not make these distinctions. There is a misguided tendency to equate brightness with beauty. 'Brilliance', as defined by some to include dark/light contrast, is a better indication of buyer appeal, but how will an instrument measure this?
. To be of value in estimating beauty, a one-to-one correspondence must be shown between certain instrument patterns (such as 'hearts & arrows')and beauty features. I have seen no proof that such correlation exists.
. I hope to see a quantitative grading system based on measurable cut parameters so that stones can be appraised equitably and that such grading favors stones of good appearance. AGS presently fails in this effort partly because of the way their tolerances are applied. I think that there is no 'best' cut, but a continuum of good proportions, such as shown by GIA, MSU, and HCA, including Garry's FIC, TIC, and BIC types to accommodate buyers' preferences. See www.diamondring.com/forums/showthread.php?t=7427 post#54112 04-08-01 10:01 PM

Last edited by beryl : 12-06-02 at 08:13 PM.
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Quote this post and reply to it Post#17 @ 12-06-02 , 08:37 PM


Quote:
Originally posted by beryl
. Slightly off the subject, but needing to correct a point of common misunderstanding ...
. As the head moves away from the stone, its 'SHADOW', by light sources behind the head gets BIGGER. however, the angular range of rays 'OBSTRUCTED' by the viewer's head gets SMALLER.
. Head 'SHADOW' and head 'OBSTRUCTION' are two different things. "SHADOW" is blockage of light incident on the surface of the gem, while "OBSTRUCTION" is blockage of rays which would be returned to the viewer's eye if his head weren't in the way (e.g.: a light-bulb on his nose).
. This was illustrated and discussed in DT. Many still do not see the difference. See http://www.diamondring.com/forums/s...ead.php?t=10173
post 79191, 9-28-01 6:36 PM


Thanks for the distinctions, an analogy may I will:

Hold your hand up between a projector and a screen and do 'hand puppets'

Hold it close to the screen and away from projector - small, intensely dark shadow - a small shadow but extensive angular obstruction of rays.

Hold if closer to projector and away from screen - larger, but more of a gray shadow - a large shadow but less extensive angular obstruction.

Dos this not presume single point light source?

But is not the hemispheric dome of the GIA brilliance study suggesting a:

Diffuse, with multiple light source all angles pointing towards the diamond?

Does shadow become constant at all distances?

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Quote this post and reply to it Post#18 @ 12-06-02 , 08:43 PM


You guys are giving me a headache*.








*Note to self...don't read light reversibility thread after long day at work.

Last edited by Serlin2 : 12-06-02 at 08:44 PM.
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Quote this post and reply to it Post#19 @ 12-06-02 , 08:47 PM


Quote:
Originally posted by beryl
. I guess 'they' assume that if you eliminate it in that one direction, you have minimized it in all other directions, and thus have a generally brighter stone.



A carefully worded 'they'

Knowing that you have looked at this area quite thoroughly i.e. 'faceting limits', how much can you assume that perpendicular, face-up elimination of leakage is also minimized in other directions? Can you model this mathematically as well?



My n=1 study of the stone I bought suggests that light reuturn is good when tilting the diamond at various angles.

Are there other good 'face-up' stones that do not do so well on the 'tilt' test?

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HCA methodology
Quote this post and reply to it Post#20 @ 12-06-02 , 09:06 PM


This might have a better place on another thread but I like to expand on Beryl's comments on the HCA (see below)


Quote:
Originally posted by beryl

. Beauty is subjective, not quantitative; what is most beautiful to one person is not to another. An instrument will not make these distinctions. There is a misguided tendency to equate brightness with beauty. 'Brilliance', as defined by some to include dark/light contrast, is a better indication of buyer appeal, but how will an instrument measure this?
. To be of value in estimating beauty, a one-to-one correspondence must be shown between certain instrument patterns (such as 'hearts & arrows')and beauty features. I have seen no proof that such correlation exists.
. I hope to see a quantitative grading system based on measurable cut parameters so that stones can be appraised equitably and that such grading favors stones of good appearance. AGS presently fails in this effort partly because of the way their tolerances are applied. I think that there is no 'best' cut, but a continuum of good proportions, such as shown by GIA, MSU, and HCA, including Garry's FIC, TIC, and BIC types to accommodate buyers' preferences. See www.diamondring.com/forums/showthread.php?t=7427 post#54112 04-08-01 10:01 PM [/B]



Would you say that a combination of Ideal-scope and HCA and visual inspection needs to be combined to have an optimal evaluation of the stone?

I was wondering if Garry could answer a question re the HCA methodology that do not appear on his website.

1) How many evaluators were involved in the diamcalc evaluation of the stone?

2) How well did the evaluators correspond in their scores (INTEROBSERVER RELIABILITY)

3) Was each evaluator prevented from knowing the other person's score/evaluation before submission. (BLINDING)

4) Were the diamcalc and virtual fire-scope evaluations done in a manner that it was impossible to know the result generated by the other? (BLINDING)

I am asking this because people would like to be reassured that observer bias was not involved in the scoring.

e.g. 'Oh look, we are now looking at that diamond with the fantastic fire-scope image, mmm I am sure it will look good on the diamcalc sim. Let's have a look.....'

I know that people are usually not that blatant, but there is a quote in medicine 'We find what we are looking for'.

Have you got any comments?

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Quote this post and reply to it Post#21 @ 12-07-02 , 03:20 AM


Garry,

I was re-reading your HCA methodology from your website.

Did you make an overall assessment of brilliance without the benefit of a the firescope simulation - i.e. in a photoreal situation?

Have you ever come across a stone which looked good under ideal-scope and turned out not-so-good? And vice-versa?

Perhaps the best standardized way of viewing a diamond is when the bride is resplendent in her wedding gown?

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Quote this post and reply to it Post#22 @ 12-07-02 , 03:25 AM


Is it a form of vanity to quote oneself?

Quote:
Originally posted by dymonite

Now to answer my original question, is the proportion of light that leaks backwards from crown lighting exactly equal to the proportion of energy that leaks forwards from pavilion lighting?

OR

Using the firescope example - is the fraction of red light leaking out the pavilion equal to the fraction of white light leaking out the crown?





Is it equally valid to examine a stone pavilion side up under ideal scope?

In this example, since white light is crown illumination lighting leaking out the pavilion, then isn't this an equally objective way of quantitating light leakage?



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Quote this post and reply to it Post#23 @ 12-07-02 , 03:37 AM


. I defer to Garry: as creator of the IdealScope, I think he should answer this.

Last edited by beryl : 12-07-02 at 03:39 AM.
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Quote this post and reply to it Post#24 @ 12-07-02 , 04:28 AM


Quote:
Originally posted by dymonite
Hold your hand up between a projector and a screen and do 'hand puppets'.
Hold it close to the screen and away from projector - small, intensely dark shadow - a small shadow but extensive angular obstruction of rays.
Hold it closer to projector and away from screen - larger, but more of a gray shadow - a large shadow but less extensive angular obstruction.
Does this not presume single point light source?

. Yes, but it applies to multiple light source directions individually and thus affects a group of them to some extent.
. I think every jeweler knows that multiple overhead point sources are best for displaying diamonds.
Quote:

But is not the hemispheric dome of the GIA brilliance study suggesting a:
Diffuse, with multiple light source all angles pointing towards the diamond?

. Yes, in their 1998 study re amount of light return. They have been severely criticized by several experts for neglecting obstruction by the viewer's head, but, 'at the end of the day', their indications as to what is 'best' do not include cuts which would have been affected by this, and so the omission is academic. See http://www.diamondring.com/forums/s...05&pagenumber=1 post#52705 @ 03-28-01 2:48 PM (was 'threadid=7305&pagenumber=1 but some of this changed when the DiamondTalk system was changed). See also http://www.diamondring.com/forums/s...ead.php?t=12719 post#105367 @ 02-03-02 03:51 PM
. Furthermore, a great deal more light is obscured, from one side, by the viewer's body. This was the subject of a sequel to "Faceting Limits". A GIA handout, at the '01 Tucson Show, shows many red reflections of a gal in a red dress wearing the diamond. Garry likes to cite a white man in a white suit vs. a black man in a black suit (or something like that).
Quote:
Does shadow become constant at all distances?

. I do not understand this question.

Last edited by beryl : 12-07-02 at 05:05 AM.
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Quote this post and reply to it Post#25 @ 12-07-02 , 04:54 AM


. 1.63 = average RI of tourmaline!
. Discussion of this does indeed justify a new thread.

Last edited by beryl : 12-07-02 at 04:56 AM.
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