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Diamonds and Lighting

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BEDAZZLED

Shiny_Rock
Joined
Sep 8, 2001
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115
What is the best light to view a diamond for optimum brilliance and fire?

My diamonds look very different in flourescent light, sunlight, low light, the light from a lamp ...
 

optimized

Shiny_Rock
Joined
Dec 28, 2002
Messages
306
I believe "diffuse sunlight" is probably the accepted standard for "accurate" viewing, meaning to look at it in "shaded natural" light if you're trying to see the "true" colors of the diamond and its optical traits. Sunlight contains the full spectrum of visible and invisible light (from infrared up through ultraviolet) so has the best color rendering. Direct sunlight can be cool for seeing the raw brilliance, but shading the sunlight allows more of the fire to be exhibited without being drowned out by the brilliance. Also, fire is especially evident in lower levels of light. I've found candlelight to be quite nice for this type of viewing.

Fun to look at them in almost any kind of light though.


-Tim
 

optimized

Shiny_Rock
Joined
Dec 28, 2002
Messages
306
Brilliance is "white light" return. Basically, it's light that enters the diamond through the table, reflects off the pavilion facets, and is then returned back to the viewer as white light. Brilliance is what makes a well-cut diamond almost seem to be illuminated from within.

Fire is "colored light" return. Fire (or dispersion) is light that enters the diamond through the table, reflects off the pavilion facets, but is then split into its component colors by the crown facets (much like a prism). This spectral light is returned to the viewer as colored light, and is what produces the wonderful fiery flashes of "rainbow" colors.

-Tim
 

Beith

Rough_Rock
Joined
Mar 5, 2003
Messages
21
Hi Tim,

I am always confused about Fire and Brilliance. If Fire is "light" enters the diamond through the table, reflects off the pavillion facets, and is returned back to the viewer "split" while exiting CROWN FACETS, then does it mean Brilliance is "light" "unsplit" while exiting the TABLE.

If that is the case, does it imply that stones with a bigger table will exhibit a higher Brilliance "white light return", while a smaller table will have a higher Fire "color light return"? This is kind of anti-tuitive since it also implies that a stone cannot have both maximum Fire and Brilliance at the same time.


I've heard that old European cut has a smaller table as compared to today's standard but they exhibit tremendous fire. How do we really evaluate then a diamond's visual performance, white light return, fire light return, or the summation of the two?

Sorry for all these questions.
 

optimized

Shiny_Rock
Joined
Dec 28, 2002
Messages
306
Hi Beith,

Okay, I'm willing to tackle this one, but it's going to be a bit of a basic physics lesson so prepare yourself.


First of all, the different facets themselves have less to do with whether returned light is white or colored than with the angle at which the light passes through the facets. Basically we're talking about refraction and dispersion here.

Refraction is the bending (or changing of direction) of light as it passes from one material to another, and is caused by the different speeds at which light travels through different materials. If light passes from one material to another at an angle roughly perpendicular to the material (i.e. "straight through" the material) the light will not be bent. However, if the light passes from one material to another at an angle, the light will "bend" (actually, change direction) as it makes the transition. This is called refraction. The faster a wavelength of light moves (i.e. the higher its frequency), the less refraction it will exhibit as it passes from one material to another.

Dispersion is basically a function of refraction. Dispersion occurs when light is refracted at significant angles while passing from one material to another, causing the light to separate into its component colors. Basically, white light consists of all wavelengths of visible light (the full visible light spectrum) all traveling together. However, these wavelengths all travel at different speeds based on their frequency, so as they pass from one medium to another they will bend different amounts. This is the basic principle that causes white light to separate into its component colors while passing through a prism. Higher frequencies of light (toward the red end of the spectrum) will bend less as they pass from one material to another while lower frequencies (toward the violet end) will bend more, thus causing the different colors to spread from each other.

So, the light that passes through a diamond will be returned as white light (brilliance) if the angle at which the light passes through the diamond remains roughly perpendicular to the surface of the diamond at the point at which it makes the transitions, while light that passes through the diamond at an angle will break into its component colors (fire). Thus, since the crown facets tend to have light passing through them at significant angles, they will predictably tend to break the light into component colors more intensely, while light passing straight through the table will stay "white" and be returned as brilliance. This is assuming a light source that is more or less directly perpendicular to the diamonds table. Light entering at angles will follow the laws of physics and be broken into colors according to the refractive index of the materials and the angles of travel, so while light passing through the table is most closely associated with brilliance, any facets that have white light traveling "straight" through them will be seen as brilliance.

This situation indeed does suggest that fire and brilliance are somewhat at odds with each other for a couple of reasons, the first being simply that white light (being made up of all component colors) is more intense than dispersed light, and thus will tend to drown out the more subtle colors (which is why I mentioned soft candlelight as a good way to see fiery performance). The other reason is that the optical conditions that cause brilliance also necessarily reduces fire (meaning, any light passing "straight through" the diamond is light that isn't passing through at an angle), which is sort of what you were talking about when you mentioned the idea of a diamond with a large table having more brilliance while smaller tables would yield more fire. It's important to note though that the table size alone is just one part of the equation. The angles of the facets have a great deal to do with the way the light interacts with the diamond, so simply saying that a diamond with a large table will be more brilliant isn't exactly accurate.

In conclusion, the sum "visual performance" of a diamond is ultimately a personal preference, so there will always be differences of opinion as to whether white or colored light is more important. Some people like lots of brilliance while others like lots of fire. Tolkowsky famously tried to strike the most pleasing balance between the two, but ultimately it comes down to what the viewer likes to see.

I hope that all makes sense but if it doesn't, let me know and I'll try again. Sometimes it's tough to write the scientific stuff.


-Tim
 

Beith

Rough_Rock
Joined
Mar 5, 2003
Messages
21
Wow, this is very very informative.
Thanks a lot. I don't mind the scientific part; in fact, I enjoy it and it helps to refresh my college physics.

Now, the more I learn, the most questions that I have.
Please bear with me. I am very thankful to people that would listen, let alone answer my silly questions.

1. Concerning the brilliantscope from GemEx, what exactly are they measuring w.r.t. Brilliance and Fire. What is the comparison standard and benchmark for High and Very High? I read from another thread that they sum the corresponding white light or color light return pixels together and the plane of reference is the one perpendicular to the table (I hope I got this right).

2. Also concerning the GEMEX brilliantscope. The cert contains a couple of light views. What are the differences between the various views: viewing angle, type of light, etc, etc? Which view is for brilliance and which for fire? In the center, there is an enlarged view with simulation. Is this center view a composite of all the others? I went through the GemEX website and did not seem to locate the definition of all these views. It is probably my oversight.

3. The idealscope gives an indication of light leakage. Does it give any indication as to white light leakage, color light leakage or just overall leakage?

4. Diamondcalc. software: I assume this is a 3D (or 2D)computer model that calculates light ray refraction and reflection of a diamond. Does it take light dispersion into account? Does it distinguish which light path will be split into its component color or the program just assumes all light paths are undispersed?

5. HCA Score: What is the reference for the comparison standard? A HCA < 2.0 is equivalent to what % light leakage? Doe it distinguish which portion is from white or color light return? Does it take in account light sources other than the one perpendicular to the diamonds table?

6. When you stated that "Tolkowsky famously tried to strike the most pleasing balance between the two", are you referring to the ideal cut?

Sorry again for another load of quesetions
I have learned a lot from this forum and all these interesting new tools and scores.


Many thanks again
 

optimized

Shiny_Rock
Joined
Dec 28, 2002
Messages
306
I'll do my best to answer the questions you've posed....

1. Concerning the brilliantscope from GemEx, what exactly are they measuring w.r.t. Brilliance and Fire. What is the comparison standard and benchmark for High and Very High? I read from another thread that they sum the corresponding white light or color light return pixels together and the plane of reference is the one perpendicular to the table (I hope I got this right).

for some decent information about the mechanics of the BScope.

2. Also concerning the GEMEX brilliantscope. The cert contains a couple of light views. What are the differences between the various views: viewing angle, type of light, etc, etc? Which view is for brilliance and which for fire? In the center, there is an enlarged view with simulation. Is this center view a composite of all the others? I went through the GemEX website and did not seem to locate the definition of all these views. It is probably my oversight.

My understanding is that the five light views are standardized views from pre-determined lighting distances, but again I would welcome any information from those with more intimate knowledge of the machine itself. There are no specific "brilliance" or "fire" views per se. The calculations of "white" pixels and "colored" pixels are derived from all of the views and then averaged. The angles of the diamond's facets will determine the performance at any given view. Thus, some diamonds tend to be more brilliant in the #1 and #3 positions, while others will look better in the #2 and #4 spots. And yes, the center view is a simple animation of the other views.

3. The idealscope gives an indication of light leakage. Does it give any indication as to white light leakage, color light leakage or just overall leakage?

Since the Idealscope is basically just showing which portions of the diamond are NOT returning light back to the viewer, it's not making any judgment of what type of light is not being returned. Basically, since all light entering the diamond is typically close to "white light," the leakage is also white. After all, the colors don't really come into the picture until the diamond's facets had its way with the white stuff.


4. Diamondcalc. software: I assume this is a 3D (or 2D)computer model that calculates light ray refraction and reflection of a diamond. Does it take light dispersion into account? Does it distinguish which light path will be split into its component color or the program just assumes all light paths are undispersed?

) lets you play with these features. Fun stuff...

5. HCA Score: What is the reference for the comparison standard? A HCA i]

The HCA is based purely on a mathematical model of what are "ideal" angles/proportions of diamond facets, so the scores are all contrasted to the theoretical "ideal" combinations. It's basically a "better" type of cut-grading system than the static AGS/AGA cut charts in that it is able to compensate for angles that may fall outside "AGS Ideal" range but complement each other well. For example, it accounts for the fact that the overall visual beauty of a stone with a less than perfect pavilion angle can be somewhat compensated for by a crown angle that complements it well, and vice versa.

The overall score attributed by the HCA is the sum of the values derived for the four individual scores it calculates (light return, fire, scintillation, spread). The "leakage" you mention is then somewhat independent of the overall score, but it IS directly represented by the "light return" score.

is loaded with information about how the HCA actually works. Definitely worth checking out.

6. When you stated that "Tolkowsky famously tried to strike the most pleasing balance between the two", are you referring to the ideal cut?

Yep, that would be the legendary "Tolkowsky Ideal Cut."

Hope all this helps....

-Tim

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Adrienne

Shiny_Rock
Joined
Feb 15, 2003
Messages
120
*bump*

This thread is a few months old, but I just read it and found it really interesting... thought a bump for the newer people was in order.

Thanks, Optimized, for writing out such a detailed and understandable explanation!
 
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