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Vision Impacts Sparkle

The science behind how we “see” the sparkle of a diamond.


Above: The dark spots are more vivid when viewed with both eyes open.

Sergey Sivovolenko - CEO of OctoNus Software, developer of the Helium and related products, Finland
Dr. Yuri ShelementievHead of Moscow State University (MSU) Gemological Center Laboratory, Russia
Garry HollowayManaging director (MD) of Holloway Diamonds and Ideal-Scope, Australia
Janak MistryMD & Group CEO of Lexus SoftMac, Surat, India
Roman SerovResearcher at MSU Gemological Center, Moscow, Russia
Stepan ZhulinResearcher at OctoNus Software, Russia
Kristina ZipaResearcher at OctoNus Software, Russia
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   The Cut Group, comprised of Sergey Sivovolenko, Yuri Shelementiev, Garry Holloway and Janak Mistry, has been cooperating for more than a decade with the aim of improving diamond cut quality and helping to decommoditize the diamond market. 

Brilliance is largely an illusion created because each eye sees a diamond from a different direction, creating brain conflict when a facet is dark for one eye and bright for the other. This breakthrough led the Cut Group to study human vision and many optical illusions in viewing diamonds. Those findings were published in The Australian Gemmologist in 2013, with a stereoscopic viewer enclosed with the issue. The research has established new definitions of brilliance, scintillation and fire, which are the basis of developing mathematical models for diamond beauty assessment. The research is testing and modifying these systems against experts’ observations in the group’s controlled light environment.
   Existing cut quality is based on “rejection” of bad stones, but consumers want diamond beauty and performance scoring and ways to compare the beauty of many different diamonds. Retailers and consumers need new language to communicate elements of diamond beauty. The Cut Group’s goal is to design beautiful new cuts and improve existing cuts. Using a music analogy, let’s add “jazz” and “rock” to the current narrow range of “classics.”

BRILLIANCE, SCINTILLATION AND FIRE
   Brilliance is largely a result of binocular rivalry, when each eye “sees” a different intensity at the same spot. Diamond photos show no brilliance, a mono movie and a still stereo image show a little, but a stereo or 3-D diamond movie has real brilliance. The group developed a controlled light box with an advanced video capacity called ViBox (see Box A, below). This finding can be tested by looking with one eye, then both, while rocking a diamond.  

Box A — Vibox and Diamond Stereo Videos

Left: ViBox with the doors open, Canon camera and figure of eight diamond holding gimbal. Right: Doors partly closed, with a stereo splitter and a 360-degree rotating stage.

Left: Stereo splitter. Right: A stereo pair of diamonds captured in Vibox with the OctoNus and Lexus watermark logos. 

   Different lighting, clothing, distance, motion and holding methods change diamond observations. Experts or consumers can compare impressions of ViBox magnified diamond videos at different times and places. Consumer “crowdsourced” preferences of diamonds for sale can be statistically analyzed.
   Software-controlled lighting enables reproducible imaging. Hundreds of still images are made into a movie as the diamond moves through various controlled motions. The stereo 3-D movies can be viewed on popular media players or TVs. Videos are auto-uploaded with optional independent grading metrics.

   Diamond brilliance is an illusion caused when perceived brightness exceeds actual brightness.
   Scintillation is short, bright flashes that appear and disappear quickly when a moving diamond is lit by small bright lights. Brilliance — and fire — are seen in both large, relatively dim illumination and small, bright lighting. Scintillating flashes can extend outside facet edges or even past the diamond’s edge, such as the “bloom” and “star” effects often seen in photographs. More scintillation is seen with both eyes, or in a stereo video, than with one eye.
   Diamond fire is separate, colored flashes characterized by brightness, saturation, size, shape, lifetime (duration) and color gradient. Fire can also have “bloom” and “star” effects. Flashes can be slower — like brilliance flashes — and need not be very bright. The research group determines a diamond’s fire value by counting ViBox movie colored flashes while accounting for the attributes mentioned above for every flash.


This single colored flash changes from red to yellow.

Figure 2, above: Two colored flashes on either side of the white flash change from orange to yellow, separated by the white, then blue and violet.

   Contrast in brightness, color and time is essential for brilliance, scintillation and fire. A polished diamond is a unique optical object; it creates all three types of contrast in our brain at the same time.
   Brilliance is a slow, but not low, contrast from point to point, from time to time. The human brain interprets slow changes as a united image with a wider range of dynamic brightness.
   Scintillation is fast contrast, changing dramatically in a very short time. Flashes appear at one place and then another so quickly our brain cannot see it as one image.
   Fire is color contrast and can be slow or fast. Slow-contrast fire results from large light sources that cause brilliance. Fast-contrast fire is created by small bright light sources as for scintillation.
   These complex definitions are not “sales language.” They are used to create software metrics and visual assessment methods to help develop high light performance diamond cuts.

Human Stereo Vision

Figure 3: The image on the left is what cyclops or a camera mounted between your eyes would see. Center and right are ViBox stereo images or what each eye sees. Viewing the image pair with stereo glasses — included with The Australian Gemmologist article — shows the difference in human perception.

   The three long black facets in each image create “binocular rivalry” in the brain, dark for one and light to the other eye, causing brilliance instead of darkness (as shown in Figure 4, below).

Gray Background

Figure 4: With stereo glasses or a 3-D screen, one eye sees the central white square and the other eye sees a black square. The squares sparkle and blink about two times a second. Diamond facets can sparkle more because they can be considerably brighter than the squares in this test. (See link to this test in Editor’s Note at end of this article.)

   In many cases, our brain receives perplexing, contradictory information from our two eyes, yet the brain always achieves a final image. In Figure 5, below, our brain ignores the reflected glare across the table of the diamond and we see the facets the other eye sees.But rock the diamond and our brain will briefly see the glare before “switching back” to the small facets. This is called the information maximization principle.
   There are limitations to diamond assessment with monoscopic systems. Stereo vision is critical in creating an accurate optical performance evaluation system and metrics. Contradictory information in both optical channels is important for assessment and development of high optical performance cuts.


Figure 5: A diamond photographed in stereo, with left and right eye views. When stationary, the brain ignores the glare on the table and we see the facets under it.

   Rays from left and right eyes entering the same point in the diamond exit in different directions.1 Obscuration (see Figure 6) of light by an observer’s head from 30 centimeters (12 inches) is approximately 8 degrees for each eye. Some existing cut grading systems assume a much greater impact from observer obstruction. Likewise, the importance placed on light leakage is exaggerated because if either eye sees the facet as bright, our brain can perceive brilliance.
   With both eyes, we see approximately twice as many colored flashes, although some cuts show a similar number of fire flashes for one eye only.

Stereoscopic Contrast For Two Eyes

Figure 6: Stereo contrast is seen when a facet is bright for one eye and dark for the other.

Conclusion
   A diamond is a unique luxury product that achieves three types of pleasing visual and illusory rivalry: binocular, spatial — e.g., the shimmering in impressionist paintings — and temporal — e.g., as in the case of a movie. But many optical illusions are not understood and so human visual brain function and perception cannot currently be quantified with metrics. Therefore, the Cut Group proposes two steps:
   1. Where human perception of diamond features matches quantifiable human vision metrics, the Cut Group creates computer performance scores.
   2. Where mathematical models or metrics are not feasible — e.g., many optical illusions and stereo vision contrast cannot be “measured” — the Cut Group checks factors like the probability of seeing stereo contrast facet by facet. This knowledge can be used to design new cuts, improve existing cuts and develop new consumer language.
   Consider a recipe with ingredients and preparation methods. A talented chef’s subjective interpretation makes a big difference. Objective metrics can only evaluate similarities in individual features of performance between two diamonds. Overall diamond beauty can currently only be assessed subjectively by expert observation. Metric algorithms can check observer consistency for diamonds with close characteristics, enabling comparison of different cuts and shapes to each other. By combining subjective and objective assessment, the researchers can begin developing high performance cuts and a new language for consumers. They also can begin to solve the problem of the inability to grade fancy cuts.
   The ViBox video system will enable thousands of highly repeatable objective diamond observations by experts and first-time buyers, creating “crowdsourced” data to produce “cut maps.” ViBox automatic flash calculation metrics will enable human preference matching to help solve problems such as: Which is better – one big, or two small flashes? This question is more complex than it seems because differences in flash size, location, brightness, color and duration must be accounted for. Crowdsourced statistics for at least 5 percent to 10 percent of diamonds sold annually will enable creation of data maps to automate performance evaluation within several years. The Cut Group uses these metrics with 3-D cut models to reject cut candidates with low scores. These metrics and models are combined with experts’ subjective evaluation of stereo DiamCalc software movies before cutting samples.
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1Bruce Harding, Gems & Gemology, 1975.

Editor’s Note: This article is a condensed version of a much longer study.
For more details visit the following sites: 
Order the full article here
The link below contains 3-D diamond demonstration files of ViBox movies, stereo tests, illusions and material referred to in the full article.
3-D diamond demonstration files
For more information, contact:
cut@octonus.com

Article from the Rapaport Magazine - August 2014. To subscribe click here.

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