Let us turn our attention to perhaps the most complex of the three sets of variables outlined above; the psycho-physiology of the observer. This important factor is the one least studied in the field of diamond cut studies.

a) Contrast.

Figure 2. The average brightness is the same for all examples

The eyes adaptation to a background shows us that brilliance is more than simply light return. Spatial contrast is also important. All 6 pictures in fig. 2 have the same light return, but are differentiated in brilliance; the difference in brilliance is perceptual and is a result of differences in contrast.

	Figure 3. Subjective brightness depends on luminance. Little leakage almost the same as no leakage

b) Subjective brightness depends on luminance; a small amount of leakage will have a much smaller effect on subjective brightness because the relationship between perceived brightness and light return is not linear. For example direct light measurement devices that implement various lighting schemes may reveal partial light leakage through a diamonds pavilion facets. Observation of a leakage area might lead the observer to conclude that this area of the diamond will not appear to sparkle, where-as the partial light return area may still be perceived as sparkling. In an example from fig. 3, one particular area in this diamond may have 50% leakage, but the same area has 85% of subjective light return into an eye.

c) Primary and secondary light sources. Some areas in a diamond are seen as bright sparkles, while other areas are still white (bright) but are not perceived as a sparkle. The nature of 'sparkle' is dependant on light sources that we could consider to be primary and secondary sources.
If we trace a light ray from an eye through a sparkling area in a diamond, and follow it further to its source, the ray would go directly into source of illumination what we call the "primary" illumination source. Follow a light ray through a white (but not sparkling area) and it may go to a surrounding wall, ceiling, floor or other object that reflects light emitted from a primary light source; that is from a "secondary" light source.

d) The stereoscopic nature of human vision results in the brain receiving two images that it 'superimposes', but our mind does not interpret this function like a simple averaging. The following test illustrates the complexity of how our brains create images. The simple averaging of two pictures (fig. 4) seen by each eye does not result in the same image that is seen with one eye from the face up position. Understanding stereovision is essential for evaluation of diamond beauty. This is a problem in the use of any direct assessment method that uses data or images from a single lens view.

Figure 4. Stereoscopic nature of human vision. a) Straight Face-up position; b) Picture for left eye; c) Picture for right eye.

The two pictures fig. 5 below illustrate a property of human stereoscopic adaptation for dark and for light backgrounds.

Figure 5. Human stereoscopic adaptation for dark (left) and for light (right) backgrounds.

e) Sparkle, pupil motion and eye reaction. A special simulation for understanding these phenomena has been constructed in Brill software. It can be played at http://www.cutstudy.com/cut/english/grading1/mirror2.htm

When you play this 'game' you will understand that your mind 'sees' or imagines it sees flashes that are not present. The same happens with diamonds, and it is caused by temporal contrast; it is an important psycho-physiological response to scintillation and is part of the BLR factors that need to be considered by any cut grading system.

From the study of human psycho-physiology we have learned that it is important to evaluate light response of diamond not objectively as a measuring device but subjectively as it is seen by human eye and is perceived by human brain.

What should be taken into account in the accurate illumination model?

Figure 6. Diamond facet as additional diaphragm for light source

As a result of these developments it is now possible to develop a system for grading the quality of diamond cut of any individually scanned diamond using 'Basic Light Responses' (BLR) of the diamond in conjunction with formalized criteria established by experts. This means an expert cut quality assessment system is now feasible that could be applied to any diamond irrespective of its shape or symmetry.

See also "Analysis of different models of illumination", "An observer model" and "Different models of illumination".

Sergey Sivovolenko, OctoNus, Moscow, Russia
Yuri Shelementiev, Gemological Center MSU, Moscow, Russia