







Faceting limits

By Bruce
L. Harding
Holden, Massachusetts, USA
Copyright GIA 




Figure
7a
(click image for big view) 

Figure
7b
(click image for big view) 
Figure 7A shows the extreme angles
of a ray reflecting off both sides of the pavilion: min
= CP (negative) and max
= 180°C3P (because D = 180°4P). The corresponding
external angles fmin and fmax
are the limits at which the viewer can see reflections through
the table from the far and near sides of the pavilion, respectively.
Angle fmin is therefore the limit for seeing
reflections from both sides at once.
These limits are noted across
the top of the faceting chart in Figure 7B for each pavilion
slope. Note that as one limit increases, the other decreases.




Figure 8a 
For ultimate liveliness it should
be possible to see reflections in the table from both sides
of the pavilion with both eyes at once. At a viewing distance
of one foot, as shown in Figure 8A, the angle between reflections
to both eyes (two different rays) is about 12°. To see reflections
from both sides with both eyes, therefore, the minimum external
table reflection angle fmin must be at least 6°.
Designs which do not satisfy this
condition are shaded light at the left of Figure 8B. 



Figure 8b
(click image for big view) 
The faceter should choose a
design from one of the lighter areas of the chart according
to that he thinks is «best»
 increase pavilion slope for wider range of «live» center,
 decrease pavilion slope for wider range of table reflections,
 increase bezel slope for greater dispersion,
 decrease bezel slope for greater brightness.











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