Standard Round Brilliant Lowboy
GemCad Derived Rendering by Bob Keller
SRB Variation "Lowboy" Angles for R.I. = 1.54 57 facets + 16 facets on girdle = 73 8-fold, mirror-image symmetry 96 index L/W = 1.000 T/W = 0.546 T/L = 0.546 P/W = 0.435 C/W = 0.121 H/W = (P+C)/W+0.02 = 0.575 P/H = 0.755 C/H = 0.210 Vol./W^3 = 0.187 Brightness at 0 degrees tilt for R.I. = 1.54
COS = 82.2% ISO = 87.2%
Pavilion pb 43.00 03-09-15-21-27-33-39-45-51-57-63-69-75-81-87-93 Cut to temporary center point g 90.00 03-09-15-21-27-33-39-45-51-57-63-69-75-81-87-93 Cut to meet pb pm 41.00 96-12-24-36-48-60-72-84 Cut to meet pb,g Crown cb 32.00 03-09-15-21-27-33-39-45-51-57-63-69-75-81-87-93 Cut to meet g - level girdle cm 28.00 96-12-24-36-48-60-72-84 Cut to meet cb,g s 13.00 06-18-30-42-54-66-78-90 Cut to meet cm,cb t 0.00 Table Cut to meet s,cm The most fundamental variations of the SRB share the same indexes for all the facets but vary the angles with which they are cut. Design analysis done with raytracing and light modeling software demonstrates that lower angles are generally brighter angles within the performance envelopes. This 'lowboy' variation is cut with the pavilion mains at 41° - half a degree above the critical angle for quartz, and also employs substantially reduced crown angles compared to the 'classic' SRB. Note that the lower angles yield COS and ISO brightness values which are approximately double that of the classic, albeit arguably at the expense of other desirable properties.
The SRB Lowboy Design File is available for GemCad users.