BOG and the Fluorite SRB

Bob Keller wrote:
<< I've just published a new feature article online at Bob's Rock
<< Shop, "A Graphical Presentation of Brightness in the Standard Round
<< Brilliant":
<< P.S. There is an as yet unpublished section to this article... Before
<< I bring that online, brownie points plus a rough prize to the first
<< USFG Faceters List member who posts the correct explanation as to why
<< GemRay calculates the greatest brightness for a fluorite SRB when its
<< pavilion mains are cut significantly *BELOW* the critical angle of
<< fluorite.


Here is BOG's take on the situation - with apologies to Arthur Conan Glenn for the lack of literary merit...

(more information about BOG, including download instructions, appears at

I fired up BOG with a standard (Vargas) SRB cut in fluorite. I then selected the pavillion (45 degrees) and crown mains (44 degrees) for reference. With only ISO brightness set as the Merit Function criterion, I ran the Merit Function Mapper and got the plot below (this took less than a minute on my three-year-old PC).

The image at left shows BOG's view of the Case of the Purloined Light Ray.

Clicking on the nominal angles (45 pavillion and 44 crown) produces a rather dull stone, with only 33% ISO brightness and extremely poor star facet performance (see GemRay rendering and ISO values in the left panel).

Sliding over to the left into the better region (red area) to a pavillion angle of 37 degrees produces much better performance - the star facets light up brilliantly. Of course, it is cut well below the critical angle and does fish-eye in the center. Nevertheless, the ISO brightness, at 72% is more than twice as high as before.

Clicking around on other parts of the map does not produce a prettier solution. Some designs, Vargas' SRB among them, just don't do well with low index material.

This example shows how BOG can help pick the best cutting angles, based on the actual stone appearance as well as more objective criteria, such as ISO brightness. For a real stone, I would also include tilt performance (at least) in the evaluation.

The actual reason for this behavious was explained by Gregg "Sherlock" Glenn in a previous post.

Page maintained by Tom Herbst, last modified 05-Oct-2002