The Nikon Java applet is cute but it is propaganda. While it may be correct that light will directly enter the objective if one moves from a particular radius of curvature towards the straight line of a cylinder it ignores how competitively made (but nor really competing in the same markets) stereo darkfield units are designed.
The original Gemological Institue of America Stereo Darkfield unit probably developed in the 1930s or 40s had a spherical reflector with a lamp mounted in it and a blocking stop to shield the objective from direct light.
In the 1950s they cooked up a nice increase in efficiency by making an ellipsoidal reflector. You will recall that an ellipse has two foci and the bulb was located at one and the gemstone at the other. This was fairly easy to have spun in sheet metal and the interior surface polished. The only real disadvantage to an ellipsoid was that it increased the depth of the unit.
In the late eighties or early nineties they updated the design replacing the spun ellipsoid with a molded plastic toroid. The plastic is plated with chrome plating and if you didn't notice that it was plastic you would think it was a metal or glass mirror. The working surfaces are completely specular
more so than the old aluminum one. They are lower in profile than the old one. But they are a complete circle not segmented like the Nikon which must show some artifact of that segmentation in the illumination uniformity. How much I don't know. Like Betty's countrymen Nikon charges so much for that particular item that they have not worked their way through the economy to where a bottom feeder such as myself can get his paws on one.
With all due respect to all the great German, American, and Japanese (and anyone else) optical companies none of them ever really built anything like the GIA base. GIA equipped it with B&L or AO stereos. The units were designed for looking at diamonds and gemstones and while one can do this with other bases for ease of use nothing else even comes close.
There is a fellow in Oregon who makes a similar unit here
http://www.gemproducts.com/
Similar units are also now coming from China from Motic who have their own website.
Some of the Chinese and other distributors and manufacturers are going to LED lighting which for Gems use is not advisable.
The two complaints that users consistently register are that when used for extended periods of time the units get rather hot so that if you drop a stone into the well you must have tongs ready to fish it out. Your fingers would get scalded.
Also the 30-35 watt lamps used don't furnish enough light for all uses. Generally enough not for dark or semitranslucent stones.
I have considered upping the wattage and adding a small size fan to draw the heat downwards and out. LEDs do solve both these problems but their spectral curves have shapes that are too funny to judge color by. I expect that in the future these matters will probably be standardized.
The subject of lighting for gemological use was addressed here in a fashion better than I could ever hope to do:
http://www.palagems.com/gem_lighting1.htm
http://www.palagems.com/gem_lighting2.htm
When Dr. Sersen wrote this article, (1990) LEDs in white were not available and Metal Halide Discharge lamps were not as available as they are now.