For decades, the only refractometer I used was a Gem Instruments, Duplex II. This is the best available in the US. In Europe, the Rayner's are regarded as the top brand. Shipping costs prohibit these from being regularly traded across the Atlantic, so most people do not have a choice of brands. However, these are both top quality instruments from highly respected manufacturers.
While writing the review on the No Name Brand Refractometer, (see “Product Reviews” in our Archives,) I read where the Duplex II is the only refractometer designed specifically for spot readings. The no name brand unit was almost useless at spot readings and I wondered how much the smaller size had to do with it.
As an experiment I purchased a Rayner Dialdex, which is about the same size as the no name brand refractometer. I did a size by side comparison of the Duplex II and the Dialdex, both to see how much size has to do with readings and to see how the instruments compared overall.
The sharpness of the shadow line depends on the polish of the facet being tested, as well as the quality of your hemisphere. In this case both hemispheres were in excellent condition. A few dozen gems of various sizes, species, and polish quality were tested on both refractometers, so the variables were minimized.
With the Duplex II, you read the RI where the shadow falls on a scale. While this picture is not very clear, the shadow is not sharp even when you can see it well. It is very hard to read to the thousandth. The addition of a Wratten filter helps some, but then you also have to remember to subtract .005 from your readings.
As you can see in this image, the Rayner has a much sharper shadow line. (The same stone was used for both pictures.) Once you have the shadow in the view finder, you turn a dial on the side. That causes a dark bar to move across the lens. You adjust it so that it is aligned with the bottom of the shadow. (You can see the bar to the right side of the lens.)
Once you have this lined up, you read the RI from the outside of the instrument. In this picture, you can clearly see that the RI is 1.641. In cases where knowing the birefringence is important, this makes a difference.
The advantage in flat facet readings goes to the Dialdex. Both instruments gave identical results, but the Dialdex was clearer when reading to the thousandth.
The real challenge, and what brought this up, are spot readings. Again, the advantage to the Duplex II is its large viewing area, which makes it easier to find the spot. However, it is challenging to get the shadow inside the spot and then read it off the scale. You can focus on the shadow or the scale, but not both and the difference is considerable.
With the duplex, it often takes more searching to find the spot and then the shadow. However, once you do it is fairly simple to align the bar to the shadow. Again, I found the readings clearer and, as a result, slightly more accurate.
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