Using a refractometer
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Author:  Kerensky [ Tue Jul 05, 2011 8:55 am ]
Post subject:  Using a refractometer

When people of like interests get together, frequently they like to exchange information on techhniques they use or specialisms that they have. Here's a little poll for all active GO forum members, be they just interested in gems, collectors, students, hobby cutters or in some aspect of the business.

Click on a button to answer 'yes' and leave blank to answer 'no'. Please answer:
- Only one of options 1 - 3,
- Only one of options 5 - 7

This means you can respond to a maximum of six only of the ten options listed. Totals for 1+2+3 = total polled. Totals for 2+3= total refractometer users polled. etc....

I hope it will be interesting to see the patterns that emerge. I've left this little poll open-ended and all who participate can change any of their answers if their pattern of refractometer use changes.

edit: The software design of the poll doesn't calculate the percentage other than for a simple list. I'll work the stats manually at 1, 7 and 30 days and post here.

Author:  Kerensky [ Fri Jul 08, 2011 8:02 pm ]
Post subject:  Re: Using a refractometer

Stats so far:

Viewed topic - 48
Polled - 10
Regular use of refractometer - 80%
Know the Bright Line method of use - 40%
Use Bright Line method exclusively - 0%
Use Bright Line & Shadow methods - 25% (of refractometer users)
Use Bright Line & Shadow methods - 50% of those knowing about BLM
Use of BLM to measure the dispersion value of cut gems - 0%

Not yet a large enough poll to be statistically significant but, nevertheless, there are interesting trends.

1. The Bright Line method of using a gemmo refractometer is not new, having been around for over three decades at least. It has a good reputation for giving results where observations using the Shadow method are hard to read or are hard to discriminate. It is claimed also to enable measurement of dispersion values (C-F) of tested gems. Yet the major gem schools do not teach the method and (perhaps unsurprisingly therefore) only 50% of our polled refractometer users know about it.
2. No user of the Bright Line method uses it to measure a gem's dispersion value. This surprising result in turn suggests one or both of the following:
- The measurment of dispersion by this method may be too time consuming to be worth the effort.
- Despite claims for it, practitioners find that the method is insufficiently accurate on the measuremnt of dispersion value to be worth the trouble.

If one of our regular Bright Line method users would like to give us a detailed view of their experiences with it, there are probably quite a few of us who would be interested in receieving such opinions, based on their experience with the method.

For those who would like to read up on details of the method and an assessment of its value, there is an excellent paper (published in the Journal of Gemmology) which is also available on line from the authors at

Author:  Kerensky [ Mon Jul 11, 2011 7:02 am ]
Post subject:  Re: Using a refractometer

Stats at 7 days of polling:

Viewed topic - 71
Polled - 13
Regular use of refractometer - 85%
Know the Bright Line method of use - 46% (55% of refractometer regular users)
Use Bright Line method exclusively - 0%
Use Bright Line & Shadow methods - 36% (of refractometer users)
Use Bright Line & Shadow methods - 66% (of those knowing about BLM)
Use of BLM to measure the dispersion value of cut gems - 0%
Use of refractometer to measure RI (only) - 18% (of refractometer users)

Wrap-up commentary on 10th August.

Anyone else finding difficulty with Hoover's description of the 'grazing angle method' (see fig 5a of )? Rather, shouldn't it be in all cases that the incident light must enter through a pavilion facet and never a crown facet (the stone being table down)? It seems to me that there must also be a 'sweet spot' angle of incidence of light relative to the table, that 'sweet spot' being variable within limits determined by the pavilion angles of the stone under test. Still looking for more background on this..... later........

Author:  Kerensky [ Sat Aug 13, 2011 4:08 pm ]
Post subject:  Re: Using a refractometer

Well, views have risen to 191 over a month but those polling remains only at 13. Accordingly, the stats as reported above rremain unaltered. In turn, this suggests that there is limited interest in or experience of the 'bright-line' method of refractometry. Though I am interested in it, I am unable to experiment with it practically since to do so would mean making a permanent modification to my refractometer (a Wildman) and I am unwilling to do that so my curiosity might be satisfied. However, I have looked in detail at how light may behave and how it cannot behave in bright line refractometry), building some tables for the reflection and refraction of light at boundaries between stones of differing RIs, contact fluid and air. For any interested, I'll be posting a short commentary of the paper in the appropriate topic area in the next couple of days or so.

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