I know that with the wine world, refractometers tend to be used to work out the level of brix in a juice so when the fruit is ready for harvesting i.e. at the maximum natural sugar content.
This is because they're more practical for field testing. With a hydrometer you'd need to crush/extract the juice before testing and with no certain way of making sure that you've removed everything that might skew the reading.......
Of course, they can be used to test and check the progress of the ferment but as soon as there is alcohol present in the mixture being tested, you have to use a table to adjust the reading as the alcohol does skew the reading etc.
The pro's seem to use hydrometers (expensive but accurate shorter scale ones) for on site testing, as most of the production facilities don't have the money or justification to spend the kind of money needed to buy the complex laboratory kit.......
If you dig around the net and look at all the info about refractometers, the differences are pretty much the same as hydrometers i.e. that they work in the same way, but will have different measuring scales - some brix, some Oeschle, etc. I never bothered to get one but considered one that was graduated in degrees Oeschle as that's the closest to specific gravity, which is what I'm most familiar with.
Whether using light like this is accurate enough for a refractometer to give a decent measurement of alcohol content, I don't know.........
Oh and don't forget, making stuff on a hobby basis is very different from making it commercially. We don't have to worry about a tiny difference that makes the tax man a lot more wealthy etc (think on the differences between the excise duty paid by producers who make to the industry standard of 40% ABV and why producers like Diageo now sell their brands at 37.5% ABV - it's big money, very very quickly).......Statistics: Posted by Phantom — Mon Aug 29, 2016 8:13 am
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