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 Post subject: Refractometer Brightness?
PostPosted: Tue Feb 11, 2020 12:47 am 
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I've been a long time reader here, but have not had much time to dedicate to gemstones until recently... so I finally decided to register!

My first post here is something I've searched for, but haven't found an answer for yet.

How bright should a refractometer be when you look into it? I bought a cheap refractometer from ebay a few years ago upon the recommendation of someone else I met who was training to be a gemologist, and thought the one they got was good for a fair price....

This one has a built in light source (LED I assume), and it is so bright that its hard to use! I'm not talking about just uncomfortably bright... but I actually see spots for a couple minutes after looking into it. This can't possibly be right is it?

So this refractometer has been sitting unused in a drawer since I bought it until recently when I picked up a used Duplex II refractometer from a local gemologist who retired last year. The hemi-cylinder has a few scratches, and it has been difficult to get a consistent reading with the Duplex II, so I pulled the cheap ebay unit out again to compare measurements and was reminded again of how obnoxiously bright it is! This time I used a pair of non-polarized sunglasses to make it more tolerable.

I think my next step is to re-polish the hemi-cylinder on the DuplexII, but I thought it was worth asking here to see if refractometers can sometimes be TOO bright? I'm wondering if I should just toss the ebay unit, or take it apart and add a diffuser or change/remove the light source entirely to make it usable.


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 Post subject: Re: Refractometer Brightness?
PostPosted: Tue Feb 11, 2020 2:09 am 
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Weird. This is not an issue I have encountered with any chinese refractometer I have tried (which has been a few now). Likely it is defective somehow.

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 Post subject: Re: Refractometer Brightness?
PostPosted: Tue Feb 11, 2020 8:09 pm 
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Ok... so I just had another thought today. I wonder if a lower voltage battery would make the light less intense, or if it would just make the spectrum of the LED wider. If i recall correctly, it has a 3v lithium battery, but I'm pretty sure ive seen 1.5v batteries the same size before.

Hearing it's not normal makes me more willing to take it apart and try to fix it. Perhaps there is an internal diffuser that got knocked out of place during shipping...


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 Post subject: Re: Refractometer Brightness?
PostPosted: Tue Feb 11, 2020 9:28 pm 
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I'm sorry, instrument repair is not my strong suit, but doesn't it seem that the light bulb itself is the culprit, not the battery.
Also are you using a polarizing lens?


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 Post subject: Re: Refractometer Brightness?
PostPosted: Tue Feb 11, 2020 10:37 pm 
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If an LED is being driven by too much voltage, they will sometimes produce a lot of light... for a relatively short lifespan. I'm curious if this could be the case here. I think it's worth trying a different battery(or hooking it up to a different power supply) before I tear it apart.

I'm no stranger to repairing things that most people wouldn't attempt... so as a last attempt I'll take it apart and figure out whats going on inside!


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 Post subject: Re: Refractometer Brightness?
PostPosted: Wed Feb 12, 2020 6:51 pm 
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I forgot to answer about the polarizing filter. Yes, I had the polarizer installed.

The 1.5v battery didnt work... so I took the refractometer apart last night to see if it was something internal. Nothing looked out of place, but the LED is pointing directly at the hemi-cylinder from about 1/8" away.

I tried installing a diffuser ( a piece of scotch tape) between the LED and the hemi-cylinder.... and now it works great! Not only us the intensity now tolerable, but the whole scale is illuminated much more evenly! I was able to get repeatable readings on it after the modification!

I think the tape is still a temporary fix. I'll probably make something better out of some translucent plastic, but this at least gets me up and running! Next step is to re-polish the Duplex II and see if I can get readings from that one too.


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 Post subject: Re: Refractometer Brightness?
PostPosted: Thu Feb 13, 2020 4:15 pm 
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Well done!


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 Post subject: Re: Refractometer Brightness?
PostPosted: Mon Feb 17, 2020 1:49 pm 
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I have long held the view that having a go, providing you take care not to get in further than you can get out, is the way to solve most instrument problems.


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 Post subject: Re: Refractometer Brightness?
PostPosted: Tue Feb 18, 2020 1:45 am 
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I usually tend to agree with that logic. My fear with the cheap Chinese instrument was that it may be glued together, or "snap fit" in a way that would cause damage upon disassembly.

In this case, I was lucky enough that I peeled back the sticker on the bottom and found some screws. Simply removing the screws was all i had to do to get it apart. The LED light source IS glued in place though, so if it ever dies, it will probably need drilled out for replacement or use of an external light source.


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 Post subject: Re: Refractometer Brightness?
PostPosted: Tue Feb 18, 2020 9:04 pm 
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I would take the Duplex II and send to Denver Gem Cutting to have the hemisphere repolished and skip buying cheap equipment that is not reliable.

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