Let’s Open a Refractometer!

Have you ever wondered what a refractometer looks like with its guts out? Just look at the image at left. Here are the inner workings of the ISG Refractometer. If you look closely at the lower right side, you will see two screws with red coloring. These are your calibration screws and they must be in exactly the right position.

If anyone has ever purchased one of those cheap US$90.00 (or less) refractometers on ebay and wondered why your readings were skewed, it’s because the company did not calibrate the refractometer…they just screwed the parts together and sold it on Ebay. To get proper readings these calibration points must be carefully positioned and glued tightly. 

The manufacturer of the ISG Refractometer thinks I am clinically OCD about calibration. I personally spot check our refractometers coming into the office to make sure of the calibration. If I find one off…well, you know how I can get about these things. We are very careful about calibration, something those cheap Ebay sellers do not care about at all! 

Now, let’s look at how light travels through a refractometer to understand how this important tool provides us with the refractive index readings.

I must first tell you this: don’t open your refractometer unless you need to. Calibration of a refractometer is very tricky and once it is out of calibration it is very difficult to get it properly set. Unless you have a compelling need to open your refractometer, such as to clean it from a spill that leaked inside, don’t do it. If you must, be very careful and do not mess with the calibration screws.

At left you see how the workings come out, and this applies to most designs of refractometer: the screws on the bottom are removed and the inner workings simply lift out the top.

Here is a look at the workings coming out of my refractometer. Watch the finger prints! Oil from your skin will smear on the glass mirrors and must be cleaned or it will have a detrimental effect on your readings.

Here are the inner workings of my refractometer. Remember, the red material on the two screws you see on the left side is glue. These are the calibration screws and are glued so they don’t slip in normal use. Don’t mess with this area unless your refractometer is needing calibration.

Let see how this works…

For our demonstration, I have used a box (don’t tell the post office I cut one of their boxes) and created a hole for our light source. The gemstone on the top is a peridot, used because it has a wide range of refractive indices. 

In order for us to see the light beam, I have used a low power laser positioned just outside the hole in the box, but directed to the first mirror as you see below. This is rather dangerous, as we will talk about later, but since I needed a visible light beam to do this whole newsletter I used the laser and directed it into the regular light source path as you see below.

Once I turned the room lights out, it was easy to see the light beam path of the laser, but quite difficult to photograph the full path. Below left is the laser traveling through the mirror system, refracting off the peridot, then traveling through the lens, reflecting off the reading scale and out where the viewing lens would normally be located. You can see these by the points of light in the image. The yellow arrow in the image below right shows where the viewing scale is located. When you look into the viewing lens of a refractometer, this is where you are looking to get your reading.

If you look closely at the image at left, you can see the two readings of this double refractive peridot. We get two readings due to the double refraction. If we were to rotate the peridot these two lines would move as the refractive indices change based on the orientation of the crystal axis. 

These two lines are also visible on the viewing scale at the same time, if you look without a polarizing filter. The purpose of using a polarizing filter with a refractometer is to allow your eyes to see one line at a time. Polarized light allows light traveling in just one direction to pass. By using a polarizing filter, and turning it at 90 degree angles, you can see one reading and then the other separately. 

Remember that the birefringence of peridot is quite high: .036, so the two lines are easy to see here wih laser light. With other gemstones we would not be able to see these two lines without the polarizing filter, even in this demonstration. Just NEVER use a laser with your refractometer.

Finally, here are our readings. Since the peridot is not turned to optimum positioning we do not have the full birefringence showing with these readings. That is why you must turn the stone and get multiple readings to make sure you get the widest variables of readings. By doing this you can use your refractometer to determine the refractive indices of the gemstone, plus the optical character and sign of the gemstone. The refractometer can be used in place of a polariscope if you are traveling and don’t have your full lab with you.

You may be asking yourself: If a laser will give such bright readings as seen above, why not use a laser to do all of my refractometer testing?”

The answer is: Blindness. Quick and permanent blindness. Looking at a laser beam traveling through a refractometer is the fastest way I know to totally burn out your retinas. Of course, I had to try it, so I did. 

While I was very careful and did not blind myself doing so, this is NEVER something to try at home. Remember, I am a Stunt Gemologist, so don’t try this at home, EVER!

Below left is the laser source at the light opening of my refractometer. The tricky part was getting it positioned properly and risking my digital camera to take the pictures. Below right are the pictures of the readings using the laser light source. Due to the tendencies of digital cameras to be red color-sensitive, the image is somewhat over-exposed. But this is as far as I got. I figured if I got these images without doing any damage to myself or the camera I should quit while I was ahead. 

I hope you enjoyed this look inside a refractometer. This gemology stuff….it’s not rocket science. Gemology is fun and rewarding as a hobby and profession. Getting an opportunity to see what is going on inside one of the most important gemological tools is just part of the fun that we like to share with everyone at the ISG.

If you would like to join us for the fun and rewarding study of gemology and jewelry appraisal, I invite you to click the ISG icon below to learn more about our world-class programs. Join today and avoid the tuition increase!

Thank you for sharing your day with us.

Robert James FGA, GG, GCA
President, International School of Gemology
the education division of the Global Claims Associates