Refractometer

by Ritika Changrani

A Refractometer is a gemmological tool used to find the ability of gemstone to bend or refract light. This measure is the refractive index of a gemstone.

A Refractometer can be used to measure the refractive index of both singly refractive (isotropic) and doubly refractive (anisotropic) gems. The refractive index helps to determine the composition and physical properties of gemstone. With the help of refractive index, it is easy to identify the type of gemstone. Also, it is possible to estimate the amount of impurity present in the gemstone by comparing the measured refractive index with the standard value of the natural gemstone. With the help of Refractometer, birefringence and optic sign of the gem can also be determined.

How does a Refractometer work?

Refractometer works on the principle of Total Internal Reflection i.e. when light travels from denser material or the material with higher refractive index to rarer material or the material with lower refractive index, the light reflects into the denser material or refracts into the rarer material depending on the angle of incidence of the light traveling. (image courtesy: www.diamondtech.com).

Refractometer is made of a metal case, polarizing filter on the top, glass hemicylinder and viewing lens at the bottom. Natural or white light is passed through the back of the unit. The light travels through the glass hemi cylinder and then to the gemstone which is placed on the glass hemicylinder using a special refractive index liquid, which seals the gemstone to the hemicylinder thereby removing the air in between them. This is done to maintain the same speed of light in both the glass hemicylinder as well as stone so that the refracted rays obtained are accurate. The light is then refracted and sent to the viewing lens through the glass hemicylinder. Through the viewing lens one can see the refracted rays of light. The bending or slowing of light caused by the gemstone appears as the light and dark area combined by thin green line. The green line thus obtained is the refractive index of the gemstone.

A Refractometer is designed such that the incident and exiting rays reach the boundary of hemicylinder at 90 degree. No refraction occurs when the light ray is at 90 degrees to the boundary. Therefore no refracted rays are visible and hence appear as dark areas on the scale. The reflected rays of light appear as the light areas on the scale of the Refractometer. The light and the dark boundary on the scale is the representation of critical angle.

How to use a Refractometer?

• Apply a small amount of refractive index liquid on the metal base near the hemicylinder. Slowly place the gemstone on the refractive index liquid. Then slide the stone to the centre of the hemicylinder. Take care that you don’t apply pressure on the stone as it may damage the hemicylinder. The liquid builds optical contact between the stone and the hemicylinder.
• Close the lid of the Refractometer to avoid external light.
• Place the light source at the back of the Refractometer. Adjust the beam until you see bright light when you observe through the viewing lens.
• Position your eye such that you can see the whole scale through the viewing lens without moving your eye. This is to avoid the parallax error in the reading.
• Move your head up and down until you can see the clear separation of light and dark areas. Note down the reading on the scale where the thin greenish line appears.
• Unscrew the polarizing filter and place it on the viewing lens.
• Take down the reading. Rotate the polarizing filter to 90 degree and note the readings.
• Polarizing filter helps to determine if the stone is singly refractive or doubly refractive. If the stone is singly refractive, only one shadow edge is seen. Note down the readings rotating the stone 135 degrees.
• If the stone is doubly refractive, shadow edges move between two values on the scale. Note down the two values, i.e. the upper and the lower value. Take down four sets of reading by rotating the stone to 45 degrees.
• If one of the reading i.e. either upper reading or the lower reading remains same, the stone is said to be uni-axial. If both the readings change after rotating the stone, the stone is said to be bi-axial.
• The refractive index is the difference between the largest higher reading and the smallest lower reading.
• f the difference is positive, the optic sign of the stone is positive; else the optic sign of the stone is negative.

Tips to Remember while using a Refractometer

• Store the equipment in a dry place to protect the device from moisture which can affect the accuracy of the equipment.
• While you move the gem stone on the glass hemicylinder, take care to move the stone slowly so that no scratch is caused on the hemicylinder. A minute scratch causes can damage it and turns costly to be repaired.
• The refractive index liquid has to be stored and sealed. Avoid its contact with hands or eyes.
• Make sure to use the tiniest drop of refractive index liquid since the liquid itself has its own refractive index and causes total internal reflection between hemicylinder and the liquid. This will in turn affect the accuracy of the reading.
• The contact liquid should be selected with proper care since it sets the limit to the stones tested. The stones with greater refractive index than the contact liquid cannot be tested and hence give negative reading.
• Make sure there is adequate amount of light. White light can be used while testing, but monochromatic yellow light is widely used. White light can give good results for single refractive stones, but for doubly refractive gems sodium light source is the best option. White light, when used with doubly refractive gemstones causes overlap of the refraction readings thereby getting wrong results. Also, the use of sodium light source clearly differentiates the boundary between dark and light areas and hence helps take the readings easily.