Arch Gemstone Refractive Index Determination

Refractometer, also known as “refractometer”, is an instrument for measuring the refractive index of transparent gemstones. The refractive index value of different gemstones is one of the main criteria for scientific identification. Therefore, the refractometer is a very important technical means in the treasure of finished Baoyu stone, and it is widely used, especially in the scope of “no damage identification”. The refractometer used in the study and identification of gemstones (Figs. 10-25) was designed and fabricated based on the principle of total reflection of light.

As described above, all light that enters the light-clearing medium from the optically dense medium is refracted when its incident angle is less than the critical angle, and total reflection occurs when the incident angle is greater than the critical angle. Based on this, a refractometer was created. It is actually an instrument for measuring the critical angle of gemstone and converting its reading directly into refractive index. Its components are mainly made of high refractive index (greater than 1.81) lead glass or rare earth glass hemispherical table (hemisphere plane upward), ruler (scale ruler), eyepiece, polarizer, etc., working principle See Figure 10-26. When light irradiates the high refractive index glass hemisphere from the front of the refractometer, the helium passes through this hemisphere and reaches the contact point where the high refractive index oil (contact liquid) and the gemstone are detected, and total reflection is generated by gemstone. At this time, the light is incident on the high-refractive-index glass, the lens, the scale, and the right-angled total-reflection glass prism of the other normal line. The prism will focus on the critical angle edge of the gemstone to be measured, and finally reach the eye of the appraiser, and then directly read the measured refractive index value of the gemstone from the scale. Originally, with the critical angle as the boundary, a bright area was formed in the total reflection area, that is, on the side of the normal line. In the region where the incident angle is less than the critical angle, a shadow region is formed on the reflective side because there is no reflection of light. In this way, the appraisers can see through the eyepiece the light and dark parts with the critical angle as the boundary.

Of course, if you say it in reverse, this dark and bright boundary can also reflect the size of the critical angle. If the angle is measurable, the critical angle of gemstone can be obtained. However, because the visual angle is very inconvenient, the angle and refractive index values ​​are previously converted when the refractometer is designed and produced. As long as the appraisers can accurately read the scale on the scale, they can directly obtain the measured refractive index value of gemstone. Long-term practice and research have shown that the use of gemstone refractometer must pay attention to five aspects.

(1) Gemstones with different refractive indices have different critical angles and are the core of the working principle of the refractometer. Generally, the critical angle of low-refractive-index gemstones is large, and the range of shadowed areas seen in the field of view is smaller than the range of bright areas; the boundary angle of high-refractive-index gemstones is small, and the range of shadow areas seen in the field of view is brighter than the bright areas thereof. Big. This shows that the interface between the shadow area and the bright area within the visual field is determined entirely by the critical angle of the gemstone being measured.

(2) Gemstones and glass hemispheres must have good optical contact. If the measured gemstone is directly placed on the glass hemisphere and no reading is obtained, the contact liquid is generally dropped on the glass hemisphere, and then the gemstone is placed on the contact liquid to form a good optical contact and obtain an ideal reading. The most common contact solution is a saturated solution of diiodomethane containing tetraiodoethylene. Pure methylene iodide has a rejection rate of approximately 1.74. If * is added to increase the saturation point, the refractive index value can be increased to about 1.79; if 18% of the yellow crystalline tetraiodoethylene is added, the refractive index value can reach 1.81. It becomes a commonly used contact solution. However, this contact solution is highly corrosive. After measuring the refractive index of gemstone as soon as possible, the contact liquid on the glass hemisphere must be completely removed to prevent it from being corroded or damaged.

(3) Select the appropriate lighting source. When daylight or ordinary light source lighting is used, the narrow spectrum showing the refractometer reading is dominated by blue-green with a very narrow yellow band. If you read at the boundary between the yellow and green lines, then the reading will be very close to the data in the gemstone refractive index table (the data on the index table is usually for sodium light). If the entrance to the light is covered, the light will pass directly through or behind the measured gem. At this time, the high reading side of the ruler will appear black. A red line between black and white indicates the location of the reading. Obviously, because of the high refractive index of the glass hemisphere and the strong dispersion, the readings in the field of view of the refractometer are often not a clear line (actually a color band), so that the reading of the refractive index value is not difficult or accurate. To solve this problem, monochromatic light sources can be used for lighting. The best monochromatic source is a sodium lamp, which emits monochromatic light with a wavelength of 589 nm, making the light and dark differences of the scale more pronounced. Since the light is monochromatic, there is no spectral interference to its clear readings, and the reading can be accurate to +-O.001. If the measured gemstone has a birefringent property, two refractive index values ​​can be measured on most facets. It must be noted, however, that monochromatic light can only make readings that are more likely to be seen in white light, and readings that are not available in white light cannot be obtained in monochromatic light. In addition to dedicated monochromatic light sources, red and green filters can also be used to eliminate color bands at the readings.

(4) The measured gemstone's refractive index must be less than the refractive index of the glass hemisphere and the contact liquid. As mentioned above, the refractive index of the glass hemisphere is generally 1.8 l, and the refractive index of the contact liquid is also 1.81, so the refractometer can only determine the gemstone with a refractive index of less than 1.80. When the refractive index of gemstone exceeds 1.80, the refractometer cannot measure it, and a reflector must be used.

(5) The measured gemstone must have a flat or curved surface that has been polished. The larger the polished surface and the better the finish, the higher the accuracy of the gemstone refractive index measurement.