Identification method
Refractive index and birefringence
(1) Abbe gemstone refractometer
1, method principle
When light enters the prism with refractive index n from the measured sample with refractive index n, the incident angle is I and the refraction angle is υ, then
Sin one
ordinary
——— = —— ………………………… ( 1)
hard
n
In the reflection refractometer, the incident angle i = 90, which is replaced by (1).
1
ordinary
——— = —— ………………………… (2)
hard
n
N = N sinυ…………( 3)
The refractive index n of the prism is known, and the refractive index n of the measured sample can be obtained by measuring the refraction angle υ.
The range of measurable sample refractive index depends on the value of n and the refractive index of contact refractive index oil.
Subtracting the minimum value from the maximum value of a birefringent sample is birefringence. In actual measurement, due to the randomness of sample orientation, the measured birefringence value is less than or equal to the theoretical value of the sample.
2. Tools
Gem refractometer, accuracy 0.002, measuring range 1.350 ~ 1.800.
3, operation steps
A. clean or wipe the test sample.
B, dropping an appropriate amount of oil drops with refractive index greater than that of the sample on the metal table at one side of the sample table.
C place the polished plane or crystal surface of the sample downward and gently put it on the oil.
D. Gently push the sample to the center of the sample table.
E. Observe that the scale of the dividing line between light and dark read by the eyepiece is the refractive index value. When the refractive index is greater than 1.80, it is expressed by >: 1.80.
F, continuously rotating the polarizer of the sample can measure a maximum value and a minimum value without isotropic body, and the difference between the two values is birefringence.
Note 1: In any of the following circumstances, the refractive index is not regarded as an important identification item:
It is difficult to measure the refractive index when the sample is too small (the plane diameter is く2mm).
B. When the inlaid metal exceeds the sample plane, the measured refractive index error is large.
C. When the sample has no smooth plane, the refractive index value cannot be determined.
The refractive index value is higher than the measuring range of refractometer.
Note 2: Birefringence is not considered as an important identification item in any of the following cases:
A. When the sample is too small, the birefringence value cannot be determined.
It is not easy to measure the refractive index when the sample is bent.
C. When the sample is aggregate, the birefringence value cannot be determined.
(2) Reflective gemstone refractometer
1, method principle
The surface reflectivity of the sample has the following approximate functional relationship with the refractive index, namely:
(n-N)2
Reflectance = ——
(n+N)2
Where: n- refractive index of the sample
N- refractive index of surrounding medium (N ≈ 1 of air).
Using near infrared light as light source, the reflectivity value of polished sample plane is measured, which is calculated by instrument or automatically converted into refractive index value.
2. Tools
Reflective gemstone refractometer with accuracy of 0.005 and measuring range of 1.300 ~ 2.999.
3, operation steps
A. clean or wipe the surface of the test sample.
B. Place the polished plane of the sample downward and horizontally on the test window of the refractometer, and cover the sample on the sample.
C. Rotate the sample horizontally for one revolution, and read the refractive index value (single refraction) or the maximum and minimum refractive index values (birefringence) of the sample from the reading disk.
D the polished plane of the sample to be measured must be larger than the measurement window and have a good finish, otherwise the measurement accuracy will be affected.