Precision RefractometersContact Liquid (Refractive liquid)

Contact Liquid (Refractive liquid)

Thirty-one types of standard contact liquid are available in 0.01 increments from 1.48 to 1.78 for the d-line (587.56nm) refractive index. Custom contact liquids can be produced to designated requirements to four-decimal-place accuracy.
In addition, the refractive index can be specified for other spectral lines (C-line, F-line).

Characteristic data - including dispersion, temperature coefficient and transmissivity data - is available on request for the refractive liquids.

For safety reasons, Shimadzu's refractive liquids are arsenic-free. Safety data sheets are available on request.

Major Methods of Use

As immersion liquids for the qualitative analysis of powders
For quality inspections on glass, crystals, etc. 

Use with KPR precision refractometers

Shimadzu's KPR Series precision refractometers use a V-block prism to measure refractive index. Contact liquid (refractive liquid) is used for these measurements.

The measurement principle of KPR Series refractometers is outlined below. A right-angle (90o) prism of the sample material is placed in the 90o groove of a V-block prism, as shown in the diagram. Light enters perpendicularly through the side face of the V-block prism. The light then exits the opposite side face at a certain angle due to refraction at the prism boundary. The refractive index of the sample can be easily calculated from the exit angle using Snell's Law.
The KPR Series is able to measure refractive index at ±0.00005 accuracy. However, in addition to instrument maintenance, accurate refractive index measurement requires restrictions on the sample installation and preparation such as:

  • Accurate cutting of the 90o angle on the sample, such that no air gap forms between the sample and the V-block prism; and
  • no scratching or unevenness that can cause light dispersion at the V-block prism or sample.

To simplify sample preparation and enhance the accuracy of refractive index measurements, KPR Series customers use a contact liquid (refractive liquid).
Applying the contact liquid (refractive liquid) when the sample is inserted in the V-block prism offers a variety of advantages. Firstly, the contact liquid (refractive liquid) fills any space between the sample and the V-block prism to prevent air gaps. In addition, the contact liquid (refractive liquid) coats the sample and V-block prism surfaces to allow light transmission without dispersion through a sample with surface irregularities. If the sample is not cut to an exact 90o angle, errors can be minimized by using a contact liquid (refractive liquid) with a refractive index close to that of the sample.

Contact liquids (refractive liquids) are supplied as accessories with a KPR Series instrument. However, a contact liquid (refractive liquid) with a different refractive index can also be used.

Dot pattern chart

Immersion method

This is a method of qualitative analysis and refractive index measurement for minerals and crystals. A microscope is used for this method. The Becke line test method requires a sample, transmission microscope, glass slide, and cover glass.
An immersion liquid containing finely ground sample powder is placed on a glass slide and covered with a cover glass. A bright halo is observed around the sample particles when viewed through a microscope with the aperture closed down. These halos are called "Becke lines."
The Becke lines move toward higher refractive index when the microscope tube moves upward; the Becke lines move toward lower refractive index when the microscope tube moves downward.
The refractive index of a sample can be measured by comparing movement of the Becke lines for several types of immersion liquid and sample refractive index.

Immersion method

Dispersion method

A dispersion method uses the difference in light dispersion to conduct qualitative analysis of the sample. A microscope is used for this method. A light source producing continuously variable wavelengths, such as spectroscope, is required. As with the Becke line test, refractive liquid containing the finely ground sample powder is placed on a glass slide and covered with a cover glass. The prepared slide is then mounted in a microscope. The dispersion value of the refractive liquid and sample differ, such that the refractive index differs according to the wavelength. Consequently, the outlines of the sample particles in the refractive liquid may become invisible at a specific wavelength (Fig. 1). By repeating the procedure with different refractive liquids, the sample refractive index can be determined at each wavelength.

Dispersion method

Immersion testing

This is a test method for gemstones. When a gemstone is immersed in a refractive liquid, its outline and surface reflection disappear, revealing any internal defects. Observing the outline of the gemstone while changing the refractive index of the immersion liquid reveals the refractive index of the gemstone. This method can be used for internal observations in transparent substances, not only gemstones. It can be applied to observing the interior of LEDs and to inspect for internal defects and striae in glass.
Shimadzu's refractive liquid can be used with plastics as well as glass and crystals. For example, when light enters a fiber, light leaks from the fiber core to the cladding. If the excess light leaking to the cladding causes a problem, the fiber coating is removed and the fiber immersed in a refractive liquid with the same refractive index as the cladding. This achieves an accurate Gaussian distribution.

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