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Refractometer

Choose one, measure everything

Anton Paar’s range of Abbemat refractometers embody over forty years of technical expertise. They measure the refractive index and concentration of liquids, gels and solids. These truly universal refractometers cover all applications in all industries. Abbemat refractometers are built with care and precision using the highest quality materials. An Abbemat is a secure investment for the future, providing reliable and accurate results for years to come.

Economy Line: “The economic Abbemat refractometer for routine measurement.”.
The Abbemat 200 refractometer makes the Abbemat series’ sophisticated measuring technology available to users with a low budget. Offering all essential features and intuitive handling, this out-of-the-box refractometer is ideal for small laboratories that require a limited number of measurements without any complex data processing.

Performance line: “Measures, measures, measures.”
The robust and easy-to-operate refractometers of the Performance line are ideal solutions for routine analysis and quality control, ranging from checks on incoming raw materials, intermediate to final products. The large display gives a clear pass/fail result for analysis of large numbers of samples when time is short.

Performance Plus line: “Ready for any job today and fit for tomorrow.”
The versatile, high-end refractometers of the Performance Plus line are designed for research and development as well as demanding quality control applications. They can be operated with a peristaltic pump or sample changer to simplify filling and are easily expanded by a wide range of accessories.

Heavy Duty line: “Measures when others fail.”
The Heavy Duty line refractometers are designed for work in harsh environments and for special applications requiring high temperature (HT model) or multiple wavelengths (MW model). The external PC monitor can be positioned away from the refractometer so you can check the results without being near the sample. To measure samples containing solid particles or air bubbles you can position the Heavy Duty Abbemat on its side to prevent sedimentation and disturbance affecting the results.

Our specialists are available for in-depth information on which refractometer suits you best. Anton Paar has a refractometer for every task.

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Measuring the curing behavior of nano-tuned gluewith Abbemat refractometers.

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Abbemat refractometers are powerful tools for determining the refractive index of nanoparticles in solution. The knowledge of the refractive index is fundamental for deriving the

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Abbemat refractometers are powerful tools for determining the refractive index of nanoparticles in solution. The knowledge of the refractive index is fundamental for deriving the right size distribution from laser diffraction data.

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AdBlue® is an aqueous urea solution and was developed tolower nitrogen oxides in diesel emissions. Internationalstandards demand that refractive index is measured for

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AdBlue® is an aqueous urea solution and was developed tolower nitrogen oxides in diesel emissions. Internationalstandards demand that refractive index is measured for thedetermination of the urea concentration in AdBlue®.

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Fuel system icing inhibitors are added to jet fuel to preventfreezing of the water content at flight altitude. Theirconcentration in water and therefore the freezing

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Fuel system icing inhibitors are added to jet fuel to preventfreezing of the water content at flight altitude. Theirconcentration in water and therefore the freezing pointcorrelates with the refractive index.

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Deuterium oxide, also known as heavy water, is used as amoderator in nuclear power plants to maintain the nuclearchain reaction. The purity and concentration of heavy watercan be

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Deuterium oxide, also known as heavy water, is used as amoderator in nuclear power plants to maintain the nuclearchain reaction. The purity and concentration of heavy watercan be analyzed with refractive index measurement.

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Transparent plastics are getting more and more important foroptical applications, like for example optical lenses or LEDdisplayes. The refractive index and its wavelengthdependence

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Transparent plastics are getting more and more important foroptical applications, like for example optical lenses or LEDdisplayes. The refractive index and its wavelengthdependence (dispersion and Abbe number) need to bedetermined for polymers used in optical applications.

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Soluble solid content and acidity are major quality parametersin tomatoes. The soluble solid content in tomato products isdetermined by refractive index measurements and

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Soluble solid content and acidity are major quality parametersin tomatoes. The soluble solid content in tomato products isdetermined by refractive index measurements and expressedas °Brix.


Refractive index

The refractive index (n) is a physical constant for a particular material. It is a dimensionless number that describes how light propagates through a specific medium.

The refractive index (n) of a medium is a measure of how much the speed of light is reduced inside the medium compared to the speed of light in vacuum under standard conditions. Standard conditions are 20 °C and 1013 mbar at 50 % relative humidity.

 

Examples of the speed of light in different media:
In vacuum     299 792 km/s
In air             299 710 km/s
In water        225 000 km/s
In sapphire   170 000 km/s

The higher the optical density of the medium is, the lower is the speed of light inside the medium and the higher its refractive index.

The refractive index is expressed as the ratio of the speed of light in the vacuum νvacuum relative to the speed of light in the medium νmedium:

The refractive index depends on the temperature T and wavelength λ of light and is expressed as nTλ . Refractometric standard conditions are a wavelength of λ= 589 nm (Sodium D-line) and a temperature of T = 20 °C. The refractive index for these conditions is commonly written as nD or nD.

Examples of different refractive indexes (wavelength = 589.3 nm, temperature = 20.0 °C):
Vacuum                       1.00000 nD
Water                          1.33299 nD
Dodecane                    1.42172 nD
Tetrachloroethylene    1.50580 nD
Bromonaphtalene       1.65784 nD
Sapphire Al2O3           1.76 nD
YAG                             1.83 nD
Diamond C                  2.42 nD

 

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Measuring principle

The change in speed at the interface between two media also causes a change in direction of the light beam, like waves on the shore. Light that is incident on a material with a different optical density will bend or refract (unless it enters the medium perpendicular to its surface).

The amount of deflection depends on the difference of the refractive indexes of the two media. At the interface not only refraction (change of speed and change of direction) takes place, but also reflection.

 

For measuring the refractive index of an unknown sample, the critical angle of total reflection is determined. Total internal reflection describes the situation in which no light refracts into the second medium and all light is instead reflected at the boundary. This occurs when the difference of optical densities and the angle of incidence is so big that light does not enter the second medium. If the angle of incidence increases up to a critical angle, the refracted light deviates at the border line of the two media. Therefore, the critical angle of total reflection is when the angle of refraction is 90° to the normal. All angles of incidence higher than this critical angle (α) show only reflection and no refraction any more. This phenomenon is referred to as total internal reflection.

 

Figure 1: Depending on the angle of the incoming light beam at the interface between two different media the light beam is partly reflected and refracted (α<αcritical) or totally reflected (α>αcritical).

 

When the light beam A crosses the interface between medium 1 (having a refractive index = n1) and medium 2 (having a refractive index n2, with n2 > n1) under the incident angle α1 the light beam will be partially refracted (angle β1) at the boundary surface, and partially reflected (beam A’).

If the angle of incidence α1 is increased there will be an angle α2 at which the refracted light beam B will travel along the boundary between medium 2 and medium 1, while part of the light beam is reflected (B’). This angle is called the critical angle of total reflection αcritical.

If the angle of incidence is increased even more (α3 > αcritical, light beam C) the whole light will be totally reflected by the medium 1 (C’) and will no longer be refracted.

This can only occur when light travels from a medium with a higher [n2 = higher refractive index] to one with a lower refractive index [n1 = lower refractive index].

For measuring the refractive index of medium 1, Snell’s law is used. Snell’s law states that the ratio of the refractive indexes n1/n2 equals the inverse ratio of the sinus of the angles α1 and β1.

When looking at the special case in which α2 = αcritical the angle α1 = β1 = 90°. Inserting these angles into Snell’s law results in

 

This means that the refractive index of medium 2 can be determined by knowing (measuring) the critical angle of total reflection αcritical and the refractive index of medium 2 only.

 

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Abbemat measuring design

Measurement of the refractive index of the sample with an Abbemat is based on the determination of the critical angle of total reflection. A sample needs to be placed in good contact to a measuring prism.

 

An LED light source irradiates light from a wide range of angles onto a prism surface in contact with the sample. Depending on the difference of the refractive indexes between sample and prism the light is partly refracted and reflected, or totally reflected (αcritical). The critical angle of total reflection is determined by measuring the reflected light intensity on a CCD array in dependence of the incident angle.

 

While the light incident beam is totally reflected (angle of incidence is larger than αcritical) all light will be totally reflected and imaged onto the CCD scanner, resulting in a high light intensity on the CCD scanner. If the angle of incidence is smaller than αcritical the light will be partially refracted and partially reflected, resulting in a lower light intensity on the CCD array for angles smaller then αcritical.

As a result a curve for many different angles of incidence with the corresponding light intensities is monitored at the CCD scanner in a static setup.

 

Figure 2: Schematic setup of an Abbemat refractometer. The measured sample is in direct contact with the prism. Depending on its refractive index, the incoming light below the critical angle of total reflection is partly transmitted to the sample, whereas for higher angles of incidence the light is totally reflected. The intensity of the reflected light is measured with a high-resolution sensor array.

 

As shown in Figure 1 a step in light intensity is observable. This determines the critical angle. The video signal taken with the CCD sensor with 1024 channels is fitted with a Fresnel-theory curve taking into account all different angles, not only the critical angle of total reflection. This technique guarantees a more accurate and reliable determination of the refraction index compared to a simple shadow line detection.

 

Figure 3: Scheme of the detection of the critical angel with a CCD sensor. The complete set of measured data is fitted with a Fresnel-theory curve to exactly determine the critical angle αcritical.

 

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Dependence of the refractive index

Wavelength

The refractive index varies for nearly all materials with the wavelength of light. This is called the dispersion relation and is characteristic for every material.

As an example in Figure 4 the dispersion relation (red curve) and the absorption (green curve) of water from the visible to infrared wavelength range is shown. In the visible wavelength range a decrease of the refractive index and nearly no absorption is observable. In the infrared wavelength range several absorption maxima and fluctuations in the refractive index appear.

 

Figure 4 Dispersion curve of water


To guarantee a high accuracy of up to 2x10-5 in refractive index measurement the wavelength has to be adjusted or determined well. Therefore, the wavelength in all Abbemat single wavelength refractometers is individually adjusted and calibrated with an interference filter up to an accuracy of ± 0.2 nm. Most of the measurements are performed with light of the sodium D-line at 589 nm. For special applications further wavelengths ranging from 436 nm up to 656 nm are available. Each single wavelength in the Abbemat multiple wavelengths model (Abbemat MW) is exactly measured (“true wavelengths”). Using the true wavelength instead of the nominal wavelength assures precise measurement of the refractive index.

Temperature

Temperature is one of the major influences on the refractive index. Therefore, the temperature of the measuring prism and the temperature of the sample have to be controlled and measured with high precision.
With the Abbemat refractometers the temperature is controlled by an internal Peltier device, there is no need for an external thermocirculator.

 

A high-precision temperature control stabilizes the temperature of measuring prism and sample. Therefore, the temperature sensor is integrated into the measuring prism very close to the sample. Due to the good heat conductivity of the prism material this temperature control guarantees a fast temperature control of the sample. The built-in temperature sensor has an absolute temperature accuracy of up to ± 0.003 °C and a temperature stability of up to ± 0.002 °C.

 

In addition to the temperature control of the sample and prism, the whole optical setup is temperature-stabilized at a constant temperature. This guarantees no distortion of optical parts and a high optical stability without the need for re-adjustments to any elements.

 

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eLearning course “Basics of Optical Analysis”

In our interactive eLearning course “Basics of Optical Analysis” you can find detailed explanations of the basics of refractometry

 

 

Order your free eLearning course “Basics of Optical Analysis”.

 

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