Refractometers

A refractometer is a scientific instrument used to measure the refractive index of different materials – a value indicating how much the phase velocity of light is smaller compared with propagation in vacuum. Various refractometer types allow measurements in liquid, solid, or gaseous samples. Refractometers have a long history and a wide range of applications in various fields such as physics, chemistry, biology, agriculture (e.g. wine making), geology, and industrial fabrication (see below for more detail).

The process of measuring refractive indices is termed refractometry. Often, the direct focus is not on the refractive index itself, but rather on a related quantity such as the concentration of a substance in a solution.

Typically, a refractometer measures the real part of the refractive index. However, specialized refractometers can measure complex refractive indices, providing insight into light's attenuation constant in the medium.

Principles of Operation

Most refractometers are based on one of a few common operation principles, although a wide range of other principles has also been demonstrated – see, for example, various papers in the bibliography below. The most importance operation principles are explained in the following.

Measuring Beam Deflection

The conceptually simplest method involves observing the direction change of a light beam due to refraction at an interface between the sample and a medium such as air or an optical material. This shift in direction is described by Snell's law (as explained in the article on refraction) and depends on the refractive indices of the two media involved.

In some refractometers, light from some light source is directed onto a prism made of a material with a known refractive index. The sample (a liquid, for example) is placed on one surface of the prism, such that the light is refracted at the interface between the prism and the sample, and the direction of the output beam is measured. From that angle, one can then calculate the refractive index of the sample using Snell's law. The instrument may directly indicate the refractive index, e.g. with a scale on which the position of the output light beam can be seen.

Using Total Internal Reflection

Common in laboratories, the Abbe refractometer (named after its inventor Ernst Abbe) employs the principle of total internal reflection. Here, a sample (typically liquid) is placed between two high refractive index prisms and illuminated via one of the prisms with divergent light. Total internal reflection prevents the light transmission at too high incidence angles and thus limits the angular range of transmitted light. From the observed maximum angle of transmitted light (obtained with high precision, using a kind of microscope), the refractive index can be obtained.

The Abbe refractometer exemplifies a critical angle refractometer, which although involving beam deflection, stands apart due to its distinct operational principle.

Interferometric Refractometers

Interferometric refractometers utilize interferometers to measure changes in optical phase. These are especially useful for tracking small changes in refractive index due to external influences. However, they are often not suitable for obtaining absolute refractive indices.

The Rayleigh interferometer is a typical device in this category of differential refractometers, using changes of an interference pattern due to phase shifts. (Note that this has nothing to do with Rayleigh scattering.)

As another example, a Fabry-Pérot interferometer may be filled with a liquid or gas sample, the refractive index of which influences the |optical frequencies of the resonances. These can be determined, for example, with a wavelength-swept laser.

Various kinds of interferometers can also be realized with fiber optics, leading to kinds of fiber-optic refractometers as discussed in the next section.

Fiber-optic Refractometers

Various kinds of refractometers are based on fiber optics, and can be based on a range of quite different operation principles. Some of them contain one or more fiber Bragg gratings, or a normal piece of optical fiber, where some part of the fiber is prepared such that the probed substances gets sufficiently close to the fiber core to interact with the light in guided modes. One may, for example, obtain a spectral shift of features of a fiber Bragg grating depending on the refractive index of the probed substances surrounding the fiber. With certain techniques, one may also utilize external influences on cladding modes rather than those directly on the light in the fiber core, e.g. by exploiting the fact that loss peaks generated by a tilted fiber Bragg grating depend on the details of cladding modes.

Wavelength Dependence

The refractive index depends on the optical wavelength (→ chromatic dispersion). For measuring it at different wavelengths, a refractometer must provide test light in the relevant wavelength range. Simple devices provide only the refractive index in the visible spectral region, possibly even with no precise indication to which wavelength exactly it applies – or for one or several well-defined wavelengths by using a quasi-monochromatic light source, such as a gas discharge lamp emitting some standard spectral line, one of which is selected with an optical filter.

Some refractometers are specifically made for operation in certain special wavelength regions, e.g. in the mid-infrared.

Specifically for analyzing chromatic dispersion, i.e., essentially the frequency dependence of the refractive index, one may in principle use refractive indices at various wavelengths, as measured with some kind of refractometer. However, there are more accurate techniques based on entirely different principles, e.g. using white light interferometers.

Refractometers of Different Sizes and Formats

Depending on the application, different formats of refractometers may be used:

• Some refractometers are high-precision bench-top laboratory devices, e.g. with a size similar to that of an optical microscope.
• There are also compact and lightweight handheld refractometers, suitable also for use in the field, but less accurate and reliable. Some of those use digital technology, while others do not require electrical power.
• For industrial use, there are inline refractometers, which are integrated into some machinery such as process lines. They are used, for example, for continuous monitoring of a refractive index during a production process.

Precision and Other Qualities of Refractometers

Enhancements such as temperature stabilization and the use of laser light for precise measurements of refraction angles at well-defined wavelengths can improve a refractometers' precision. In some cases, digital technology is used to improve the accuracy and/or the ease of use.

While simple refractometers may reach a refractive index accuracy only of the order of 0.01, precision devices can be several orders of magnitudes better – for example, reaching a precision at the 10⁻⁸ level. Particularly high precision is achieved with some interferometric differential refractometers. Note that resolution is not necessarily the same as accuracy, as the reliability of the readings may not be perfect.

Of course, precision is not the only relevant performance parameter. Some other aspects are:

• The measurement speed can be important, e.g. for inline refractometers in industrial production. In principle, millions or even billions of measurements per second are possible with certain types of instruments, while even just the handling for the introduction of a sample may limit the speed of other devices to the order of only one measurement per minute.
• Another limiting factor may be the covered wavelength range.
• Many refractometers are limited to certain types of substances, such as liquids, work only with materials having a refractive index within a certain region, and can be used only for samples up to a certain maximum size.
• The convenience of handling, influencing the required amount of training of the users, is of course of interest. Portability and ruggedness can be vital for use in the field.
• The same holds for the durability and required maintenance.

Applications of Refractometers

Refractometers have a wide range of applications in various fields. Some examples:

• In the optics industry, the refractive index of optical materials is often of high interest, and therefore needs to be measured accurately.
• In the food and beverage industry, refractometers are used to measure the sugar content in fruit juices, honey, and wine. One exploits the fact that the sugar concentration influences the refractive index more than the concentration of various other substances.
• The same principle is also often used in the pharmaceutical industry for determining the concentration of active ingredients in solutions, or in some fields for measuring the salinity (salt concentration) of seawater or the protein concentration in biological samples.
• In the oil and gas industry, refractometers serve for measuring the density of petroleum products.
• In geology, refractometers can be used for identifying minerals because those can differ very much in terms of their refractive index, even when looking quite similar.

More to Learn

Encyclopedia articles:

• refractive index
• refraction
• total internal reflection