Refractometer

Picture and Use of a Handheld Refractometer

Handheld refractometer

The handheld refractometer (HHR) uses the refraction of light through a prism to display the refractive index of a solution.

Figure 1: Handheld refractometer

The operation of the refractometer is based on the physical principle of light refraction. Light slows down as it passes into more optically dense media, and speeds up as it passes into less optically dense media. The change in speed is accompanied by a change in direction, and at a certain angle of incidence, the light does not refract in the second medium at all, but is entirely reflected. The angle at which this occurs is known as the critical angle, and it is this angle that the refractometer measures. Handheld refractometers are critical angle refractometers.

How to understand the critical angle

An easy way to visualize this phenomenon is in terms of the perceived and actual location of a fish below the surface of a body of water. The refraction of light leaving the water and entering the air causes us to perceive the location (using our eye) of the fish as closer than it actually is as shown in Figure 2.

Figure 2: Refraction of the light reflected off the fish (from the sun) causes us to perceive the position of the fish as closer than it actually is.

If we bring our eye closer and closer to the water, the angle 𝛼 becomes 90º. At the point when angle 𝛼 becomes 90º, the angle 𝛽 is the critical angle.

Picture and Use of a Handheld Refractometer

Handheld refractometers are one of the most popular analytical devices. They are used in many places - like in wine growing and wine making, both by professionals and amateurs, in beer brewing, in garages to check battery electrolyte and cooling liquid quality, and so on. They are very simple in use and give almost instant result, without tedious and costly laboratory procedures to follow.

Handheld refractometers are in most cases critical angle refractometers, not much different from the immersion refractometer (which - by itself - is a variant of Abbé refractometer).

Figure 3: Handheld refractometer - working principle. Note that real devices usually contain additional optical elements, like lenses and optical wedges, that help to obtain sharp shadow boundary.

Instead of having an illuminating prism, handheld refractometers have an illuminator flap which produces a diffused light at a grazing angle and helps to keep the sample in place. Light passes through the sample, enters the measuring prism and possibly other lenses, and finally falls on the measuring scale where it can be read. Depending on the reason for using the refractometer, its scale can be graduated in Brix degrees, percentage of alcohol or glycol percentage, etc.

To take care of temperature differences, simple handheld refractometers have to be either calibrated before taking measurements (using calibration screw and distilled water), or the result have to be converted using a temperature corrections table (which requires separate temperature measurement). However, many refractometers have built in temperature compensation - either scale or additional optical wedge are mounted on the bimetallic strip, which bends when the temperature changes, compensating for changes of refractive index. That makes them much easier to use.

Measurement technique is very simple. First, you open the illuminator flap (it is connected to the device by a small hinge) and put a sample on the measurement prism surface. To put the sample on th prism you can use a pipette, but when taking measurements in the field even squeezing a few drops of juice from the fruit will do. After the flap is closed, you look through the eyepiece, and read result from the scale. That's all. For easier reading it may be necessary to place the refractometer in the direction of some light source (like Sun or lamp), but during a day ambient light is usually strong enough. After finishing measurement, you should wipe dry prism and flap with a clean, soft cloth.

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