What Is a Polarimeter?

A polarimeter is a scientific instrument for measuring the quantity of light received at a specific point. This depends on what direction or polarization the various light waves may have in reaching the source. The process of the polarization of light was first discovered in 1808 by Etienne-Louis Malus, a French physicist, while one of the first functional polarimeters to measure the effect was made by Jean-Baptiste Biot, another French physicist, in 1816. They were continually refined until the mid 1800s, when they reached a level of sophistication that remained largely unchanged into the late 20^th century. Progression of the polarimeter design from the 1980s onward has led to the digital polarimeter and automatic polarimeter that are computer-controlled and have electronic readouts.

Since a polarimeter measures the refraction, or bending of light through a medium, they are largely chemistry and physics instruments. The samples used to measure the effect must be partially transparent. They have a wide variety of shapes and sizes, but the basic principle is the same. A beam of unpolarized light is reflected off of mirrors and thereby refracted through solid crystals or transparent liquid samples that break it up into polarized light.

As light waves are polarized in a basic polarimeter, they are channeled through a tube 4 inches (10 centimeters) in diameter that contains the chemical being studied. If the compound has polarizing properties, the brightness of the light will diminish as its angle of exit changes from the tube. This angle is then determined by rotating the axis of the analyzer at the end of the tube. If the change in angle is considered positive, or towards the right, its called dextrorotatory, and, if its towards the left, its referred to as levorotatory. The size of the angle of rotation is determined both by the length of the tube and the type and concentration of the compound that the light is being passed through, known as an enantiomer.

In fine tolerance applications, such as ophthalmology, the laser polarimeter or optical polarimeter is built into an ophthalmoscope and uses a near-infrared laser to determine the ability of the cornea to compensate for polarized light. This is useful in tracking such degenerative eye conditions as glaucoma. The results are then analyzed using statistical software to try to predict the onset of glaucoma before physical symptoms are present in the patient.

Since many compounds display a rotation of light that passes through them, the polarimeter has a wide array of applications in the pharmaceutical, food, and chemical industry in general. They are routinely used to determine the purity levels of medicines such as antibiotics, the concentrations of sugar molecules and flavorings in various manufactured foods, and to identify polymer concentrations in the plastics industry.