In our next post in our series on using light as a measurement tool, we explore one of light’s more hidden properties, its polarization, and how it is used in some powerful optical measurement techniques.
But first, what is polarized light? Light consists of electric and magnetic fields oscillating perpendicularly to the direction in which the light is travelling. If the electric field oscillations are aligned in a well-defined direction, the light is polarized. Light emitted from common sources such as LEDs, halogen bulbs, and many lasers, contains a mix of all polarization directions, and is said to be randomly polarized, or unpolarized.
Revealing information about surfaces and materials
The benefits of using polarized light stem from the fact that the polarization is often changed as it passes through a material or reflects from a surface. Many optical measurement systems do not consider light’s polarization and can’t detect these changes. But if a measurement system can pick out a specific polarization direction from unpolarized light, or employ an already polarized light source, then these changes can be measured to reveal information about the surface or material with which the light has interacted.
Furthermore, when working with polarized light, a simple polarizing filter can block all the unchanged light before it reaches the detector, retaining only the light that has interacted with the object or surface being measured.
Let’s look at some techniques in more detail.
Nevit Dilmen, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Measuring mechanical stress with a polariscope
A polariscope is a simple system of 2 crossed polarizing filters. It can reveal mechanical stress in transparent materials such as plastics or glasses. Stress inside these materials gives rise to birefringence, which causes the polarization direction to rotate as light passes through the object. Only the rotated polarization can pass through the second filter and can be seen as interference fringes with a colour and shape that is dependent on the stress. Scaled-down models of real-world structures like bridges can be made in transparent plastic and measured in a polariscope to show internal stresses under loading.
Biological and chemical applications for a polarimeter
A polarimeter, which measures an angular change in polarization, can detect the presence and concentration of chiral molecules in a liquid. Chirality is a property of some sugars, vitamins, polymers, and many other optically active materials, which causes the polarization direction of light to be rotated as it passes through. Polarimetry has applications in biological and chemical research and industry, pharmaceuticals, and in the food industry.
Nanometer accuracy for research applications
Ellipsometry is a more advanced measurement, often used in semiconductor and thin-film research and industry to measure the optical properties of a material or coating. By measuring the polarization change of a light beam while controlling the incidence polarization, angle and wavelength, the data is fitted to an analytical model to estimate the refractive index, optical absorption, or the film thickness down to nanometer accuracy.
Improving image quality for machine vision applications
Machine vision and imaging systems can also be improved by using simple polarizing filters. Strong specular reflections and glare from flat or polished parts on a production line, for example, can be reduced to ensure the image is not saturated and surface details are not lost.
User:NEON_ja, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons
High contrast imaging with polarized light microscopy
Polarized light microscopy employs polarized light to enhance the contrast of the specimen being imaged. DIC microscopy is one such technique, where polarized light is split into two beams before passing through the specimen at different points. The different optical thicknesses in the specimen cause a phase shift between the two beams, which then interfere as they are recombined. The resulting image highlights differences in optical thickness across the specimen to give higher contrast, especially at edges.
Polarizing filters are simple, robust and inexpensive
In general, thinking about polarized light for optical measurement can bring big benefits and can reveal details and properties that are otherwise invisible. Polarizing filters can be simple, compact, robust and relatively inexpensive, and can often be simply added into an existing measurement system.
DVel have engineers and PhDs with expertise in many different measurement techniques. Get in touch if you want to know more about how we can help you with measurements and technical innovation!
By Graham Triggs-Ramm, Measurement Specialist DVel