Like a distant relative, polarisation is mostly ignored. Some people are completely unaware of its existence or just a few facts are known. Even for optics or physics graduates, polarisation may have been a side comment or footnote. As optical applications continuously increase, especially in life science, it may be a good idea to take a closer look. From endoscopy over aesthetic surgery to microscopy for diagnostics, for any developer or researcher working with light or imaging, polarisation could be the answer for improving applications or developing new ones.
Polarisation is all around; it describes the orientation of the electric field vector used to define light. The vector oscillates perpendicularly to the propagation direction. Light is called unpolarised if the direction fluctuates randomly in time. Sunlight, LED spotlights, halogen lighting and many other light sources emit unpolarised light. Lasers are an example of polarised light, with a defined direction of the electrical field.
The key property used for most applications is the changed polarisation direction by every scattering, reflection and refraction. Every substance reflecting light or being translucent - even tiny dust particles in the air - can change the polarisation direction for some part of the light. Polarisers work this way, with the difference that at one point, only the defined polarisation remains.
Defined polarised light carries additional information and is used by analysing reflected or transmitted light. For example, cancer cells cause a different polarisation than the surrounding tissue. Veins and arteries up to 10mm under the skin can be made visible with polarisers and near-infrared (NIR) light, because blood absorbs NIR light while the surrounding tissue reflects it back to the sensor. This can improve shots and blood taking for patients and nurses, especially if done without a peripheral venous catheter. Submicroscopic collagen fibres can be made visible with a light microscope and polarisers, reducing laboratory equipment. Or information can be lost by reflections in case of optical sensors or imaging, which can have problems with glare. A polariser can cancel out only the electrical field and, therefore, the light causing the disturbing glare. In endoscopy analysing the digestive system, the glare of body fluids can reduce visibility. This glare can be reduced or eliminated completely by a polariser.
For other imaging applications, like 3D scanners for dental impressions, a polariser reduces glare and improves the contrast significantly. Polarised glasses help to see all pigments and vessels during the removal of pigmentation marks; without these glasses, they would not be visible to the naked eye.
This is just a short selection of applications for using polarisation in life science, and more are being developed to improve diagnostics and treatments for patients and staff. These possibilities of polarisation should not be ignored but examined in detail, as there is more to be discovered. However, there might be a wrong polariser type for your application, so get to know the whole family and their characteristics before you choose.