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164 || AWSAR Awarded Popular Science Stories - 2019
cannot be seen through naked eyes. But it is the visible light of the electromagnetic spectrum that we are focusing upon. We use devices called polarizers and waveplates to alter the polarization state of light when we rotate them.
And do you know how we know what the exact state of polarization is? It is a play between physics and mathematics. A term called ‘Stokes vector’ gives information about the polarization state of light at the input and output, and they both are related by another term called the ‘Mueller matrix’, using which we can find out about the object. This method is called Mueller polarimetry and the Mueller matrix is like the ‘guru’ of polarization, who knows everything about the polarization properties of the object.”
“Whenthepolarizedlightpassesthrough an object, it could lose its polarization, that is when the electric field vibration is no longer in the same direction. This
could be the result of scattering
happening inside and is
measured by a term called depolarization. The light could
also undergo changes in the
phase, which can be measured
by retardance. The amount
of light that is transmitted
could give us information on
something called diattenuation.
There are many methods
to break down the Mueller
matrix and find out about the depolarization, retardance and
diattenuation effects.”
“So can we find the Mueller matrix of the human body?” they reflected.
“Well, we can find the Mueller matrix of smaller tissue regions since light can only travel a short distance inside these tissues, after which it either scatters or gets absorbed. In our laboratory, we perform studies on tissue
samples and optical phantoms....” “Phantom??” they collectively interrupted. “Phantoms are materials that we create to
mimic the properties of the real tissue sample. For example, we add certain substances with the same absorption and scattering properties as different tissues, to give an idea of what could happen in the real biological tissues,” Amused, I responded.
“Oh. But why?” one of the children asked.
“We need to perform a lot of tests with any medical instrument before it gets used by doctors in real clinical settings. Once we find the response of the phantoms and other samples, we can know for sure if the instrument is ready to test real tissues and we can also use them as a basis to predict the tissue behaviour. In our experiments, we developed phantoms to mimic normal and diseased tissues. By measuring the Mueller matrix and finding out about the basic polarization
parameters of both normal and abnormal samples, we can identify the abnormal. In one of our studies, we obtained the amount of depolarization that was measured by a term called depolarization index from the Mueller matrix and observed that it is directly related to the changes in scattering that happened in cancer mimicking phantoms. In another study with skin tissues, we used the Mueller matrix to differentiate between normal skin and
scleroderma. Scleroderma refers to a group of diseases in which collagen, a protein that gives structural strength is produced in excess in the skin. This is seen as hardened skin in people with scleroderma. Since these collagen structures are responsible for scattering (A: What are they scattering? Light?) within the cell, the scleroderma skin sample shows
We developed phantoms to mimic normal and diseased tissues. By measuring the Mueller matrix and finding out about the basic polarization parameters of both normal and abnormal samples, we can identify the abnormal.