Summary: Digital cameras normally use color filter arrays (and some image processing) to turn the underlying monochrome sensor into a color sensor. These guys did it with an array of very tiny polarizers, making a camera that sees polarization of light rather than its color. This turns out to be useful for a lot of things. What about cancerous cells makes them polarize the light differently? Cells have skeletons. Cancer cell skeletons are different than normal cell skeletons. Tiny ordered things, which includes cell skekelons, reflect different polarizations of light in different ways depending on the exact structure and orientation. If you shine unpolarized light on cells and detect the polarization of the light coming back, that reflected light looks different if it is from normal or cancerous cells.