Easy method to determine refractive indices of microspheres and in micro-regions of inhomogeneous media.

We describe an innovative method which can accurately determine the refractive index (RI) of individual microspheres by immersing the microspheres in a medium and analyzing their phase-contrast microscopic images. Compared with the current techniques for microsphere RI measurement, the method has several advantages: it is simple and easy and it cannot only measure the RI of each individual microsphere but also perform measurement simultaneously on all the microspheres in the same field of view. In measurement, microspheres are not required to be suspended in a specific liquid but in any medium with known RI which is appropriate for the microspheres or even just in atmosphere. By using microspheres with known RI as sensors, the method can also be used for rapid in situ measurement of the local RI of inhomogeneous media. In this paper, we describe the principle of the method and the experiments of using the method to measure the RI of individual microspheres. Its applications for sensing instantaneous RI/concentration/temperature variation in critical situations such as anywhere in mixing flows or living biological specimens are also presented.