Abstract: A method and apparatus for performing an assay process, featuring providing microbeads in a solution, each microbead having a particle substrate with a grating with a superposition of different predetermined regular periodic variations of the index of refraction disposed in the particle along a grating axis and indicative of a code; placing the microbeads on an alignment substrate; reading codes of the microbeads and the position thereof on the alignment substrate; reading the fluorescence on each microbead and the position order thereof on the alignment substrate; and determining an assay result based on bead position order and bead code of the earlier reading steps.