1. A particle image-capturing method, comprising: A) controlling a light-emitting diode unit to emit light so as to illuminate fluid through an annular condenser, wherein particles in the fluid scatter the light; and B) controlling an image-capturing unit to capture images of the scattered light through a microscope; wherein steps A) and B) are performed simultaneously and repeatedly in one cycle of operation, and the light-emitting diode unit emits a respective distinct color each time step A) is repeated in one cycle of operation.