Abstract: A device for characterizing a cell or particle includes a channel having an inlet and an outlet, the channel containing a moving fluid therein for carrying the cell or particle from the inlet to the outlet. The device includes a detector for detecting the presence of a cell or particle along portion of the channel, the detector including a first detecting position, a second detecting position, and a third detecting position. The device further includes a light source providing an optical gradient disposed within the channel and between the second and third detecting positions. A control system is coupled to the detector to receive and process detected signals from the detector. During operation, the amount of time that a cell or particle takes to flow through a first distance (i.e., its time-of-flight) is measured. The cell or particle is then flowed past a second, downstream distance in the presence of an optical gradient and its time-of-flight is measured.