While two or more analytes within an optical cavity move relative to an array of photosensing elements, the cavity provides output light that has a position/time varying intensity function that depends on optical characteristics of the analytes and on the relative movement. The output light is photosensed to obtain sensing results that depend on the position/time varying intensity function. The sensing results are used to obtain information about at least one of the analytes. The relative movement can, for example, be caused by moving analytes within channels within the cavity, such as by causing flow of a medium that carries the analytes through the channels. Or the analytes can be in wells of a biochip, with the cavity defined by reflective slides on opposite surfaces of the biochip, and the slides and biochip can be caused to move together relative to the array.