Spatially modulated light emanating from an object moving along a flow path is used to determine various object characteristics including object length along the flow direction. Light emanating from at least one object moving along in a flow path along a flow direction of a spatial filter is sensed. The intensity of the sensed light is time modulated according to features of the spatial filter. A time varying electrical signal is generated which includes a plurality of pulses in response to the sensed light. Pulse widths of at least some of the pulses are measured at a fraction of a local extremum of the pulses. The length of the object along the flow direction is determined based on the measured pulse widths.