On-line detection of mesophase particles employs a laser diode light source to illuminate a target area with a pulsed laser linearly or circularly polarized probe beam. Analysis of images determines extent of presence the birefringent mesophase particles, which are precursors to coking in catalytic hydrocracking processes. The inherently polarized low-coherence, unfocused but sufficiently collimated, pulsed laser beam yield sharp imaging with high depth of field of very small mesophase particles that are present in a moving, dark reactor liquid environment.
