An apparatus for detecting a size of particles in a spray jet includes a substantially square frame. A first emitter unit and a first detection unit, and a second emitter unit and a second detection unit, are respectively provided at opposite corners of the frame. Two light beams that cross each other in a focus are generated by the respective emitter units. Passage of a particle through the focus is detected based on light intensity signals from the first and second detection units. Using another set of light beams that cross each other and that are separated from the above-noted light beams in the direction of flight of the particle, passage of the particle through a second common focus can be detected. Particle velocity and particle size can be obtained based on the time difference between the passage of the particle through the two foci.