Method of measuring properties of particles and corresponding apparatus

1. A method of measuring properties of particles, comprising the steps of:

generating a beam of radiation which is propagated along an optical axis;

illuminating with the beam an observation region which is transited by a particle, a portion of the beam giving rise to radiation which is scattered by scattering interaction of the portion of the beam with the particle, and another portion being transmitted substantially undisturbed along the optical axis through the observation region; and

detecting, in a detection plane disposed along the optical axis, a plurality of radiation intensity values which are determined by the interference between the scattered radiation and the transmitted radiation;

wherein said detection of the radiation intensity values in the detection plane is carried out according to a time sequence of acquisitions corresponding to successive transit positions of said particle through the observation region, and

wherein on the basis of said time sequence of acquisitions, the trend of a parameter of asymmetry of the distribution of the plurality of radiation intensity values with respect to the optical axis, due to the successive transit positions of the particle, is determined as a function of time,

depending on the trend of the parameter of asymmetry which has been determined as a function of time, the trends of phase delay and amplitude of the scattered radiation with respect to the transmitted radiation are determined as a function of time, and

the properties of the particle are determined on the basis of the trends of the phase delay and amplitude of the scattered radiation as a function of time.

2. A method according to claim 1 , wherein the radiation beam is focused in such a way so that its minimum diameter region is disposed in the vicinity of the observation region.

3. A method according to claim 2 , wherein the observation region is disposed within the Rayleigh zone of said beam, the radiation beam being focused so that the extent of the Rayleigh zone is greater than the width of the observation region in the direction of the optical axis.

4. A method according to claim 1 , wherein said detection of the radiation intensity values in the detection plane is carried out in such a way to measure one value of radiation intensity detected as a whole by each of four quadrants of the detection plane, said quadrants being defined by a pair of reciprocally perpendicular axes which intersect each other at the optical axis, one of said axes being disposed parallel to the projection of the transit direction of the particle on the detection plane.

5. A method according to claim 4 , wherein the asymmetry parameter is determined by comparing the values of radiation intensity detected as a whole by the corresponding quadrants in the same acquisition.

6. A method according to claim 5 , wherein the determination of the trends of phase delay and amplitude of the scattered radiation with respect to the transmitted radiation is carried out selectively in the case of the trend of the asymmetry parameter determined as a function of time corresponding to a substantially diametrical transit of the particle.

7. Apparatus arranged for implementing a measurement method according to claim 1 , comprising:

a source of the radiation beam, suitable for illuminating the observation region;

a sensor disposed on the detection plane and along the optical axis in a manner to detect simultaneously a plurality of radiation intensity values determined by the interference between the scattered radiation and the transmitted radiation and to make available a signal indicative of the detection, said detection of the radiation intensity values in the detection plane being carried out according to a time sequence of acquisitions corresponding to successive transit positions of the particle through the observation region; and

a processor adapted to process said signal, the processor being programmed to:

determine, on the basis of said time sequence of acquisitions, the trend of a parameter of asymmetry of the distribution of the plurality of radiation intensity values with respect to the optical axis, due to the successive transit positions of the particle, as a function of time,

determine, depending on the trend of the parameter of asymmetry which has been determined as a function of time, the trends of phase delay and amplitude of the scattered radiation with respect to the transmitted radiation as a function of time, and

determine the properties of the particle on the basis of the trends of the phase delay and amplitude of the scattered radiation as a function of time.

8. Apparatus according to claim 7 , wherein said sensor comprises a quadrant sensor comprising a sensitive surface subdivided into four quadrants which are adapted to detect respective values of said plurality of radiation intensity values, said quadrants being delimited by a pair of reciprocally perpendicular axes which intersect each other at the optical axis, one of said axes being disposed parallel to the projection of the transit direction of the particle on the detection plane.

9. Apparatus according to claim 8 , wherein said processor is programmed to determine the asymmetry parameter by comparing the values of radiation intensity detected as a whole by the corresponding quadrants in the same acquisition.

10. Apparatus according to claim 9 , wherein said processor is programmed to determine the trends of phase delay and amplitude of the scattered radiation with respect to the transmitted radiation selectively in the case of the trend of the asymmetry parameter determined as a function of time corresponding to a substantially diametrical transit of the particle.